Version: 247.2.3

Also, only read assemblies once from disk

(cherry picked from commit 443a8dfca65be7d60c4bd46181b4c749b4756114)

Directory.Packages.props

@@ -57,7 +57,7 @@
     <PackageVersion Include="SharpZstd.Interop" Version="1.5.2-beta2" />
     <PackageVersion Include="SixLabors.ImageSharp" Version="3.1.6" />
     <PackageVersion Include="SpaceWizards.HttpListener" Version="0.1.1" />
-    <PackageVersion Include="SpaceWizards.NFluidsynth" Version="0.1.1" />
+    <PackageVersion Include="SpaceWizards.NFluidsynth" Version="0.2.2" />
     <PackageVersion Include="SpaceWizards.SharpFont" Version="1.0.2" />
     <PackageVersion Include="SpaceWizards.Sodium" Version="0.2.1" />
     <PackageVersion Include="TerraFX.Interop.Windows" Version="10.0.26100.1" />

MSBuild/Robust.Engine.Version.props

@@ -1,4 +1,4 @@
 <Project>
 
-    <!-- This file automatically reset by Tools/version.py -->
+    <!-- This file automatically reset by Tools/version.py -->
 

RELEASE-NOTES.md

@@ -54,6 +54,102 @@ END TEMPLATE-->
 *None yet*
 
 
+## 247.2.3
+
+
+## 247.2.2
+
+
+## 247.2.1
+
+
+## 247.2.0
+
+### New features
+
+* Added functions for copying components to `IEntityManager` and `EntitySystem`.
+* Sound played from sound collections is now sent as "collection ID + index" over the network instead of the final filename.
+  * This enables integration of future accessibility systems.
+  * Added a new `ResolvedSoundSpecifier` to represent played sounds. Methods that previously took a filename now take a `ResolvedSoundSpecifier`, with an implicit cast from string being interpreted as a raw filename.
+* `VisibilitySystem` has been made accessible to shared as `SharedVisibilitySystem`.
+* `ScrollContainer` now has properties exposing `Value` and `ValueTarget` on its internal scroll bars.
+
+### Bugfixes
+
+* Fix prototype hot reload crashing when adding a new component already exists on an entity.
+* Fix maps failing to save in some cases related to tilemap IDs.
+* Fix `Regex.Escape(string)` not being available in sandbox.
+* Prototypes that parent themselves directly won't cause the game to hang on an infinite loop anymore.
+* Fixed disconnecting during a connection attempt leaving the client stuck in a phantom state.
+
+### Internal
+
+* More warning cleanup.
+
+## 247.1.0
+
+### New features
+
+* Added support for `Color[]` shader uniforms
+* Added optional minimumDistance parameter to `SharedJointSystem.CreateDistanceJoint()`
+
+### Bugfixes
+
+* Fixed `EntitySystem.DirtyFields()` not actually marking fields as dirty.
+
+### Other
+
+* Updated the Yamale map file format validator to support v7 map/grid files.
+
+
+## 247.0.0
+
+### Breaking changes
+
+* `ITileDefinitionManager.AssignAlias` and general tile alias functionality has been removed. `TileAliasPrototype` still exist, but are only used during entity deserialization.
+* `IMapManager.AddUninitializedMap` has been removed. Use the map-init options on `CreateMap()` instead.
+* Re-using a MapId will now log a warning. This may cause some integration tests to fail if they are configured to fail
+  when warnings are logged.
+* The minimum supported map format / version has been increased from 2 to 3.
+* The server-side `MapLoaderSystem` and associated classes & structs has been moved to `Robust.Shared`, and has been significantly modified.
+  * The `TryLoad` and `Save` methods have been replaced with grid, map, generic entity variants. I.e, `SaveGrid`, `SaveMap`, and `SaveEntities`.
+  * Most of the serialization logic and methods have been moved out of `MapLoaderSystem` and into new `EntitySerializer`
+    and `EntityDeserializer` classes, which also replace the old `MapSerializationContext`.
+  * The `MapLoadOptions` class has been split into `MapLoadOptions`, `SerializationOptions`, and `DeserializationOptions`
+    structs.
+* The interaction between PVS overrides and visibility masks / layers have changed:
+  * Any forced entities (i.e., `PvsOverrideSystem.AddForceSend()`) now ignore visibility masks.
+  * Any global & session overrides (`PvsOverrideSystem.AddGlobalOverride()` & `PvsOverrideSystem.AddSessionOverride()`) now respect visibility masks.
+  * Entities added via the `ExpandPvsEvent` respect visibility masks.
+  * The mask used for any global/session overrides can be modified via `ExpandPvsEvent.Mask`.
+* Toolshed Changes:
+  * The signature of Toolshed type parsers have changed. Instead of taking in an optional command argument name string, they now take in a `CommandArgument` struct.
+  * Toolshed commands can no longer contain a '|', as this symbol is now used for explicitly piping the output of one command to another. command pipes. The existing `|` and '|~' commands have been renamed to `bitor` and `bitnotor`.
+  * Semicolon terminated command blocks in toolshed commands no longer return anything. I.e., `i { i 2 ; }` is no longer a valid command, as the block has no return value.
+
+### New features
+
+* The current map format/version has increased from 6 to 7 and now contains more information to try support serialization of maps with null-space entities and full game saves.
+* `IEntitySystemManager` now provides access to the system `IDependencyCollection`.
+* Toolshed commands now support optional and `params T[]` arguments. optional / variable length commands can be terminated using ';' or '|'.
+
+### Bugfixes
+
+* Fixed entity deserialization for components with a data fields that have a AlwaysPushInheritance Attribute
+* Audio entities attached to invisible / masked entities should no longer be able to temporarily make those entities visible to all players.
+* The map-like Toolshed commands now work when a collection is piped in.
+* Fixed a bug in toolshed that could cause it to preferentially use the incorrect command implementation.
+  * E.g., passing a concrete enumerable type would previously use the command implementation that takes in an unconstrained generic parameter `T` instead of a dedicated `IEnumeerable<T>` implementation.
+
+### Other
+
+* `MapChangedEvent` has been marked as obsolete, and should be replaced with `MapCreatedEvent` and `MapRemovedEvent.
+* The default auto-completion hint for Toolshed commands have been changed and somewhat standardized. Most parsers should now generate a hint of the form:
+  * `<name (Type)>` for mandatory arguments
+  * `[name (Type)]` for optional arguments
+  * `[name (Type)]...` for variable length arguments (i.e., for `params T[]`)
+
+
 ## 246.0.0
 
 ### Breaking changes

Resources/Locale/en-US/toolshed-commands.ftl

@@ -42,8 +42,7 @@ command-description-as =
 command-description-count =
     Counts the amount of entries in it's input, returning an integer.
 command-description-map =
-    Maps the input over the given block, with the provided expected return type.
-    This command may be modified to not need an explicit return type in the future.
+    Maps the input over the given block.
 command-description-select =
     Selects N objects or N% of objects from the input.
     One can additionally invert this command with not to make it select everything except N objects instead.
@@ -149,7 +148,7 @@ command-description-max =
     Returns the maximum of two values.
 command-description-BitAndCommand =
     Performs bitwise AND.
-command-description-BitOrCommand =
+command-description-bitor =
     Performs bitwise OR.
 command-description-BitXorCommand =
     Performs bitwise XOR.
@@ -203,11 +202,11 @@ command-description-mappos =
 command-description-pos =
     Returns an entity's coordinates.
 command-description-tp-coords =
-    Teleports the target to the given coordinates.
+    Teleports the given entities to the target coordinates.
 command-description-tp-to =
-    Teleports the target to the given other entity.
+    Teleports the given entities to the target entity.
 command-description-tp-into =
-    Teleports the target "into" the given other entity, attaching it at (0 0) relative to it.
+    Teleports the given entities "into" the target entity, attaching it at (0 0) relative to it.
 command-description-comp-get =
     Gets the given component from the given entity.
 command-description-comp-add =
@@ -277,7 +276,7 @@ command-description-ModVecCommand =
     Performs the modulus operation over the input with the given constant right-hand value.
 command-description-BitAndNotCommand =
     Performs bitwise AND-NOT over the input.
-command-description-BitOrNotCommand =
+command-description-bitornot =
     Performs bitwise OR-NOT over the input.
 command-description-BitXnorCommand =
     Performs bitwise XNOR over the input.

Robust.Client.WebView/Cef/Program.cs

@@ -6,7 +6,7 @@ using Xilium.CefGlue;
 
 namespace Robust.Client.WebView.Cef
 {
-    public static class Program
+    internal static class Program
     {
         // This was supposed to be the main entry for the subprocess program... It doesn't work.
         public static int Main(string[] args)

Robust.Client.WebView/Cef/WebViewManagerCef.cs

@@ -5,6 +5,7 @@ using System.Net;
 using System.Reflection;
 using System.Text;
 using Robust.Client.Console;
+using Robust.Client.Utility;
 using Robust.Shared.Configuration;
 using Robust.Shared.ContentPack;
 using Robust.Shared.IoC;
@@ -24,6 +25,7 @@ namespace Robust.Client.WebView.Cef
 
         [Dependency] private readonly IDependencyCollection _dependencyCollection = default!;
         [Dependency] private readonly IPrototypeManager _prototypeManager = default!;
+        [Dependency] private readonly IGameControllerInternal _gameController = default!;
         [Dependency] private readonly IResourceManagerInternal _resourceManager = default!;
         [Dependency] private readonly IClientConsoleHost _consoleHost = default!;
         [Dependency] private readonly IConfigurationManager _cfg = default!;
@@ -61,7 +63,10 @@ namespace Robust.Client.WebView.Cef
 
             var cachePath = "";
             if (_resourceManager.UserData is WritableDirProvider userData)
-                cachePath = userData.GetFullPath(new ResPath("/cef_cache"));
+            {
+                var rootDir = UserDataDir.GetRootUserDataDir(_gameController);
+                cachePath = Path.Combine(rootDir, "cef_cache", "0");
+            }
 
             var settings = new CefSettings()
             {

Robust.Client/Animations/AnimationTrackPlaySound.cs

@@ -40,11 +40,7 @@ namespace Robust.Client.Animations
                 var keyFrame = KeyFrames[keyFrameIndex];
 
                 var audioParams = keyFrame.AudioParamsFunc.Invoke();
-                var audio = new SoundPathSpecifier(keyFrame.Resource)
-                {
-                    Params = audioParams
-                };
-                IoCManager.Resolve<IEntitySystemManager>().GetEntitySystem<AudioSystem>().PlayEntity(audio, Filter.Local(), entity, true);
+                IoCManager.Resolve<IEntitySystemManager>().GetEntitySystem<AudioSystem>().PlayEntity(keyFrame.Specifier, Filter.Local(), entity, true, audioParams);
             }
 
             return (keyFrameIndex, playingTime);
@@ -55,7 +51,7 @@ namespace Robust.Client.Animations
             /// <summary>
             ///     The RSI state to play when this keyframe gets triggered.
             /// </summary>
-            public readonly string Resource;
+            public readonly ResolvedSoundSpecifier Specifier;
 
             /// <summary>
             ///     A function that returns the audio parameter to be used.
@@ -69,9 +65,9 @@ namespace Robust.Client.Animations
             /// </summary>
             public readonly float KeyTime;
 
-            public KeyFrame(string resource, float keyTime, Func<AudioParams>? audioParams = null)
+            public KeyFrame(ResolvedSoundSpecifier specifier, float keyTime, Func<AudioParams>? audioParams = null)
             {
-                Resource = resource;
+                Specifier = specifier;
                 KeyTime = keyTime;
                 AudioParamsFunc = audioParams ?? (() => AudioParams.Default);
             }

Robust.Client/Audio/AudioSystem.cs

@@ -415,6 +415,16 @@ public sealed partial class AudioSystem : SharedAudioSystem
         return occlusion;
     }
 
+    private bool TryGetAudio(ResolvedSoundSpecifier specifier, [NotNullWhen(true)] out AudioResource? audio)
+    {
+        var filename = GetAudioPath(specifier);
+        if (_resourceCache.TryGetResource(new ResPath(filename), out audio))
+            return true;
+
+        Log.Error($"Server tried to play audio file {filename} which does not exist.");
+        return false;
+    }
+
     private bool TryGetAudio(string filename, [NotNullWhen(true)] out AudioResource? audio)
     {
         if (_resourceCache.TryGetResource(new ResPath(filename), out audio))
@@ -433,15 +443,15 @@ public sealed partial class AudioSystem : SharedAudioSystem
         return false;
     }
 
-    public override (EntityUid Entity, AudioComponent Component)? PlayPvs(string? filename, EntityCoordinates coordinates,
+    public override (EntityUid Entity, AudioComponent Component)? PlayPvs(ResolvedSoundSpecifier? specifier, EntityCoordinates coordinates,
         AudioParams? audioParams = null)
     {
-        return PlayStatic(filename, Filter.Local(), coordinates, true, audioParams);
+        return PlayStatic(specifier, Filter.Local(), coordinates, true, audioParams);
     }
 
-    public override (EntityUid Entity, AudioComponent Component)? PlayPvs(string? filename, EntityUid uid, AudioParams? audioParams = null)
+    public override (EntityUid Entity, AudioComponent Component)? PlayPvs(ResolvedSoundSpecifier? specifier, EntityUid uid, AudioParams? audioParams = null)
     {
-        return PlayEntity(filename, Filter.Local(), uid, true, audioParams);
+        return PlayEntity(specifier, Filter.Local(), uid, true, audioParams);
     }
 
     /// <inheritdoc />
@@ -477,21 +487,21 @@ public sealed partial class AudioSystem : SharedAudioSystem
     /// </summary>
     /// <param name="filename">The resource path to the OGG Vorbis file to play.</param>
     /// <param name="audioParams"></param>
-    private (EntityUid Entity, AudioComponent Component)? PlayGlobal(string? filename, AudioParams? audioParams = null, bool recordReplay = true)
+    private (EntityUid Entity, AudioComponent Component)? PlayGlobal(ResolvedSoundSpecifier? specifier, AudioParams? audioParams = null, bool recordReplay = true)
     {
-        if (string.IsNullOrEmpty(filename))
+        if (specifier is null)
             return null;
 
         if (recordReplay && _replayRecording.IsRecording)
         {
             _replayRecording.RecordReplayMessage(new PlayAudioGlobalMessage
             {
-                FileName = filename,
+                Specifier = specifier,
                 AudioParams = audioParams ?? AudioParams.Default
             });
         }
 
-        return TryGetAudio(filename, out var audio) ? PlayGlobal(audio, audioParams) : default;
+        return TryGetAudio(specifier, out var audio) ? PlayGlobal(audio, specifier, audioParams) : default;
     }
 
     /// <summary>
@@ -499,9 +509,9 @@ public sealed partial class AudioSystem : SharedAudioSystem
     /// </summary>
     /// <param name="stream">The audio stream to play.</param>
     /// <param name="audioParams"></param>
-    public (EntityUid Entity, AudioComponent Component)? PlayGlobal(AudioStream stream, AudioParams? audioParams = null)
+    public (EntityUid Entity, AudioComponent Component)? PlayGlobal(AudioStream stream, ResolvedSoundSpecifier? specifier, AudioParams? audioParams = null)
     {
-        var (entity, component) = CreateAndStartPlayingStream(audioParams, stream);
+        var (entity, component) = CreateAndStartPlayingStream(audioParams, specifier, stream);
         component.Global = true;
         component.Source.Global = true;
         DirtyField(entity, component, nameof(AudioComponent.Global));
@@ -513,22 +523,22 @@ public sealed partial class AudioSystem : SharedAudioSystem
     /// </summary>
     /// <param name="filename">The resource path to the OGG Vorbis file to play.</param>
     /// <param name="entity">The entity "emitting" the audio.</param>
-    private (EntityUid Entity, AudioComponent Component)? PlayEntity(string? filename, EntityUid entity, AudioParams? audioParams = null, bool recordReplay = true)
+    private (EntityUid Entity, AudioComponent Component)? PlayEntity(ResolvedSoundSpecifier? specifier, EntityUid entity, AudioParams? audioParams = null, bool recordReplay = true)
     {
-        if (string.IsNullOrEmpty(filename))
+        if (specifier is null)
             return null;
 
         if (recordReplay && _replayRecording.IsRecording)
         {
             _replayRecording.RecordReplayMessage(new PlayAudioEntityMessage
             {
-                FileName = filename,
+                Specifier = specifier,
                 NetEntity = GetNetEntity(entity),
                 AudioParams = audioParams ?? AudioParams.Default
             });
         }
 
-        return TryGetAudio(filename, out var audio) ? PlayEntity(audio, entity, audioParams) : default;
+        return TryGetAudio(specifier, out var audio) ? PlayEntity(audio, entity, specifier, audioParams) : default;
     }
 
     /// <summary>
@@ -537,7 +547,7 @@ public sealed partial class AudioSystem : SharedAudioSystem
     /// <param name="stream">The audio stream to play.</param>
     /// <param name="entity">The entity "emitting" the audio.</param>
     /// <param name="audioParams"></param>
-    public (EntityUid Entity, AudioComponent Component)? PlayEntity(AudioStream stream, EntityUid entity, AudioParams? audioParams = null)
+    public (EntityUid Entity, AudioComponent Component)? PlayEntity(AudioStream stream, EntityUid entity, ResolvedSoundSpecifier? specifier, AudioParams? audioParams = null)
     {
         if (TerminatingOrDeleted(entity))
         {
@@ -545,7 +555,7 @@ public sealed partial class AudioSystem : SharedAudioSystem
             return null;
         }
 
-        var playing = CreateAndStartPlayingStream(audioParams, stream);
+        var playing = CreateAndStartPlayingStream(audioParams, specifier, stream);
         _xformSys.SetCoordinates(playing.Entity, new EntityCoordinates(entity, Vector2.Zero));
 
         return playing;
@@ -557,22 +567,22 @@ public sealed partial class AudioSystem : SharedAudioSystem
     /// <param name="filename">The resource path to the OGG Vorbis file to play.</param>
     /// <param name="coordinates">The coordinates at which to play the audio.</param>
     /// <param name="audioParams"></param>
-    private (EntityUid Entity, AudioComponent Component)? PlayStatic(string? filename, EntityCoordinates coordinates, AudioParams? audioParams = null, bool recordReplay = true)
+    private (EntityUid Entity, AudioComponent Component)? PlayStatic(ResolvedSoundSpecifier? specifier, EntityCoordinates coordinates, AudioParams? audioParams = null, bool recordReplay = true)
     {
-        if (string.IsNullOrEmpty(filename))
+        if (specifier is null)
             return null;
 
         if (recordReplay && _replayRecording.IsRecording)
         {
             _replayRecording.RecordReplayMessage(new PlayAudioPositionalMessage
             {
-                FileName = filename,
+                Specifier = specifier,
                 Coordinates = GetNetCoordinates(coordinates),
                 AudioParams = audioParams ?? AudioParams.Default
             });
         }
 
-        return TryGetAudio(filename, out var audio) ? PlayStatic(audio, coordinates, audioParams) : default;
+        return TryGetAudio(specifier, out var audio) ? PlayStatic(audio, coordinates, specifier, audioParams) : default;
     }
 
     /// <summary>
@@ -581,7 +591,7 @@ public sealed partial class AudioSystem : SharedAudioSystem
     /// <param name="stream">The audio stream to play.</param>
     /// <param name="coordinates">The coordinates at which to play the audio.</param>
     /// <param name="audioParams"></param>
-    public (EntityUid Entity, AudioComponent Component)? PlayStatic(AudioStream stream, EntityCoordinates coordinates, AudioParams? audioParams = null)
+    public (EntityUid Entity, AudioComponent Component)? PlayStatic(AudioStream stream, EntityCoordinates coordinates, ResolvedSoundSpecifier? specifier, AudioParams? audioParams = null)
     {
         if (TerminatingOrDeleted(coordinates.EntityId))
         {
@@ -589,33 +599,33 @@ public sealed partial class AudioSystem : SharedAudioSystem
             return null;
         }
 
-        var playing = CreateAndStartPlayingStream(audioParams, stream);
+        var playing = CreateAndStartPlayingStream(audioParams, specifier, stream);
         _xformSys.SetCoordinates(playing.Entity, coordinates);
         return playing;
     }
 
     /// <inheritdoc />
-    public override (EntityUid Entity, AudioComponent Component)? PlayGlobal(string? filename, Filter playerFilter, bool recordReplay, AudioParams? audioParams = null)
+    public override (EntityUid Entity, AudioComponent Component)? PlayGlobal(ResolvedSoundSpecifier? specifier, Filter playerFilter, bool recordReplay, AudioParams? audioParams = null)
     {
-        return PlayGlobal(filename, audioParams);
+        return PlayGlobal(specifier, audioParams);
     }
 
     /// <inheritdoc />
-    public override (EntityUid Entity, AudioComponent Component)? PlayEntity(string? filename, Filter playerFilter, EntityUid entity, bool recordReplay, AudioParams? audioParams = null)
+    public override (EntityUid Entity, AudioComponent Component)? PlayEntity(ResolvedSoundSpecifier? specifier, Filter playerFilter, EntityUid entity, bool recordReplay, AudioParams? audioParams = null)
     {
-        return PlayEntity(filename, entity, audioParams);
+        return PlayEntity(specifier, entity, audioParams);
     }
 
     /// <inheritdoc />
-    public override (EntityUid Entity, AudioComponent Component)? PlayStatic(string? filename, Filter playerFilter, EntityCoordinates coordinates, bool recordReplay, AudioParams? audioParams = null)
+    public override (EntityUid Entity, AudioComponent Component)? PlayStatic(ResolvedSoundSpecifier? specifier, Filter playerFilter, EntityCoordinates coordinates, bool recordReplay, AudioParams? audioParams = null)
     {
-        return PlayStatic(filename, coordinates, audioParams);
+        return PlayStatic(specifier, coordinates, audioParams);
     }
 
     /// <inheritdoc />
-    public override (EntityUid Entity, AudioComponent Component)? PlayGlobal(string? filename, ICommonSession recipient, AudioParams? audioParams = null)
+    public override (EntityUid Entity, AudioComponent Component)? PlayGlobal(ResolvedSoundSpecifier? specifier, ICommonSession recipient, AudioParams? audioParams = null)
     {
-        return PlayGlobal(filename, audioParams);
+        return PlayGlobal(specifier, audioParams);
     }
 
     public override void LoadStream<T>(Entity<AudioComponent> entity, T stream)
@@ -629,39 +639,39 @@ public sealed partial class AudioSystem : SharedAudioSystem
     }
 
     /// <inheritdoc />
-    public override (EntityUid Entity, AudioComponent Component)? PlayGlobal(string? filename, EntityUid recipient, AudioParams? audioParams = null)
+    public override (EntityUid Entity, AudioComponent Component)? PlayGlobal(ResolvedSoundSpecifier? specifier, EntityUid recipient, AudioParams? audioParams = null)
     {
-        return PlayGlobal(filename, audioParams);
+        return PlayGlobal(specifier, audioParams);
     }
 
     /// <inheritdoc />
-    public override (EntityUid Entity, AudioComponent Component)? PlayEntity(string? filename, ICommonSession recipient, EntityUid uid, AudioParams? audioParams = null)
+    public override (EntityUid Entity, AudioComponent Component)? PlayEntity(ResolvedSoundSpecifier? specifier, ICommonSession recipient, EntityUid uid, AudioParams? audioParams = null)
     {
-        return PlayEntity(filename, uid, audioParams);
+        return PlayEntity(specifier, uid, audioParams);
     }
 
     /// <inheritdoc />
-    public override (EntityUid Entity, AudioComponent Component)? PlayEntity(string? filename, EntityUid recipient, EntityUid uid, AudioParams? audioParams = null)
+    public override (EntityUid Entity, AudioComponent Component)? PlayEntity(ResolvedSoundSpecifier? specifier, EntityUid recipient, EntityUid uid, AudioParams? audioParams = null)
     {
-        return PlayEntity(filename, uid, audioParams);
+        return PlayEntity(specifier, uid, audioParams);
     }
 
     /// <inheritdoc />
-    public override (EntityUid Entity, AudioComponent Component)? PlayStatic(string? filename, ICommonSession recipient, EntityCoordinates coordinates, AudioParams? audioParams = null)
+    public override (EntityUid Entity, AudioComponent Component)? PlayStatic(ResolvedSoundSpecifier? specifier, ICommonSession recipient, EntityCoordinates coordinates, AudioParams? audioParams = null)
     {
-        return PlayStatic(filename, coordinates, audioParams);
+        return PlayStatic(specifier, coordinates, audioParams);
     }
 
     /// <inheritdoc />
-    public override (EntityUid Entity, AudioComponent Component)? PlayStatic(string? filename, EntityUid recipient, EntityCoordinates coordinates, AudioParams? audioParams = null)
+    public override (EntityUid Entity, AudioComponent Component)? PlayStatic(ResolvedSoundSpecifier? specifier, EntityUid recipient, EntityCoordinates coordinates, AudioParams? audioParams = null)
     {
-        return PlayStatic(filename, coordinates, audioParams);
+        return PlayStatic(specifier, coordinates, audioParams);
     }
 
-    private (EntityUid Entity, AudioComponent Component) CreateAndStartPlayingStream(AudioParams? audioParams, AudioStream stream)
+    private (EntityUid Entity, AudioComponent Component) CreateAndStartPlayingStream(AudioParams? audioParams, ResolvedSoundSpecifier? specifier, AudioStream stream)
     {
         var audioP = audioParams ?? AudioParams.Default;
-        var entity = SetupAudio(null, audioP, initialize: false, length: stream.Length);
+        var entity = SetupAudio(specifier, audioP, initialize: false, length: stream.Length);
         LoadStream(entity, stream);
         EntityManager.InitializeAndStartEntity(entity);
         var comp = entity.Comp;
@@ -694,17 +704,17 @@ public sealed partial class AudioSystem : SharedAudioSystem
 
     private void OnEntityCoordinates(PlayAudioPositionalMessage ev)
     {
-        PlayStatic(ev.FileName, GetCoordinates(ev.Coordinates), ev.AudioParams, false);
+        PlayStatic(ev.Specifier, GetCoordinates(ev.Coordinates), ev.AudioParams, false);
     }
 
     private void OnEntityAudio(PlayAudioEntityMessage ev)
     {
-        PlayEntity(ev.FileName, GetEntity(ev.NetEntity), ev.AudioParams, false);
+        PlayEntity(ev.Specifier, GetEntity(ev.NetEntity), ev.AudioParams, false);
     }
 
     private void OnGlobalAudio(PlayAudioGlobalMessage ev)
     {
-        PlayGlobal(ev.FileName, ev.AudioParams, false);
+        PlayGlobal(ev.Specifier, ev.AudioParams, false);
     }
 
     protected override TimeSpan GetAudioLengthImpl(string filename)

Robust.Client/BaseClient.cs

@@ -115,10 +115,6 @@ namespace Robust.Client
         /// <inheritdoc />
         public void DisconnectFromServer(string reason)
         {
-            DebugTools.Assert(RunLevel > ClientRunLevel.Initialize);
-            DebugTools.Assert(_net.IsConnected);
-            // run level changed in OnNetDisconnect()
-            // are both of these *really* needed?
             _net.ClientDisconnect(reason);
         }
 

Robust.Client/GameController/GameController.cs

@@ -382,7 +382,7 @@ namespace Robust.Client
 
             _prof.Initialize();
 
-            _resManager.Initialize(Options.LoadConfigAndUserData ? userDataDir : null);
+            _resManager.Initialize(Options.LoadConfigAndUserData ? userDataDir : null, hideUserDataDir: true);
 
             var mountOptions = _commandLineArgs != null
                 ? MountOptions.Merge(_commandLineArgs.MountOptions, Options.MountOptions)

Robust.Client/GameObjects/EntitySystems/MapSystem.cs

@@ -17,7 +17,7 @@ public sealed class MapSystem : SharedMapSystem
     {
         // Client-side map entities use negative map Ids to avoid conflict with server-side maps.
         var id = new MapId(--LastMapId);
-        while (MapManager.MapExists(id))
+        while (MapExists(id) || UsedIds.Contains(id))
         {
             id = new MapId(--LastMapId);
         }

Robust.Client/GameObjects/EntitySystems/VisibilitySystem.cs

@@ -0,0 +1,8 @@
+using Robust.Shared.GameObjects;
+
+namespace Robust.Client.GameObjects;
+
+public sealed class VisibilitySystem : SharedVisibilitySystem
+{
+
+}

Robust.Client/Graphics/Clyde/Clyde.Rendering.cs

@@ -523,6 +523,9 @@ namespace Robust.Client.Graphics.Clyde
                     case Color color:
                         program.SetUniform(name, color);
                         break;
+                    case Color[] colorArr:
+                        program.SetUniform(name, colorArr);
+                        break;
                     case int i:
                         program.SetUniform(name, i);
                         break;

Robust.Client/Graphics/Clyde/Clyde.Shaders.cs

@@ -485,6 +485,13 @@ namespace Robust.Client.Graphics.Clyde
                 data.Parameters[name] = value;
             }
 
+            private protected override void SetParameterImpl(string name, Color[] value)
+            {
+                var data = Parent._shaderInstances[Handle];
+                data.ParametersDirty = true;
+                data.Parameters[name] = value;
+            }
+
             private protected override void SetParameterImpl(string name, int value)
             {
                 var data = Parent._shaderInstances[Handle];

Robust.Client/Graphics/Clyde/ClydeHeadless.cs

@@ -369,6 +369,10 @@ namespace Robust.Client.Graphics.Clyde
             {
             }
 
+            private protected override void SetParameterImpl(string name, Color[] value)
+            {
+            }
+
             private protected override void SetParameterImpl(string name, int value)
             {
             }

Robust.Client/Graphics/Clyde/GLObjects/Clyde.ShaderProgram.cs

@@ -334,7 +334,7 @@ namespace Robust.Client.Graphics.Clyde
             }
 
             [MethodImpl(MethodImplOptions.AggressiveInlining)]
-            private void SetUniformDirect(int slot, in Color color, bool convertToLinear=true)
+            private void SetUniformDirect(int slot, in Color color, bool convertToLinear = true)
             {
                 var converted = color;
                 if (convertToLinear)
@@ -349,6 +349,39 @@ namespace Robust.Client.Graphics.Clyde
                 }
             }
 
+            public void SetUniform(string uniformName, Color[] colors)
+            {
+                var uniformId = GetUniform(uniformName);
+                SetUniformDirect(uniformId, colors);
+            }
+
+            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+            private void SetUniformDirect(int slot, Color[] colors, bool convertToLinear = true)
+            {
+                scoped Span<Color> colorsToPass;
+                if (convertToLinear)
+                {
+                    colorsToPass = stackalloc Color[colors.Length];
+                    for (int i = 0; i < colors.Length; i++)
+                    {
+                        colorsToPass[i] = Color.FromSrgb(colors[i]);
+                    }
+                }
+                else
+                {
+                    colorsToPass = colors;
+                }
+
+                unsafe
+                {
+                    fixed (Color* ptr = &colorsToPass[0])
+                    {
+                        GL.Uniform4(slot, colorsToPass.Length, (float*)ptr);
+                        _clyde.CheckGlError();
+                    }
+                }
+            }
+
             public void SetUniform(string uniformName, in Vector3 vector)
             {
                 var uniformId = GetUniform(uniformName);

Robust.Client/Graphics/Shaders/ParsedShader.cs

@@ -221,11 +221,12 @@ namespace Robust.Client.Graphics
 
         public static bool TypeSupportsArrays(this ShaderDataType type)
         {
-            // TODO: add support for int, and vec3/4 arrays
+            // TODO: add support for int, and vec3 arrays
             return
                 (type == ShaderDataType.Float) ||
                 (type == ShaderDataType.Vec2) ||
-                (type == ShaderDataType.Bool);
+                (type == ShaderDataType.Bool) ||
+                (type == ShaderDataType.Vec4);
         }
 
         [SuppressMessage("ReSharper", "StringLiteralTypo")]

Robust.Client/Graphics/Shaders/ShaderInstance.cs

@@ -113,6 +113,13 @@ namespace Robust.Client.Graphics
             SetParameterImpl(name, value);
         }
 
+        public void SetParameter(string name, Color[] value)
+        {
+            EnsureAlive();
+            EnsureMutable();
+            SetParameterImpl(name, value);
+        }
+
         public void SetParameter(string name, Vector4 value)
         {
             EnsureAlive();
@@ -223,6 +230,7 @@ namespace Robust.Client.Graphics
         private protected abstract void SetParameterImpl(string name, Vector3 value);
         private protected abstract void SetParameterImpl(string name, Vector4 value);
         private protected abstract void SetParameterImpl(string name, Color value);
+        private protected abstract void SetParameterImpl(string name, Color[] value);
         private protected abstract void SetParameterImpl(string name, int value);
         private protected abstract void SetParameterImpl(string name, Vector2i value);
         private protected abstract void SetParameterImpl(string name, bool value);

Robust.Client/Placement/PlacementMode.cs

@@ -144,7 +144,7 @@ namespace Robust.Client.Placement
             if (mousePos.MapId == MapId.Nullspace)
                 yield break;
 
-            var (_, (x, y)) = EntityCoordinates.FromMap(pManager.StartPoint.EntityId, mousePos, transformSys, pManager.EntityManager) - pManager.StartPoint;
+            var (_, (x, y)) = transformSys.ToCoordinates(pManager.StartPoint.EntityId, mousePos) - pManager.StartPoint;
             float iterations;
             Vector2 distance;
             if (Math.Abs(x) > Math.Abs(y))
@@ -176,7 +176,7 @@ namespace Robust.Client.Placement
             if (mousePos.MapId == MapId.Nullspace)
                 yield break;
 
-            var placementdiff = EntityCoordinates.FromMap(pManager.StartPoint.EntityId, mousePos, transformSys, pManager.EntityManager) - pManager.StartPoint;
+            var placementdiff = transformSys.ToCoordinates(pManager.StartPoint.EntityId, mousePos) - pManager.StartPoint;
 
             var xSign = Math.Sign(placementdiff.X);
             var ySign = Math.Sign(placementdiff.Y);
@@ -264,13 +264,15 @@ namespace Robust.Client.Placement
         protected EntityCoordinates ScreenToCursorGrid(ScreenCoordinates coords)
         {
             var mapCoords = pManager.EyeManager.PixelToMap(coords.Position);
+            var transformSys = pManager.EntityManager.System<SharedTransformSystem>();
+
             if (!pManager.MapManager.TryFindGridAt(mapCoords, out var gridUid, out var grid))
             {
-                return EntityCoordinates.FromMap(pManager.MapManager, mapCoords);
+
+                return transformSys.ToCoordinates(mapCoords);
             }
 
-            var transformSys = pManager.EntityManager.System<SharedTransformSystem>();
-            return EntityCoordinates.FromMap(gridUid, mapCoords, transformSys, pManager.EntityManager);
+            return transformSys.ToCoordinates(gridUid, mapCoords);
         }
     }
 }

Robust.Client/UserInterface/Controls/ScrollContainer.cs

@@ -28,6 +28,30 @@ namespace Robust.Client.UserInterface.Controls
         public int ScrollSpeedX { get; set; } = 50;
         public int ScrollSpeedY { get; set; } = 50;
 
+        public float VScroll
+        {
+            get => _vScrollBar.Value;
+            set => _vScrollBar.Value = value;
+        }
+
+        public float VScrollTarget
+        {
+            get => _vScrollBar.ValueTarget;
+            set => _vScrollBar.ValueTarget = value;
+        }
+
+        public float HScroll
+        {
+            get => _hScrollBar.Value;
+            set => _hScrollBar.Value = value;
+        }
+
+        public float HScrollTarget
+        {
+            get => _hScrollBar.ValueTarget;
+            set => _hScrollBar.ValueTarget = value;
+        }
+
         private bool _reserveScrollbarSpace;
         public bool ReserveScrollbarSpace
         {

Robust.Server/Audio/AudioSystem.cs

@@ -81,53 +81,58 @@ public sealed partial class AudioSystem : SharedAudioSystem
     }
 
     /// <inheritdoc />
-    public override (EntityUid Entity, AudioComponent Component)? PlayGlobal(string? filename, Filter playerFilter, bool recordReplay, AudioParams? audioParams = null)
+    public override (EntityUid Entity, AudioComponent Component)? PlayGlobal(ResolvedSoundSpecifier? specifier, Filter playerFilter, bool recordReplay, AudioParams? audioParams = null)
     {
-        if (string.IsNullOrEmpty(filename))
+        if (specifier is null)
             return null;
 
-        var entity = SetupAudio(filename, audioParams);
+        var entity = SetupAudio(specifier, audioParams);
         AddAudioFilter(entity, entity.Comp, playerFilter);
         entity.Comp.Global = true;
         return (entity, entity.Comp);
     }
 
     /// <inheritdoc />
-    public override (EntityUid Entity, AudioComponent Component)? PlayEntity(string? filename, Filter playerFilter, EntityUid uid, bool recordReplay, AudioParams? audioParams = null)
+    public override (EntityUid Entity, AudioComponent Component)? PlayEntity(ResolvedSoundSpecifier? specifier, Filter playerFilter, EntityUid uid, bool recordReplay, AudioParams? audioParams = null)
     {
-        if (string.IsNullOrEmpty(filename))
+        if (specifier is null)
             return null;
 
         if (TerminatingOrDeleted(uid))
             return null;
 
-        var entity = SetupAudio(filename, audioParams);
+        var entity = SetupAudio(specifier, audioParams);
         // Move it after setting it up
         XformSystem.SetCoordinates(entity, new EntityCoordinates(uid, Vector2.Zero));
+
+        // TODO AUDIO
+        // Add methods that allow for custom audio range.
+        // Some methods try to reduce the audio range, resulting in a custom filter which then unnecessarily has to
+        // use PVS overrides. PlayEntity with a reduced range shouldn't need PVS overrides at all.
         AddAudioFilter(entity, entity.Comp, playerFilter);
 
         return (entity, entity.Comp);
     }
 
     /// <inheritdoc />
-    public override (EntityUid Entity, AudioComponent Component)? PlayPvs(string? filename, EntityUid uid, AudioParams? audioParams = null)
+    public override (EntityUid Entity, AudioComponent Component)? PlayPvs(ResolvedSoundSpecifier? specifier, EntityUid uid, AudioParams? audioParams = null)
     {
-        if (string.IsNullOrEmpty(filename))
+        if (specifier is null)
             return null;
 
         if (TerminatingOrDeleted(uid))
             return null;
 
-        var entity = SetupAudio(filename, audioParams);
+        var entity = SetupAudio(specifier, audioParams);
         XformSystem.SetCoordinates(entity, new EntityCoordinates(uid, Vector2.Zero));
 
         return (entity, entity.Comp);
     }
 
     /// <inheritdoc />
-    public override (EntityUid Entity, AudioComponent Component)? PlayStatic(string? filename, Filter playerFilter, EntityCoordinates coordinates, bool recordReplay, AudioParams? audioParams = null)
+    public override (EntityUid Entity, AudioComponent Component)? PlayStatic(ResolvedSoundSpecifier? specifier, Filter playerFilter, EntityCoordinates coordinates, bool recordReplay, AudioParams? audioParams = null)
     {
-        if (string.IsNullOrEmpty(filename))
+        if (specifier is null)
             return null;
 
         if (TerminatingOrDeleted(coordinates.EntityId))
@@ -139,7 +144,7 @@ public sealed partial class AudioSystem : SharedAudioSystem
         if (!coordinates.IsValid(EntityManager))
             return null;
 
-        var entity = SetupAudio(filename, audioParams);
+        var entity = SetupAudio(specifier, audioParams);
         XformSystem.SetCoordinates(entity, coordinates);
         AddAudioFilter(entity, entity.Comp, playerFilter);
 
@@ -147,10 +152,10 @@ public sealed partial class AudioSystem : SharedAudioSystem
     }
 
     /// <inheritdoc />
-    public override (EntityUid Entity, AudioComponent Component)? PlayPvs(string? filename, EntityCoordinates coordinates,
+    public override (EntityUid Entity, AudioComponent Component)? PlayPvs(ResolvedSoundSpecifier? specifier, EntityCoordinates coordinates,
         AudioParams? audioParams = null)
     {
-        if (string.IsNullOrEmpty(filename))
+        if (specifier is null)
             return null;
 
         if (TerminatingOrDeleted(coordinates.EntityId))
@@ -163,7 +168,7 @@ public sealed partial class AudioSystem : SharedAudioSystem
             return null;
 
         // TODO: Transform TryFindGridAt mess + optimisation required.
-        var entity = SetupAudio(filename, audioParams);
+        var entity = SetupAudio(specifier, audioParams);
         XformSystem.SetCoordinates(entity, coordinates);
 
         return (entity, entity.Comp);
@@ -186,7 +191,7 @@ public sealed partial class AudioSystem : SharedAudioSystem
         if (sound == null)
             return null;
 
-        var audio = PlayPvs(GetSound(sound), source, audioParams ?? sound.Params);
+        var audio = PlayPvs(ResolveSound(sound), source, audioParams ?? sound.Params);
 
         if (audio == null)
             return null;
@@ -201,7 +206,7 @@ public sealed partial class AudioSystem : SharedAudioSystem
         if (sound == null)
             return null;
 
-        var audio = PlayPvs(GetSound(sound), coordinates, audioParams ?? sound.Params);
+        var audio = PlayPvs(ResolveSound(sound), coordinates, audioParams ?? sound.Params);
 
         if (audio == null)
             return null;
@@ -210,12 +215,12 @@ public sealed partial class AudioSystem : SharedAudioSystem
         return audio;
     }
 
-    public override (EntityUid Entity, AudioComponent Component)? PlayGlobal(string? filename, ICommonSession recipient, AudioParams? audioParams = null)
+    public override (EntityUid Entity, AudioComponent Component)? PlayGlobal(ResolvedSoundSpecifier? filename, ICommonSession recipient, AudioParams? audioParams = null)
     {
         return PlayGlobal(filename, Filter.SinglePlayer(recipient), false, audioParams);
     }
 
-    public override (EntityUid Entity, AudioComponent Component)? PlayGlobal(string? filename, EntityUid recipient, AudioParams? audioParams = null)
+    public override (EntityUid Entity, AudioComponent Component)? PlayGlobal(ResolvedSoundSpecifier? filename, EntityUid recipient, AudioParams? audioParams = null)
     {
         if (TryComp(recipient, out ActorComponent? actor))
             return PlayGlobal(filename, actor.PlayerSession, audioParams);
@@ -223,12 +228,12 @@ public sealed partial class AudioSystem : SharedAudioSystem
         return null;
     }
 
-    public override (EntityUid Entity, AudioComponent Component)? PlayEntity(string? filename, ICommonSession recipient, EntityUid uid, AudioParams? audioParams = null)
+    public override (EntityUid Entity, AudioComponent Component)? PlayEntity(ResolvedSoundSpecifier? filename, ICommonSession recipient, EntityUid uid, AudioParams? audioParams = null)
     {
         return PlayEntity(filename, Filter.SinglePlayer(recipient), uid, false, audioParams);
     }
 
-    public override (EntityUid Entity, AudioComponent Component)? PlayEntity(string? filename, EntityUid recipient, EntityUid uid, AudioParams? audioParams = null)
+    public override (EntityUid Entity, AudioComponent Component)? PlayEntity(ResolvedSoundSpecifier? filename, EntityUid recipient, EntityUid uid, AudioParams? audioParams = null)
     {
         if (TryComp(recipient, out ActorComponent? actor))
             return PlayEntity(filename, actor.PlayerSession, uid, audioParams);
@@ -236,12 +241,12 @@ public sealed partial class AudioSystem : SharedAudioSystem
         return null;
     }
 
-    public override (EntityUid Entity, AudioComponent Component)? PlayStatic(string? filename, ICommonSession recipient, EntityCoordinates coordinates, AudioParams? audioParams = null)
+    public override (EntityUid Entity, AudioComponent Component)? PlayStatic(ResolvedSoundSpecifier? filename, ICommonSession recipient, EntityCoordinates coordinates, AudioParams? audioParams = null)
     {
         return PlayStatic(filename, Filter.SinglePlayer(recipient), coordinates, false, audioParams);
     }
 
-    public override (EntityUid Entity, AudioComponent Component)? PlayStatic(string? filename, EntityUid recipient, EntityCoordinates coordinates, AudioParams? audioParams = null)
+    public override (EntityUid Entity, AudioComponent Component)? PlayStatic(ResolvedSoundSpecifier? filename, EntityUid recipient, EntityCoordinates coordinates, AudioParams? audioParams = null)
     {
         if (TryComp(recipient, out ActorComponent? actor))
             return PlayStatic(filename, actor.PlayerSession, coordinates, audioParams);

Robust.Server/BaseServer.cs

@@ -297,7 +297,7 @@ namespace Robust.Server
                 : null;
 
             // Set up the VFS
-            _resources.Initialize(dataDir);
+            _resources.Initialize(dataDir, hideUserDataDir: false);
 
             var mountOptions = _commandLineArgs != null
                 ? MountOptions.Merge(_commandLineArgs.MountOptions, Options.MountOptions) : Options.MountOptions;

Robust.Server/Console/Commands/MapCommands.cs

@@ -1,14 +1,15 @@
 using System.Linq;
 using System.Numerics;
-using Robust.Server.GameObjects;
-using Robust.Server.Maps;
 using Robust.Shared.Console;
 using Robust.Shared.ContentPack;
+using Robust.Shared.EntitySerialization;
+using Robust.Shared.EntitySerialization.Systems;
 using Robust.Shared.GameObjects;
 using Robust.Shared.IoC;
 using Robust.Shared.Localization;
 using Robust.Shared.Map;
 using Robust.Shared.Maths;
+using Robust.Shared.Utility;
 
 namespace Robust.Server.Console.Commands
 {
@@ -42,7 +43,7 @@ namespace Robust.Server.Console.Commands
                 return;
             }
 
-            _ent.System<MapLoaderSystem>().Save(uid, args[1]);
+            _ent.System<MapLoaderSystem>().TrySaveGrid(uid, new ResPath(args[1]));
             shell.WriteLine("Save successful. Look in the user data directory.");
         }
 
@@ -63,7 +64,6 @@ namespace Robust.Server.Console.Commands
     public sealed class LoadGridCommand : LocalizedCommands
     {
         [Dependency] private readonly IEntitySystemManager _system = default!;
-        [Dependency] private readonly IMapManager _map = default!;
         [Dependency] private readonly IResourceManager _resource = default!;
 
         public override string Command => "loadgrid";
@@ -91,13 +91,14 @@ namespace Robust.Server.Console.Commands
                 return;
             }
 
-            if (!_map.MapExists(mapId))
+            var sys = _system.GetEntitySystem<SharedMapSystem>();
+            if (!sys.MapExists(mapId))
             {
                 shell.WriteError("Target map does not exist.");
                 return;
             }
 
-            var loadOptions = new MapLoadOptions();
+            Vector2 offset = default;
             if (args.Length >= 4)
             {
                 if (!float.TryParse(args[2], out var x))
@@ -112,9 +113,10 @@ namespace Robust.Server.Console.Commands
                     return;
                 }
 
-                loadOptions.Offset = new Vector2(x, y);
+                offset = new Vector2(x, y);
             }
 
+            Angle rot = default;
             if (args.Length >= 5)
             {
                 if (!float.TryParse(args[4], out var rotation))
@@ -123,9 +125,10 @@ namespace Robust.Server.Console.Commands
                     return;
                 }
 
-                loadOptions.Rotation = Angle.FromDegrees(rotation);
+                rot = Angle.FromDegrees(rotation);
             }
 
+            var opts = DeserializationOptions.Default;
             if (args.Length >= 6)
             {
                 if (!bool.TryParse(args[5], out var storeUids))
@@ -134,10 +137,11 @@ namespace Robust.Server.Console.Commands
                     return;
                 }
 
-                loadOptions.StoreMapUids = storeUids;
+                opts.StoreYamlUids = storeUids;
             }
 
-            _system.GetEntitySystem<MapLoaderSystem>().Load(mapId, args[1], loadOptions);
+            var path = new ResPath(args[1]);
+            _system.GetEntitySystem<MapLoaderSystem>().TryLoadGrid(mapId, path, out _, opts, offset, rot);
         }
 
         public override CompletionResult GetCompletion(IConsoleShell shell, string[] args)
@@ -149,7 +153,6 @@ namespace Robust.Server.Console.Commands
     public sealed class SaveMap : LocalizedCommands
     {
         [Dependency] private readonly IEntitySystemManager _system = default!;
-        [Dependency] private readonly IMapManager _map = default!;
         [Dependency] private readonly IResourceManager _resource = default!;
 
         public override string Command => "savemap";
@@ -189,13 +192,14 @@ namespace Robust.Server.Console.Commands
             if (mapId == MapId.Nullspace)
                 return;
 
-            if (!_map.MapExists(mapId))
+            var sys = _system.GetEntitySystem<SharedMapSystem>();
+            if (!sys.MapExists(mapId))
             {
                 shell.WriteError(Loc.GetString("cmd-savemap-not-exist"));
                 return;
             }
 
-            if (_map.IsMapInitialized(mapId) &&
+            if (sys.IsInitialized(mapId) &&
                 ( args.Length < 3  || !bool.TryParse(args[2], out var force) || !force))
             {
                 shell.WriteError(Loc.GetString("cmd-savemap-init-warning"));
@@ -203,7 +207,7 @@ namespace Robust.Server.Console.Commands
             }
 
             shell.WriteLine(Loc.GetString("cmd-savemap-attempt", ("mapId", mapId), ("path", args[1])));
-            _system.GetEntitySystem<MapLoaderSystem>().SaveMap(mapId, args[1]);
+            _system.GetEntitySystem<MapLoaderSystem>().TrySaveMap(mapId, new ResPath(args[1]));
             shell.WriteLine(Loc.GetString("cmd-savemap-success"));
         }
     }
@@ -211,7 +215,6 @@ namespace Robust.Server.Console.Commands
     public sealed class LoadMap : LocalizedCommands
     {
         [Dependency] private readonly IEntitySystemManager _system = default!;
-        [Dependency] private readonly IMapManager _map = default!;
         [Dependency] private readonly IResourceManager _resource = default!;
 
         public override string Command => "loadmap";
@@ -267,61 +270,49 @@ namespace Robust.Server.Console.Commands
                 return;
             }
 
-            if (_map.MapExists(mapId))
+            var sys = _system.GetEntitySystem<SharedMapSystem>();
+            if (sys.MapExists(mapId))
             {
                 shell.WriteError(Loc.GetString("cmd-loadmap-exists", ("mapId", mapId)));
                 return;
             }
 
-            var loadOptions = new MapLoadOptions();
-
-            float x = 0, y = 0;
-            if (args.Length >= 3)
+            float x = 0;
+            if (args.Length >= 3 && !float.TryParse(args[2], out x))
             {
-                if (!float.TryParse(args[2], out x))
-                {
-                    shell.WriteError(Loc.GetString("cmd-parse-failure-float", ("arg", args[2])));
-                    return;
-                }
+                shell.WriteError(Loc.GetString("cmd-parse-failure-float", ("arg", args[2])));
+                return;
             }
 
-            if (args.Length >= 4)
+            float y = 0;
+            if (args.Length >= 4 && !float.TryParse(args[3], out y))
             {
+                shell.WriteError(Loc.GetString("cmd-parse-failure-float", ("arg", args[3])));
+                return;
+            }
+            var offset = new Vector2(x, y);
 
-                if (!float.TryParse(args[3], out y))
-                {
-                    shell.WriteError(Loc.GetString("cmd-parse-failure-float", ("arg", args[3])));
-                    return;
-                }
+            float rotation = 0;
+            if (args.Length >= 5 && !float.TryParse(args[4], out rotation))
+            {
+                shell.WriteError(Loc.GetString("cmd-parse-failure-float", ("arg", args[4])));
+                return;
+            }
+            var rot = new Angle(rotation);
+
+            bool storeUids = false;
+            if (args.Length >= 6 && !bool.TryParse(args[5], out storeUids))
+            {
+                shell.WriteError(Loc.GetString("cmd-parse-failure-bool", ("arg", args[5])));
+                return;
             }
 
-            loadOptions.Offset = new Vector2(x, y);
+            var opts = new DeserializationOptions {StoreYamlUids = storeUids};
 
-            if (args.Length >= 5)
-            {
-                if (!float.TryParse(args[4], out var rotation))
-                {
-                    shell.WriteError(Loc.GetString("cmd-parse-failure-float", ("arg", args[4])));
-                    return;
-                }
+            var path = new ResPath(args[1]);
+            _system.GetEntitySystem<MapLoaderSystem>().TryLoadMapWithId(mapId, path, out _, out _, opts, offset, rot);
 
-                loadOptions.Rotation = new Angle(rotation);
-            }
-
-            if (args.Length >= 6)
-            {
-                if (!bool.TryParse(args[5], out var storeUids))
-                {
-                    shell.WriteError(Loc.GetString("cmd-parse-failure-bool", ("arg", args[5])));
-                    return;
-                }
-
-                loadOptions.StoreMapUids = storeUids;
-            }
-
-            _system.GetEntitySystem<MapLoaderSystem>().TryLoad(mapId, args[1], out _, loadOptions);
-
-            if (_map.MapExists(mapId))
+            if (sys.MapExists(mapId))
                 shell.WriteLine(Loc.GetString("cmd-loadmap-success", ("mapId", mapId), ("path", args[1])));
             else
                 shell.WriteLine(Loc.GetString("cmd-loadmap-error", ("path", args[1])));

Robust.Server/GameObjects/EntitySystems/MapSystem.cs

@@ -21,7 +21,7 @@ namespace Robust.Server.GameObjects
         protected override MapId GetNextMapId()
         {
             var id = new MapId(++LastMapId);
-            while (MapManager.MapExists(id))
+            while (MapExists(id) || UsedIds.Contains(id))
             {
                 id = new MapId(++LastMapId);
             }

Robust.Server/GameObjects/EntitySystems/VisibilitySystem.cs

@@ -6,7 +6,7 @@ using Robust.Shared.ViewVariables;
 
 namespace Robust.Server.GameObjects
 {
-    public sealed class VisibilitySystem : EntitySystem
+    public sealed class VisibilitySystem : SharedVisibilitySystem
     {
         [Dependency] private readonly PvsSystem _pvs = default!;
         [Dependency] private readonly IViewVariablesManager _vvManager = default!;
@@ -40,7 +40,7 @@ namespace Robust.Server.GameObjects
             EntityManager.EntityInitialized -= OnEntityInit;
         }
 
-        public void AddLayer(Entity<VisibilityComponent?> ent, ushort layer, bool refresh = true)
+        public override void AddLayer(Entity<VisibilityComponent?> ent, ushort layer, bool refresh = true)
         {
             ent.Comp ??= _visibilityQuery.CompOrNull(ent.Owner) ?? AddComp<VisibilityComponent>(ent.Owner);
 
@@ -53,7 +53,7 @@ namespace Robust.Server.GameObjects
                 RefreshVisibility(ent);
         }
 
-        public void RemoveLayer(Entity<VisibilityComponent?> ent, ushort layer, bool refresh = true)
+        public override void RemoveLayer(Entity<VisibilityComponent?> ent, ushort layer, bool refresh = true)
         {
             if (!_visibilityQuery.Resolve(ent.Owner, ref ent.Comp, false))
                 return;
@@ -67,7 +67,7 @@ namespace Robust.Server.GameObjects
                 RefreshVisibility(ent);
         }
 
-        public void SetLayer(Entity<VisibilityComponent?> ent, ushort layer, bool refresh = true)
+        public override void SetLayer(Entity<VisibilityComponent?> ent, ushort layer, bool refresh = true)
         {
             ent.Comp ??= _visibilityQuery.CompOrNull(ent.Owner) ?? AddComp<VisibilityComponent>(ent.Owner);
 
@@ -90,14 +90,14 @@ namespace Robust.Server.GameObjects
             RefreshVisibility(ent.Owner, null, ent.Comp);
         }
 
-        public void RefreshVisibility(EntityUid uid,
+        public override void RefreshVisibility(EntityUid uid,
             VisibilityComponent? visibilityComponent = null,
             MetaDataComponent? meta = null)
         {
             RefreshVisibility((uid, visibilityComponent, meta));
         }
 
-        public void RefreshVisibility(Entity<VisibilityComponent?, MetaDataComponent?> ent)
+        public override void RefreshVisibility(Entity<VisibilityComponent?, MetaDataComponent?> ent)
         {
             if (!_metaQuery.Resolve(ent, ref ent.Comp2, false))
                 return;

Robust.Server/GameObjects/IServerEntityManagerInternal.cs

@@ -1,21 +0,0 @@
-using Robust.Shared.GameObjects;
-using Robust.Shared.Prototypes;
-
-namespace Robust.Server.GameObjects
-{
-    internal interface IServerEntityManagerInternal : IServerEntityManager
-    {
-        // These methods are used by the map loader to do multi-stage entity construction during map load.
-        // I would recommend you refer to the MapLoader for usage.
-
-        EntityUid AllocEntity(EntityPrototype? prototype);
-
-        void FinishEntityLoad(EntityUid entity, IEntityLoadContext? context = null);
-
-        void FinishEntityLoad(EntityUid entity, EntityPrototype? prototype, IEntityLoadContext? context = null);
-
-        void FinishEntityInitialization(EntityUid entity, MetaDataComponent? meta = null);
-
-        void FinishEntityStartup(EntityUid entity);
-    }
-}

Robust.Server/GameObjects/MapSaveIdComponent.cs

@@ -1,17 +0,0 @@
-using Robust.Shared.GameObjects;
-
-namespace Robust.Server.GameObjects
-{
-    /// <summary>
-    ///     Metadata component used to keep consistent UIDs inside map files cross saving.
-    /// </summary>
-    /// <remarks>
-    ///     This component stores the previous map UID of entities from map load.
-    ///     This can then be used to re-serialize the entity with the same UID for the merge driver to recognize.
-    /// </remarks>
-    [RegisterComponent, UnsavedComponent]
-    public sealed partial class MapSaveIdComponent : Component
-    {
-        public int Uid { get; set; }
-    }
-}

Robust.Server/GameObjects/ServerEntityManager.cs

@@ -27,7 +27,7 @@ namespace Robust.Server.GameObjects
     /// Manager for entities -- controls things like template loading and instantiation
     /// </summary>
     [UsedImplicitly] // DI Container
-    public sealed class ServerEntityManager : EntityManager, IServerEntityManagerInternal
+    public sealed class ServerEntityManager : EntityManager, IServerEntityManager
     {
         private static readonly Gauge EntitiesCount = Metrics.CreateGauge(
             "robust_entities_count",
@@ -61,32 +61,6 @@ namespace Robust.Server.GameObjects
             _pvs = System<PvsSystem>();
         }
 
-        EntityUid IServerEntityManagerInternal.AllocEntity(EntityPrototype? prototype)
-        {
-            return AllocEntity(prototype, out _);
-        }
-
-        void IServerEntityManagerInternal.FinishEntityLoad(EntityUid entity, IEntityLoadContext? context)
-        {
-            LoadEntity(entity, context);
-        }
-
-        void IServerEntityManagerInternal.FinishEntityLoad(EntityUid entity, EntityPrototype? prototype, IEntityLoadContext? context)
-        {
-            LoadEntity(entity, context, prototype);
-        }
-
-        void IServerEntityManagerInternal.FinishEntityInitialization(EntityUid entity, MetaDataComponent? meta)
-        {
-            InitializeEntity(entity, meta);
-        }
-
-        [Obsolete("Use StartEntity")]
-        void IServerEntityManagerInternal.FinishEntityStartup(EntityUid entity)
-        {
-            StartEntity(entity);
-        }
-
         internal override EntityUid CreateEntity(string? prototypeName, out MetaDataComponent metadata, IEntityLoadContext? context = null)
         {
             if (prototypeName == null)

Robust.Server/GameStates/PvsData.cs

@@ -184,6 +184,9 @@ internal struct PvsMetadata
     public NetEntity NetEntity;
 
     public GameTick LastModifiedTick;
+
+    // TODO PVS maybe store as int?
+    // Theres extra space anyways, and the mask checks always need to convert to an int first, so it'd probably be faster too.
     public ushort VisMask;
     public EntityLifeStage LifeStage;
 #if DEBUG

Robust.Server/GameStates/PvsOverrideSystem.cs

@@ -29,7 +29,8 @@ public sealed class PvsOverrideSystem : SharedPvsOverrideSystem
         base.Initialize();
         EntityManager.EntityDeleted += OnDeleted;
         _player.PlayerStatusChanged += OnPlayerStatusChanged;
-        SubscribeLocalEvent<MapChangedEvent>(OnMapChanged);
+        SubscribeLocalEvent<MapRemovedEvent>(OnMapRemoved);
+        SubscribeLocalEvent<MapCreatedEvent>(OnMapCreated);
         SubscribeLocalEvent<GridInitializeEvent>(OnGridCreated);
         SubscribeLocalEvent<GridRemovalEvent>(OnGridRemoved);
 
@@ -133,7 +134,7 @@ public sealed class PvsOverrideSystem : SharedPvsOverrideSystem
 
     /// <summary>
     /// Forces the entity, all of its parents, and all of its children to ignore normal PVS range limitations,
-    /// causing them to always be sent to all clients.
+    /// causing them to be sent to all clients. This will still respect visibility masks, it only overrides the range.
     /// </summary>
     public override void AddGlobalOverride(EntityUid uid)
     {
@@ -159,8 +160,9 @@ public sealed class PvsOverrideSystem : SharedPvsOverrideSystem
     /// This causes an entity and all of its parents to always be sent to all players.
     /// </summary>
     /// <remarks>
-    /// This differs from <see cref="AddGlobalOverride"/> as it does not send children, and will ignore a players usual
-    /// PVS budget. You generally shouldn't use this unless an entity absolutely always needs to be sent to all clients.
+    /// This differs from <see cref="AddGlobalOverride"/> as it does not send children, will ignore a players usual
+    /// PVS budget, and ignores visibility masks. You generally shouldn't use this unless an entity absolutely always
+    /// needs to be sent to all clients.
     /// </remarks>
     public void AddForceSend(EntityUid uid)
     {
@@ -176,11 +178,12 @@ public sealed class PvsOverrideSystem : SharedPvsOverrideSystem
     }
 
     /// <summary>
-    /// This causes an entity and all of its parents to always be sent to a player..
+    /// This causes an entity and all of its parents to always be sent to a player.
     /// </summary>
     /// <remarks>
-    /// This differs from <see cref="AddSessionOverride"/> as it does not send children, and will ignore a players usual
-    /// PVS budget. You generally shouldn't use this unless an entity absolutely always needs to be sent to a client.
+    /// This differs from <see cref="AddSessionOverride"/> as it does not send children, will ignore a players usual
+    /// PVS budget, and ignores visibility masks. You generally shouldn't use this unless an entity absolutely always
+    /// needs to be sent to a client.
     /// </remarks>
     public void AddForceSend(EntityUid uid, ICommonSession session)
     {
@@ -206,7 +209,7 @@ public sealed class PvsOverrideSystem : SharedPvsOverrideSystem
 
     /// <summary>
     /// Forces the entity, all of its parents, and all of its children to ignore normal PVS range limitations for a
-    /// specific session.
+    /// specific session. This will still respect visibility masks, it only overrides the range.
     /// </summary>
     public override void AddSessionOverride(EntityUid uid, ICommonSession session)
     {
@@ -235,15 +238,17 @@ public sealed class PvsOverrideSystem : SharedPvsOverrideSystem
 
     /// <summary>
     /// Forces the entity, all of its parents, and all of its children to ignore normal PVS range limitations,
-    /// causing them to always be sent to all clients.
+    /// causing them to always be sent to the specified clients. This will still respect visibility masks, it only
+    /// overrides the range.
     /// </summary>
     public override void AddSessionOverrides(EntityUid uid, Filter filter)
     {
+        _hasOverride.Add(uid);
         base.AddSessionOverrides(uid, filter);
 
         foreach (var session in filter.Recipients)
         {
-            AddSessionOverride(uid, session);
+            SessionOverrides.GetOrNew(session).Add(uid);
         }
     }
 
@@ -270,14 +275,6 @@ public sealed class PvsOverrideSystem : SharedPvsOverrideSystem
 
     #region Map/Grid Events
 
-    private void OnMapChanged(MapChangedEvent ev)
-    {
-        if (ev.Created)
-            OnMapCreated(ev);
-        else
-            OnMapDestroyed(ev);
-    }
-
     private void OnGridRemoved(GridRemovalEvent ev)
     {
         RemoveForceSend(ev.EntityUid);
@@ -290,12 +287,12 @@ public sealed class PvsOverrideSystem : SharedPvsOverrideSystem
         AddForceSend(ev.EntityUid);
     }
 
-    private void OnMapDestroyed(MapChangedEvent ev)
+    private void OnMapRemoved(MapRemovedEvent ev)
     {
         RemoveForceSend(ev.Uid);
     }
 
-    private void OnMapCreated(MapChangedEvent ev)
+    private void OnMapCreated(MapCreatedEvent ev)
     {
         // TODO PVS remove this requirement.
         // I think this just required refactoring client game state logic so it doesn't sending maps/grids to nullspace.

Robust.Server/GameStates/PvsSystem.Chunks.cs

@@ -303,11 +303,8 @@ internal sealed partial class PvsSystem
         RemoveRoot(ev.EntityUid);
     }
 
-    private void OnMapChanged(MapChangedEvent ev)
+    private void OnMapChanged(MapRemovedEvent ev)
     {
-        if (!ev.Destroyed)
-            return;
-
         RemoveRoot(ev.Uid);
     }
 

Robust.Server/GameStates/PvsSystem.Overrides.cs

@@ -19,14 +19,15 @@ internal sealed partial class PvsSystem
 
     private void AddAllOverrides(PvsSession session)
     {
-        var mask = session.VisMask;
         var fromTick = session.FromTick;
-        RaiseExpandEvent(session, fromTick);
+        var mask = RaiseExpandEvent(session, fromTick);
 
         foreach (ref var ent in CollectionsMarshal.AsSpan(_cachedGlobalOverride))
         {
             ref var meta = ref _metadataMemory.GetRef(ent.Ptr.Index);
             meta.Validate(ent.Meta);
+
+            // PVS overrides still respect visibility masks
             if ((mask & meta.VisMask) == meta.VisMask)
                 AddEntity(session, ref ent, ref meta, fromTick);
         }
@@ -36,7 +37,7 @@ internal sealed partial class PvsSystem
 
         foreach (var uid in sessionOverrides)
         {
-            RecursivelyAddOverride(session, uid, fromTick, addChildren: true);
+            RecursivelyAddOverride(session, uid, fromTick, addChildren: true, mask);
         }
     }
 
@@ -45,22 +46,23 @@ internal sealed partial class PvsSystem
     /// </summary>
     private void AddForcedEntities(PvsSession session)
     {
+        // Forced overrides do not respect visibility masks, so we set all bits.
+        var mask = -1;
+
         // Ignore PVS budgets
         session.Budget = new() {NewLimit = int.MaxValue, EnterLimit = int.MaxValue};
 
-        var mask = session.VisMask;
         var fromTick = session.FromTick;
         foreach (ref var ent in CollectionsMarshal.AsSpan(_cachedForceOverride))
         {
             ref var meta = ref _metadataMemory.GetRef(ent.Ptr.Index);
             meta.Validate(ent.Meta);
-            if ((mask & meta.VisMask) == meta.VisMask)
-                AddEntity(session, ref ent, ref meta, fromTick);
+            AddEntity(session, ref ent, ref meta, fromTick);
         }
 
         foreach (var uid in session.Viewers)
         {
-            RecursivelyAddOverride(session, uid, fromTick, addChildren: false);
+            RecursivelyAddOverride(session, uid, fromTick, addChildren: false, mask);
         }
 
         if (!_pvsOverride.SessionForceSend.TryGetValue(session.Session, out var sessionForce))
@@ -68,13 +70,13 @@ internal sealed partial class PvsSystem
 
         foreach (var uid in sessionForce)
         {
-            RecursivelyAddOverride(session, uid, fromTick, addChildren: false);
+            RecursivelyAddOverride(session, uid, fromTick, addChildren: false, mask);
         }
     }
 
-    private void RaiseExpandEvent(PvsSession session, GameTick fromTick)
+    private int RaiseExpandEvent(PvsSession session, GameTick fromTick)
     {
-        var expandEvent = new ExpandPvsEvent(session.Session);
+        var expandEvent = new ExpandPvsEvent(session.Session, session.VisMask);
 
         if (session.Session.AttachedEntity != null)
             RaiseLocalEvent(session.Session.AttachedEntity.Value, ref expandEvent, true);
@@ -85,23 +87,25 @@ internal sealed partial class PvsSystem
         {
             foreach (var uid in expandEvent.Entities)
             {
-                RecursivelyAddOverride(session, uid, fromTick, addChildren: false);
+                RecursivelyAddOverride(session, uid, fromTick, addChildren: false, expandEvent.VisMask);
             }
         }
 
         if (expandEvent.RecursiveEntities == null)
-            return;
+            return expandEvent.VisMask;
 
         foreach (var uid in expandEvent.RecursiveEntities)
         {
-            RecursivelyAddOverride(session, uid, fromTick, addChildren: true);
+            RecursivelyAddOverride(session, uid, fromTick, addChildren: true, expandEvent.VisMask);
         }
+
+        return expandEvent.VisMask;
     }
 
     /// <summary>
     /// Recursively add an entity and all of its parents to the to-send set. This optionally also adds all children.
     /// </summary>
-    private bool RecursivelyAddOverride(PvsSession session, EntityUid uid, GameTick fromTick, bool addChildren)
+    private bool RecursivelyAddOverride(PvsSession session, EntityUid uid, GameTick fromTick, bool addChildren, int mask)
     {
         if (!_xformQuery.TryGetComponent(uid, out var xform))
         {
@@ -116,17 +120,20 @@ internal sealed partial class PvsSystem
         // to the toSend set, it doesn't guarantee that its parents have been. E.g., if a player ghost just teleported
         // to follow a far away entity, the player's own entity is still being sent, but we need to ensure that we also
         // send the new parents, which may otherwise be delayed because of the PVS budget.
-        if (parent.IsValid() && !RecursivelyAddOverride(session, parent, fromTick, false))
+        if (parent.IsValid() && !RecursivelyAddOverride(session, parent, fromTick, false, mask))
             return false;
 
         if (!_metaQuery.TryGetComponent(uid, out var meta))
             return false;
 
+        if ((mask & meta.VisibilityMask) != meta.VisibilityMask)
+            return false;
+
         if (!AddEntity(session, (uid, meta), fromTick))
             return false;
 
         if (addChildren)
-            RecursivelyAddChildren(session, xform, fromTick);
+            RecursivelyAddChildren(session, xform, fromTick, mask);
 
         return true;
     }
@@ -134,7 +141,7 @@ internal sealed partial class PvsSystem
     /// <summary>
     /// Recursively add an entity and all of its children to the to-send set.
     /// </summary>
-    private void RecursivelyAddChildren(PvsSession session, TransformComponent xform, GameTick fromTick)
+    private void RecursivelyAddChildren(PvsSession session, TransformComponent xform, GameTick fromTick, int mask)
     {
         foreach (var child in xform._children)
         {
@@ -145,10 +152,14 @@ internal sealed partial class PvsSystem
             }
 
             var metadata = _metaQuery.GetComponent(child);
-            if (!AddEntity(session, (child, metadata), fromTick))
-                return;
 
-            RecursivelyAddChildren(session, childXform, fromTick);
+            if ((mask & metadata.VisibilityMask) != metadata.VisibilityMask)
+                continue;
+
+            if (!AddEntity(session, (child, metadata), fromTick))
+                return; // Budget was exceeded (or some error occurred) -> return instead of continue.
+
+            RecursivelyAddChildren(session, childXform, fromTick, mask);
         }
     }
 

Robust.Server/GameStates/PvsSystem.ToSendSet.cs

@@ -142,18 +142,32 @@ internal sealed partial class PvsSystem
 
         if (meta.EntityLifeStage >= EntityLifeStage.Terminating)
         {
-            var rep = new EntityStringRepresentation(entity);
-            Log.Error($"Attempted to add a deleted entity to PVS send set: '{rep}'. Deletion queued: {EntityManager.IsQueuedForDeletion(uid)}. Trace:\n{Environment.StackTrace}");
-
             // This can happen if some entity was some removed from it's parent while that parent was being deleted.
             // As a result the entity was marked for deletion but was never actually properly deleted.
-            EntityManager.QueueDeleteEntity(uid);
+
+            bool queued;
+            lock (_toDelete)
+            {
+                queued = EntityManager.IsQueuedForDeletion(uid) || _toDelete.Contains(uid);
+                if (!queued)
+                    _toDelete.Add(uid);
+            }
+
+            var rep = new EntityStringRepresentation(entity);
+            Log.Error($"Attempted to add a deleted entity to PVS send set: '{rep}'. Deletion queued: {queued}. Trace:\n{Environment.StackTrace}");
             return false;
         }
 
         data.LastSeen = _gameTiming.CurTick;
         session.ToSend!.Add(entity.Comp.PvsData);
 
+        // TODO PVS PERFORMANCE
+        // Investigate whether its better to defer actually creating the entity state & populating session.States here?
+        // I.e., should be be constructing the to-send list & to-get-states lists, and then separately getting all states
+        // after we have gotten all entities? If the CPU can focus on only processing data in session.DataMemory without
+        // having to access miscellaneous component info, maybe it will be faster?
+        // Though for that to work I guess it also has to avoid accessing the metadata component's lifestage?
+
         if (session.RequestedFull)
         {
             var state = GetFullEntityState(session.Session, uid, meta);

Robust.Server/GameStates/PvsSystem.cs

@@ -94,6 +94,8 @@ internal sealed partial class PvsSystem : EntitySystem
     /// </summary>
     private readonly List<GameTick> _deletedTick = new();
 
+    private readonly HashSet<EntityUid> _toDelete = new();
+
     /// <summary>
     /// The sessions that are currently being processed. Note that this is in general used by parallel & async tasks.
     /// Hence player disconnection processing is deferred and only run via <see cref="ProcessDisconnections"/>.
@@ -127,7 +129,7 @@ internal sealed partial class PvsSystem : EntitySystem
         _metaQuery = GetEntityQuery<MetaDataComponent>();
         _xformQuery = GetEntityQuery<TransformComponent>();
 
-        SubscribeLocalEvent<MapChangedEvent>(OnMapChanged);
+        SubscribeLocalEvent<MapRemovedEvent>(OnMapChanged);
         SubscribeLocalEvent<GridRemovalEvent>(OnGridRemoved);
         SubscribeLocalEvent<TransformComponent, TransformStartupEvent>(OnTransformStartup);
 
@@ -195,6 +197,12 @@ internal sealed partial class PvsSystem : EntitySystem
         // Construct & serialize the game state for each player (and for the replay).
         SerializeStates();
 
+        foreach (var uid in _toDelete)
+        {
+            EntityManager.QueueDeleteEntity(uid);
+        }
+        _toDelete.Clear();
+
         // Compress & send the states.
         SendStates();
 
@@ -465,18 +473,27 @@ internal sealed partial class PvsSystem : EntitySystem
 }
 
 [ByRefEvent]
-public struct ExpandPvsEvent(ICommonSession session)
+public struct ExpandPvsEvent(ICommonSession session, int mask)
 {
     public readonly ICommonSession Session = session;
 
     /// <summary>
-    /// List of entities that will get added to this session's PVS set.
+    /// List of entities that will get added to this session's PVS set. This will still respect visibility masks.
     /// </summary>
     public List<EntityUid>? Entities;
 
     /// <summary>
     /// List of entities that will get added to this session's PVS set. Unlike <see cref="Entities"/> this will also
-    /// recursively add all children of the given entity.
+    /// recursively add all children of the given entity. This will still respect visibility masks.
     /// </summary>
     public List<EntityUid>? RecursiveEntities;
+
+    /// <summary>
+    /// Visibility mask to use when adding entities. Defaults to the usual visibility mask for that client.
+    /// </summary>
+    /// <remarks>
+    /// Note that this mask will affect all global & session overrides from <see cref="PvsOverrideSystem"/> for this
+    /// client, not just the entities in <see cref="Entities"/> and <see cref="RecursiveEntities"/>.
+    /// </remarks>
+    public int VisMask = mask;
 }

Robust.Server/Maps/LoadedMapComponent.cs

@@ -1,12 +0,0 @@
-using Robust.Shared.GameObjects;
-
-namespace Robust.Server.Maps;
-
-/// <summary>
-/// Added to Maps that were loaded by MapLoaderSystem. If not present then this map was created externally.
-/// </summary>
-[RegisterComponent]
-public sealed partial class LoadedMapComponent : Component
-{
-
-}

Robust.Server/Maps/MapLoadOptions.cs

@@ -1,59 +0,0 @@
-using System.Numerics;
-using JetBrains.Annotations;
-using Robust.Shared.Maths;
-
-namespace Robust.Server.Maps
-{
-    [PublicAPI]
-    public sealed class MapLoadOptions
-    {
-        /// <summary>
-        ///     If true, UID components will be created for loaded entities
-        ///     to maintain consistency upon subsequent savings.
-        /// </summary>
-        public bool StoreMapUids { get; set; }
-
-        /// <summary>
-        ///     Offset to apply to the loaded objects.
-        /// </summary>
-        public Vector2 Offset
-        {
-            get => _offset;
-            set
-            {
-                TransformMatrix = Matrix3Helpers.CreateTransform(value, Rotation);
-                _offset = value;
-            }
-        }
-
-        private Vector2 _offset = Vector2.Zero;
-
-        /// <summary>
-        ///     Rotation to apply to the loaded objects as a collective, around 0, 0.
-        /// </summary>
-        /// <remarks>Setting this overrides </remarks>
-        public Angle Rotation
-        {
-            get => _rotation;
-            set
-            {
-                TransformMatrix = Matrix3Helpers.CreateTransform(Offset, value);
-                _rotation = value;
-            }
-        }
-
-        private Angle _rotation = Angle.Zero;
-
-        public Matrix3x2 TransformMatrix { get; set; } = Matrix3x2.Identity;
-
-        /// <summary>
-        /// If there is a map entity serialized should we also load it.
-        /// </summary>
-        /// <remarks>
-        /// This should be set to false if you want to load a map file onto an existing map and do not wish to overwrite the existing entity.
-        /// </remarks>
-        public bool LoadMap { get; set; } = true;
-
-        public bool DoMapInit = false;
-    }
-}

Robust.Server/ServerIoC.cs

@@ -67,7 +67,6 @@ namespace Robust.Server
             deps.Register<IResourceManagerInternal, ResourceManager>();
             deps.Register<EntityManager, ServerEntityManager>();
             deps.Register<IServerEntityManager, ServerEntityManager>();
-            deps.Register<IServerEntityManagerInternal, ServerEntityManager>();
             deps.Register<IServerGameStateManager, ServerGameStateManager>();
             deps.Register<IReplayRecordingManager, ReplayRecordingManager>();
             deps.Register<IReplayRecordingManagerInternal, ReplayRecordingManager>();

Robust.Shared.Maths/Properties/AssemblyInfo.cs

@@ -6,4 +6,3 @@
 
 [assembly: InternalsVisibleTo("Robust.Client")]
 [assembly: InternalsVisibleTo("Robust.UnitTesting")]
-[assembly: InternalsVisibleTo("Content.Benchmarks")]

Robust.Shared/Audio/ResolvedSoundSpecifier.cs

@@ -0,0 +1,90 @@
+using System;
+using JetBrains.Annotations;
+using Robust.Shared.Utility;
+using Robust.Shared.Serialization;
+using Robust.Shared.Serialization.Manager.Attributes;
+using Robust.Shared.Prototypes;
+
+namespace Robust.Shared.Audio;
+
+/// <summary>
+/// Represents a path to a sound resource, either as a literal path or as a collection ID and index.
+/// </summary>
+/// <seealso cref="ResolvedPathSpecifier"/>
+/// <seealso cref="ResolvedCollectionSpecifier"/>
+[Serializable, NetSerializable]
+public abstract partial class ResolvedSoundSpecifier {
+    [Obsolete("String literals for sounds are deprecated, use a SoundSpecifier or ResolvedSoundSpecifier as appropriate instead")]
+    public static implicit operator ResolvedSoundSpecifier(string s) => new ResolvedPathSpecifier(s);
+    [Obsolete("String literals for sounds are deprecated, use a SoundSpecifier or ResolvedSoundSpecifier as appropriate instead")]
+    public static implicit operator ResolvedSoundSpecifier(ResPath s) => new ResolvedPathSpecifier(s);
+
+    /// <summary>
+    /// Returns whether <c>s</c> is null, or if it contains an empty path/collection ID.
+    /// </summary>
+    public static bool IsNullOrEmpty(ResolvedSoundSpecifier? s) {
+        return s switch {
+            null => true,
+            ResolvedPathSpecifier path => path.Path.ToString() == "",
+            ResolvedCollectionSpecifier collection => string.IsNullOrEmpty(collection.Collection),
+            _ => throw new ArgumentOutOfRangeException("s", s, "argument is not a ResolvedPathSpecifier or a ResolvedCollectionSpecifier"),
+        };
+    }
+}
+
+/// <summary>
+/// Represents a path to a sound resource as a literal path.
+/// </summary>
+/// <seealso cref="ResolvedCollectionSpecifier"/>
+[Serializable, NetSerializable]
+public sealed partial class ResolvedPathSpecifier : ResolvedSoundSpecifier {
+    /// <summary>
+    /// The resource path of the sound.
+    /// </summary>
+    public ResPath Path { get; private set; }
+
+    override public string ToString() =>
+        $"ResolvedPathSpecifier({Path})";
+
+    [UsedImplicitly]
+    private ResolvedPathSpecifier()
+    {
+    }
+    public ResolvedPathSpecifier(ResPath path)
+    {
+        Path = path;
+    }
+    public ResolvedPathSpecifier(string path) : this(new ResPath(path))
+    {
+    }
+}
+
+/// <summary>
+/// Represents a path to a sound resource as a collection ID and index.
+/// </summary>
+/// <seealso cref="ResolvedPathSpecifier"/>
+[Serializable, NetSerializable]
+public sealed partial class ResolvedCollectionSpecifier : ResolvedSoundSpecifier {
+    /// <summary>
+    /// The ID of the <see cref="SoundCollectionPrototype">sound collection</see> to look up.
+    /// </summary>
+    public ProtoId<SoundCollectionPrototype>? Collection { get; private set; }
+    /// <summary>
+    /// The index of the file in the associated sound collection to play.
+    /// </summary>
+    public int Index { get; private set; }
+
+    override public string ToString() =>
+        $"ResolvedCollectionSpecifier({Collection}, {Index})";
+
+    [UsedImplicitly]
+    private ResolvedCollectionSpecifier()
+    {
+    }
+
+    public ResolvedCollectionSpecifier(string collection, int index)
+    {
+        Collection = collection;
+        Index = index;
+    }
+}

Robust.Shared/Audio/SoundSpecifier.cs

@@ -27,6 +27,9 @@ public sealed partial class SoundPathSpecifier : SoundSpecifier
     [DataField(Node, customTypeSerializer: typeof(ResPathSerializer), required: true)]
     public ResPath Path { get; private set; }
 
+    override public string ToString() =>
+        $"SoundPathSpecifier({Path})";
+
     [UsedImplicitly]
     private SoundPathSpecifier()
     {
@@ -52,6 +55,9 @@ public sealed partial class SoundCollectionSpecifier : SoundSpecifier
     [DataField(Node, customTypeSerializer: typeof(PrototypeIdSerializer<SoundCollectionPrototype>), required: true)]
     public string? Collection { get; private set; }
 
+    override public string ToString() =>
+        $"SoundCollectionSpecifier({Collection})";
+
     [UsedImplicitly]
     public SoundCollectionSpecifier() { }
 

Robust.Shared/Audio/Systems/SharedAudioSystem.cs

@@ -283,33 +283,60 @@ public abstract partial class SharedAudioSystem : EntitySystem
     }
 
     /// <summary>
-    /// Resolves the filepath to a sound file.
+    /// Resolve a sound specifier so it can be consistently played back on all clients.
     /// </summary>
-    public string GetSound(SoundSpecifier specifier)
+    public ResolvedSoundSpecifier ResolveSound(SoundSpecifier specifier)
     {
         switch (specifier)
         {
             case SoundPathSpecifier path:
-                return path.Path == default ? string.Empty : path.Path.ToString();
+                return new ResolvedPathSpecifier(path.Path == default ? string.Empty : path.Path.ToString());
 
             case SoundCollectionSpecifier collection:
             {
                 if (collection.Collection == null)
-                    return string.Empty;
+                    return new ResolvedPathSpecifier(string.Empty);
 
                 var soundCollection = ProtoMan.Index<SoundCollectionPrototype>(collection.Collection);
-                return RandMan.Pick(soundCollection.PickFiles).ToString();
+                var index = RandMan.Next(soundCollection.PickFiles.Count);
+                return new ResolvedCollectionSpecifier(collection.Collection, index);
             }
         }
 
-        return string.Empty;
+        return new ResolvedPathSpecifier(string.Empty);
+    }
+
+    /// <summary>
+    /// Resolves the filepath to a sound file.
+    /// </summary>
+    [Obsolete("Use ResolveSound() and pass around resolved sound specifiers instead.")]
+    public string GetSound(SoundSpecifier specifier)
+    {
+        var resolved = ResolveSound(specifier);
+        return GetAudioPath(resolved);
     }
 
     #region AudioParams
 
-    protected Entity<AudioComponent> SetupAudio(string? fileName, AudioParams? audioParams, bool initialize = true, TimeSpan? length = null)
+    [return: NotNullIfNotNull(nameof(specifier))]
+    public string? GetAudioPath(ResolvedSoundSpecifier? specifier)
+    {
+        return specifier switch {
+            ResolvedPathSpecifier path =>
+                path.Path.ToString(),
+            ResolvedCollectionSpecifier collection =>
+                collection.Collection is null ?
+                    string.Empty :
+                    ProtoMan.Index<SoundCollectionPrototype>(collection.Collection).PickFiles[collection.Index].ToString(),
+            null => null,
+            _ => throw new ArgumentOutOfRangeException("specifier", specifier, "argument is not a ResolvedPathSpecifier or a ResolvedCollectionSpecifier"),
+        };
+    }
+
+    protected Entity<AudioComponent> SetupAudio(ResolvedSoundSpecifier? specifier, AudioParams? audioParams, bool initialize = true, TimeSpan? length = null)
     {
         var uid = EntityManager.CreateEntityUninitialized("Audio", MapCoordinates.Nullspace);
+        var fileName = GetAudioPath(specifier);
         DebugTools.Assert(!string.IsNullOrEmpty(fileName) || length is not null);
         MetadataSys.SetEntityName(uid, $"Audio ({fileName})", raiseEvents: false);
         audioParams ??= AudioParams.Default;
@@ -395,8 +422,9 @@ public abstract partial class SharedAudioSystem : EntitySystem
     /// <summary>
     /// Gets the timespan of the specified audio.
     /// </summary>
-    public TimeSpan GetAudioLength(string filename)
+    public TimeSpan GetAudioLength(ResolvedSoundSpecifier specifier)
     {
+        var filename = GetAudioPath(specifier) ?? string.Empty;
         if (!filename.StartsWith("/"))
             throw new ArgumentException("Path must be rooted");
 
@@ -429,7 +457,7 @@ public abstract partial class SharedAudioSystem : EntitySystem
     /// </summary>
     /// <param name="filename">The resource path to the OGG Vorbis file to play.</param>
     /// <param name="playerFilter">The set of players that will hear the sound.</param>
-    public abstract (EntityUid Entity, Components.AudioComponent Component)? PlayGlobal(string? filename, Filter playerFilter, bool recordReplay, AudioParams? audioParams = null);
+    public abstract (EntityUid Entity, Components.AudioComponent Component)? PlayGlobal(ResolvedSoundSpecifier? filename, Filter playerFilter, bool recordReplay, AudioParams? audioParams = null);
 
     /// <summary>
     /// Play an audio file globally, without position.
@@ -438,7 +466,7 @@ public abstract partial class SharedAudioSystem : EntitySystem
     /// <param name="playerFilter">The set of players that will hear the sound.</param>
     public (EntityUid Entity, Components.AudioComponent Component)? PlayGlobal(SoundSpecifier? sound, Filter playerFilter, bool recordReplay, AudioParams? audioParams = null)
     {
-        return sound == null ? null : PlayGlobal(GetSound(sound), playerFilter, recordReplay, audioParams ?? sound.Params);
+        return sound == null ? null : PlayGlobal(ResolveSound(sound), playerFilter, recordReplay, audioParams ?? sound.Params);
     }
 
     /// <summary>
@@ -446,7 +474,7 @@ public abstract partial class SharedAudioSystem : EntitySystem
     /// </summary>
     /// <param name="filename">The resource path to the OGG Vorbis file to play.</param>
     /// <param name="recipient">The player that will hear the sound.</param>
-    public abstract (EntityUid Entity, Components.AudioComponent Component)? PlayGlobal(string? filename, ICommonSession recipient, AudioParams? audioParams = null);
+    public abstract (EntityUid Entity, Components.AudioComponent Component)? PlayGlobal(ResolvedSoundSpecifier? filename, ICommonSession recipient, AudioParams? audioParams = null);
 
     /// <summary>
     /// Play an audio file globally, without position.
@@ -455,7 +483,7 @@ public abstract partial class SharedAudioSystem : EntitySystem
     /// <param name="recipient">The player that will hear the sound.</param>
     public (EntityUid Entity, Components.AudioComponent Component)? PlayGlobal(SoundSpecifier? sound, ICommonSession recipient, AudioParams? audioParams = null)
     {
-        return sound == null ? null : PlayGlobal(GetSound(sound), recipient, audioParams ?? sound.Params);
+        return sound == null ? null : PlayGlobal(ResolveSound(sound), recipient, audioParams ?? sound.Params);
     }
 
     public abstract void LoadStream<T>(Entity<AudioComponent> entity, T stream);
@@ -465,7 +493,7 @@ public abstract partial class SharedAudioSystem : EntitySystem
     /// </summary>
     /// <param name="filename">The resource path to the OGG Vorbis file to play.</param>
     /// <param name="recipient">The player that will hear the sound.</param>
-    public abstract (EntityUid Entity, Components.AudioComponent Component)? PlayGlobal(string? filename, EntityUid recipient, AudioParams? audioParams = null);
+    public abstract (EntityUid Entity, Components.AudioComponent Component)? PlayGlobal(ResolvedSoundSpecifier? filename, EntityUid recipient, AudioParams? audioParams = null);
 
     /// <summary>
     /// Play an audio file globally, without position.
@@ -474,7 +502,7 @@ public abstract partial class SharedAudioSystem : EntitySystem
     /// <param name="recipient">The player that will hear the sound.</param>
     public (EntityUid Entity, Components.AudioComponent Component)? PlayGlobal(SoundSpecifier? sound, EntityUid recipient, AudioParams? audioParams = null)
     {
-        return sound == null ? null : PlayGlobal(GetSound(sound), recipient, audioParams ?? sound.Params);
+        return sound == null ? null : PlayGlobal(ResolveSound(sound), recipient, audioParams ?? sound.Params);
     }
 
     /// <summary>
@@ -483,7 +511,7 @@ public abstract partial class SharedAudioSystem : EntitySystem
     /// <param name="filename">The resource path to the OGG Vorbis file to play.</param>
     /// <param name="playerFilter">The set of players that will hear the sound.</param>
     /// <param name="uid">The UID of the entity "emitting" the audio.</param>
-    public abstract (EntityUid Entity, Components.AudioComponent Component)? PlayEntity(string? filename, Filter playerFilter, EntityUid uid, bool recordReplay, AudioParams? audioParams = null);
+    public abstract (EntityUid Entity, Components.AudioComponent Component)? PlayEntity(ResolvedSoundSpecifier? filename, Filter playerFilter, EntityUid uid, bool recordReplay, AudioParams? audioParams = null);
 
     /// <summary>
     /// Play an audio file following an entity.
@@ -491,7 +519,7 @@ public abstract partial class SharedAudioSystem : EntitySystem
     /// <param name="filename">The resource path to the OGG Vorbis file to play.</param>
     /// <param name="recipient">The player that will hear the sound.</param>
     /// <param name="uid">The UID of the entity "emitting" the audio.</param>
-    public abstract (EntityUid Entity, Components.AudioComponent Component)? PlayEntity(string? filename, ICommonSession recipient, EntityUid uid, AudioParams? audioParams = null);
+    public abstract (EntityUid Entity, Components.AudioComponent Component)? PlayEntity(ResolvedSoundSpecifier? filename, ICommonSession recipient, EntityUid uid, AudioParams? audioParams = null);
 
     /// <summary>
     /// Play an audio file following an entity.
@@ -499,7 +527,7 @@ public abstract partial class SharedAudioSystem : EntitySystem
     /// <param name="filename">The resource path to the OGG Vorbis file to play.</param>
     /// <param name="recipient">The player that will hear the sound.</param>
     /// <param name="uid">The UID of the entity "emitting" the audio.</param>
-    public abstract (EntityUid Entity, Components.AudioComponent Component)? PlayEntity(string? filename, EntityUid recipient, EntityUid uid, AudioParams? audioParams = null);
+    public abstract (EntityUid Entity, Components.AudioComponent Component)? PlayEntity(ResolvedSoundSpecifier? filename, EntityUid recipient, EntityUid uid, AudioParams? audioParams = null);
 
     /// <summary>
     /// Play an audio file following an entity.
@@ -509,7 +537,7 @@ public abstract partial class SharedAudioSystem : EntitySystem
     /// <param name="uid">The UID of the entity "emitting" the audio.</param>
     public (EntityUid Entity, Components.AudioComponent Component)? PlayEntity(SoundSpecifier? sound, Filter playerFilter, EntityUid uid, bool recordReplay, AudioParams? audioParams = null)
     {
-        return sound == null ? null : PlayEntity(GetSound(sound), playerFilter, uid, recordReplay, audioParams ?? sound.Params);
+        return sound == null ? null : PlayEntity(ResolveSound(sound), playerFilter, uid, recordReplay, audioParams ?? sound.Params);
     }
 
     /// <summary>
@@ -520,7 +548,7 @@ public abstract partial class SharedAudioSystem : EntitySystem
     /// <param name="uid">The UID of the entity "emitting" the audio.</param>
     public (EntityUid Entity, Components.AudioComponent Component)? PlayEntity(SoundSpecifier? sound, ICommonSession recipient, EntityUid uid, AudioParams? audioParams = null)
     {
-        return sound == null ? null : PlayEntity(GetSound(sound), recipient, uid, audioParams ?? sound.Params);
+        return sound == null ? null : PlayEntity(ResolveSound(sound), recipient, uid, audioParams ?? sound.Params);
     }
 
     /// <summary>
@@ -531,7 +559,7 @@ public abstract partial class SharedAudioSystem : EntitySystem
     /// <param name="uid">The UID of the entity "emitting" the audio.</param>
     public (EntityUid Entity, Components.AudioComponent Component)? PlayEntity(SoundSpecifier? sound, EntityUid recipient, EntityUid uid, AudioParams? audioParams = null)
     {
-        return sound == null ? null : PlayEntity(GetSound(sound), recipient, uid, audioParams ?? sound.Params);
+        return sound == null ? null : PlayEntity(ResolveSound(sound), recipient, uid, audioParams ?? sound.Params);
     }
 
     /// <summary>
@@ -541,7 +569,7 @@ public abstract partial class SharedAudioSystem : EntitySystem
     /// <param name="uid">The UID of the entity "emitting" the audio.</param>
     public (EntityUid Entity, Components.AudioComponent Component)? PlayPvs(SoundSpecifier? sound, EntityUid uid, AudioParams? audioParams = null)
     {
-        return sound == null ? null : PlayPvs(GetSound(sound), uid, audioParams ?? sound.Params);
+        return sound == null ? null : PlayPvs(ResolveSound(sound), uid, audioParams ?? sound.Params);
     }
 
     /// <summary>
@@ -551,7 +579,7 @@ public abstract partial class SharedAudioSystem : EntitySystem
     /// <param name="coordinates">The EntityCoordinates to attach the audio source to.</param>
     public (EntityUid Entity, Components.AudioComponent Component)? PlayPvs(SoundSpecifier? sound, EntityCoordinates coordinates, AudioParams? audioParams = null)
     {
-        return sound == null ? null : PlayPvs(GetSound(sound), coordinates, audioParams ?? sound.Params);
+        return sound == null ? null : PlayPvs(ResolveSound(sound), coordinates, audioParams ?? sound.Params);
     }
 
     /// <summary>
@@ -559,7 +587,7 @@ public abstract partial class SharedAudioSystem : EntitySystem
     /// </summary>
     /// <param name="sound">The sound specifier that points the audio file(s) that should be played.</param>
     /// <param name="coordinates">The EntityCoordinates to attach the audio source to.</param>
-    public abstract (EntityUid Entity, Components.AudioComponent Component)? PlayPvs(string? filename,
+    public abstract (EntityUid Entity, Components.AudioComponent Component)? PlayPvs(ResolvedSoundSpecifier? filename,
         EntityCoordinates coordinates, AudioParams? audioParams = null);
 
     /// <summary>
@@ -567,7 +595,7 @@ public abstract partial class SharedAudioSystem : EntitySystem
     /// </summary>
     /// <param name="filename">The resource path to the OGG Vorbis file to play.</param>
     /// <param name="uid">The UID of the entity "emitting" the audio.</param>
-    public abstract (EntityUid Entity, Components.AudioComponent Component)? PlayPvs(string? filename, EntityUid uid,
+    public abstract (EntityUid Entity, Components.AudioComponent Component)? PlayPvs(ResolvedSoundSpecifier? filename, EntityUid uid,
         AudioParams? audioParams = null);
 
     /// <summary>
@@ -604,7 +632,7 @@ public abstract partial class SharedAudioSystem : EntitySystem
     /// <param name="filename">The resource path to the OGG Vorbis file to play.</param>
     /// <param name="playerFilter">The set of players that will hear the sound.</param>
     /// <param name="coordinates">The coordinates at which to play the audio.</param>
-    public abstract (EntityUid Entity, Components.AudioComponent Component)? PlayStatic(string? filename, Filter playerFilter, EntityCoordinates coordinates, bool recordReplay, AudioParams? audioParams = null);
+    public abstract (EntityUid Entity, Components.AudioComponent Component)? PlayStatic(ResolvedSoundSpecifier? filename, Filter playerFilter, EntityCoordinates coordinates, bool recordReplay, AudioParams? audioParams = null);
 
     /// <summary>
     /// Play an audio file at a static position.
@@ -612,7 +640,7 @@ public abstract partial class SharedAudioSystem : EntitySystem
     /// <param name="filename">The resource path to the OGG Vorbis file to play.</param>
     /// <param name="recipient">The player that will hear the sound.</param>
     /// <param name="coordinates">The coordinates at which to play the audio.</param>
-    public abstract (EntityUid Entity, Components.AudioComponent Component)? PlayStatic(string? filename, ICommonSession recipient, EntityCoordinates coordinates, AudioParams? audioParams = null);
+    public abstract (EntityUid Entity, Components.AudioComponent Component)? PlayStatic(ResolvedSoundSpecifier? filename, ICommonSession recipient, EntityCoordinates coordinates, AudioParams? audioParams = null);
 
     /// <summary>
     /// Play an audio file at a static position.
@@ -620,7 +648,7 @@ public abstract partial class SharedAudioSystem : EntitySystem
     /// <param name="filename">The resource path to the OGG Vorbis file to play.</param>
     /// <param name="recipient">The player that will hear the sound.</param>
     /// <param name="coordinates">The coordinates at which to play the audio.</param>
-    public abstract (EntityUid Entity, Components.AudioComponent Component)? PlayStatic(string? filename, EntityUid recipient, EntityCoordinates coordinates, AudioParams? audioParams = null);
+    public abstract (EntityUid Entity, Components.AudioComponent Component)? PlayStatic(ResolvedSoundSpecifier? filename, EntityUid recipient, EntityCoordinates coordinates, AudioParams? audioParams = null);
 
     /// <summary>
     /// Play an audio file at a static position.
@@ -630,7 +658,7 @@ public abstract partial class SharedAudioSystem : EntitySystem
     /// <param name="coordinates">The coordinates at which to play the audio.</param>
     public (EntityUid Entity, Components.AudioComponent Component)? PlayStatic(SoundSpecifier? sound, Filter playerFilter, EntityCoordinates coordinates, bool recordReplay, AudioParams? audioParams = null)
     {
-        return sound == null ? null : PlayStatic(GetSound(sound), playerFilter, coordinates, recordReplay, audioParams);
+        return sound == null ? null : PlayStatic(ResolveSound(sound), playerFilter, coordinates, recordReplay, audioParams);
     }
 
     /// <summary>
@@ -641,7 +669,7 @@ public abstract partial class SharedAudioSystem : EntitySystem
     /// <param name="coordinates">The coordinates at which to play the audio.</param>
     public (EntityUid Entity, Components.AudioComponent Component)? PlayStatic(SoundSpecifier? sound, ICommonSession recipient, EntityCoordinates coordinates, AudioParams? audioParams = null)
     {
-        return sound == null ? null : PlayStatic(GetSound(sound), recipient, coordinates, audioParams ?? sound.Params);
+        return sound == null ? null : PlayStatic(ResolveSound(sound), recipient, coordinates, audioParams ?? sound.Params);
     }
 
     /// <summary>
@@ -652,7 +680,7 @@ public abstract partial class SharedAudioSystem : EntitySystem
     /// <param name="coordinates">The coordinates at which to play the audio.</param>
     public (EntityUid Entity, Components.AudioComponent Component)? PlayStatic(SoundSpecifier? sound, EntityUid recipient, EntityCoordinates coordinates, AudioParams? audioParams = null)
     {
-        return sound == null ? null : PlayStatic(GetSound(sound), recipient, coordinates, audioParams ?? sound.Params);
+        return sound == null ? null : PlayStatic(ResolveSound(sound), recipient, coordinates, audioParams ?? sound.Params);
     }
 
     // These are just here for replays now.
@@ -665,7 +693,7 @@ public abstract partial class SharedAudioSystem : EntitySystem
     [NetSerializable, Serializable]
     protected abstract class AudioMessage : EntityEventArgs
     {
-        public string FileName = string.Empty;
+        public ResolvedSoundSpecifier Specifier = new ResolvedPathSpecifier(string.Empty);
         public AudioParams AudioParams;
     }
 

Robust.Shared/ComponentTrees/ComponentTreeSystem.cs

@@ -59,7 +59,7 @@ public abstract class ComponentTreeSystem<TTreeComp, TComp> : EntitySystem
         UpdatesAfter.Add(typeof(SharedTransformSystem));
         UpdatesAfter.Add(typeof(SharedPhysicsSystem));
 
-        SubscribeLocalEvent<MapChangedEvent>(MapManagerOnMapCreated);
+        SubscribeLocalEvent<MapCreatedEvent>(MapManagerOnMapCreated);
         SubscribeLocalEvent<GridInitializeEvent>(MapManagerOnGridCreated);
 
         SubscribeLocalEvent<TComp, ComponentStartup>(OnCompStartup);
@@ -143,11 +143,8 @@ public abstract class ComponentTreeSystem<TTreeComp, TComp> : EntitySystem
         RemComp(uid, component);
     }
 
-    private void MapManagerOnMapCreated(MapChangedEvent e)
+    private void MapManagerOnMapCreated(MapCreatedEvent e)
     {
-        if (e.Destroyed || e.Map == MapId.Nullspace)
-            return;
-
         EnsureComp<TTreeComp>(e.Uid);
     }
 

Robust.Shared/Console/Commands/MapCommands.cs

@@ -138,7 +138,7 @@ internal sealed class ListMapsCommand : LocalizedEntityCommands
     {
         var msg = new StringBuilder();
 
-        foreach (var mapId in _map.GetAllMapIds().OrderBy(id => id.Value))
+        foreach (var mapId in _mapSystem.GetAllMapIds().OrderBy(id => id.Value))
         {
             if (!_mapSystem.TryGetMap(mapId, out var mapUid))
                 continue;

Robust.Shared/ContentPack/AssemblyTypeChecker.Config.cs

@@ -88,6 +88,7 @@ namespace Robust.Shared.ContentPack
             public string SystemAssemblyName = default!;
             public HashSet<VerifierError> AllowedVerifierErrors = default!;
             public List<string> WhitelistedNamespaces = default!;
+            public List<string> AllowedAssemblyPrefixes = default!;
             public Dictionary<string, Dictionary<string, TypeConfig>> Types = default!;
         }
 

Robust.Shared/ContentPack/AssemblyTypeChecker.cs

@@ -131,6 +131,16 @@ namespace Robust.Shared.ContentPack
                 return false;
             }
 
+#pragma warning disable RA0004
+            var loadedConfig = _config.Result;
+#pragma warning restore RA0004
+
+            if (!loadedConfig.AllowedAssemblyPrefixes.Any(allowedNamePrefix => asmName.StartsWith(allowedNamePrefix)))
+            {
+                _sawmill.Error($"Assembly name '{asmName}' is not allowed for a content assembly");
+                return false;
+            }
+
             if (VerifyIL)
             {
                 if (!DoVerifyIL(asmName, resolver, peReader, reader))
@@ -179,10 +189,6 @@ namespace Robust.Shared.ContentPack
                 return true;
             }
 
-#pragma warning disable RA0004
-            var loadedConfig = _config.Result;
-#pragma warning restore RA0004
-
             var badRefs = new ConcurrentBag<EntityHandle>();
 
             // We still do explicit type reference scanning, even though the actual whitelists work with raw members.

Robust.Shared/ContentPack/DirLoader.cs

@@ -60,7 +60,7 @@ namespace Robust.Shared.ContentPack
 
             internal string GetPath(ResPath relPath)
             {
-                return Path.GetFullPath(Path.Combine(_directory.FullName, relPath.ToRelativeSystemPath()));
+                return PathHelpers.SafeGetResourcePath(_directory.FullName, relPath);
             }
 
             /// <inheritdoc />

Robust.Shared/ContentPack/IResourceManagerInternal.cs

@@ -14,7 +14,11 @@ namespace Robust.Shared.ContentPack
         /// The directory to use for user data.
         /// If null, a virtual temporary file system is used instead.
         /// </param>
-        void Initialize(string? userData);
+        /// <param name="hideUserDataDir">
+        /// If true, <see cref="IWritableDirProvider.RootDir"/> will be hidden on
+        /// <see cref="IResourceManager.UserData"/>.
+        /// </param>
+        void Initialize(string? userData, bool hideUserDataDir);
 
         /// <summary>
         ///     Mounts a single stream as a content file. Useful for unit testing.

Robust.Shared/ContentPack/IWritableDirProvider.cs

@@ -13,7 +13,7 @@ namespace Robust.Shared.ContentPack
     {
         /// <summary>
         /// The root path of this provider.
-        /// Can be null if it's a virtual provider.
+        /// Can be null if it's a virtual provider or the path is protected (e.g. on the client).
         /// </summary>
         string? RootDir { get; }
 

Robust.Shared/ContentPack/ModLoader.cs

@@ -93,19 +93,23 @@ namespace Robust.Shared.ContentPack
         {
             var sw = Stopwatch.StartNew();
             Sawmill.Debug("LOADING modules");
-            var files = new Dictionary<string, (ResPath Path, string[] references)>();
+            var files = new Dictionary<string, (ResPath Path, MemoryStream data, string[] references)>();
 
             // Find all modules we want to load.
             foreach (var fullPath in paths)
             {
                 using var asmFile = _res.ContentFileRead(fullPath);
-                var refData = GetAssemblyReferenceData(asmFile);
+                var ms = new MemoryStream();
+                asmFile.CopyTo(ms);
+
+                ms.Position = 0;
+                var refData = GetAssemblyReferenceData(ms);
                 if (refData == null)
                     continue;
 
                 var (asmRefs, asmName) = refData.Value;
 
-                if (!files.TryAdd(asmName, (fullPath, asmRefs)))
+                if (!files.TryAdd(asmName, (fullPath, ms, asmRefs)))
                 {
                     Sawmill.Error("Found multiple modules with the same assembly name " +
                                   $"'{asmName}', A: {files[asmName].Path}, B: {fullPath}.");
@@ -122,10 +126,10 @@ namespace Robust.Shared.ContentPack
 
                 Parallel.ForEach(files, pair =>
                 {
-                    var (name, (path, _)) = pair;
+                    var (name, (_, data, _)) = pair;
 
-                    using var stream = _res.ContentFileRead(path);
-                    if (!typeChecker.CheckAssembly(stream, resolver))
+                    data.Position = 0;
+                    if (!typeChecker.CheckAssembly(data, resolver))
                     {
                         throw new TypeCheckFailedException($"Assembly {name} failed type checks.");
                     }
@@ -137,14 +141,15 @@ namespace Robust.Shared.ContentPack
             var nodes = TopologicalSort.FromBeforeAfter(
                 files,
                 kv => kv.Key,
-                kv => kv.Value.Path,
+                kv => kv.Value,
                 _ => Array.Empty<string>(),
                 kv => kv.Value.references,
                 allowMissing: true); // missing refs would be non-content assemblies so allow that.
 
             // Actually load them in the order they depend on each other.
-            foreach (var path in TopologicalSort.Sort(nodes))
+            foreach (var item in TopologicalSort.Sort(nodes))
             {
+                var (path, memory, _) = item;
                 Sawmill.Debug($"Loading module: '{path}'");
                 try
                 {
@@ -156,9 +161,9 @@ namespace Robust.Shared.ContentPack
                     }
                     else
                     {
-                        using var assemblyStream = _res.ContentFileRead(path);
+                        memory.Position = 0;
                         using var symbolsStream = _res.ContentFileReadOrNull(path.WithExtension("pdb"));
-                        LoadGameAssembly(assemblyStream, symbolsStream, skipVerify: true);
+                        LoadGameAssembly(memory, symbolsStream, skipVerify: true);
                     }
                 }
                 catch (Exception e)
@@ -174,7 +179,7 @@ namespace Robust.Shared.ContentPack
 
         private (string[] refs, string name)? GetAssemblyReferenceData(Stream stream)
         {
-            using var reader = ModLoader.MakePEReader(stream);
+            using var reader = ModLoader.MakePEReader(stream, leaveOpen: true);
             var metaReader = reader.GetMetadataReader();
 
             var name = metaReader.GetString(metaReader.GetAssemblyDefinition().Name);

Robust.Shared/ContentPack/PathHelpers.cs

@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 using System.IO;
 using System.Runtime.InteropServices;
+using Robust.Shared.Utility;
 
 namespace Robust.Shared.ContentPack
 {
@@ -63,5 +64,27 @@ namespace Robust.Shared.ContentPack
             !OperatingSystem.IsWindows()
             && !OperatingSystem.IsMacOS();
 
+
+        internal static string SafeGetResourcePath(string baseDir, ResPath path)
+        {
+            var relSysPath = path.ToRelativeSystemPath();
+            if (relSysPath.Contains("\\..") || relSysPath.Contains("/.."))
+            {
+                // Hard cap on any exploit smuggling a .. in there.
+                // Since that could allow leaving sandbox.
+                throw new InvalidOperationException($"This branch should never be reached. Path: {path}");
+            }
+
+            var retPath = Path.GetFullPath(Path.Join(baseDir, relSysPath));
+            // better safe than sorry check
+            if (!retPath.StartsWith(baseDir))
+            {
+                // Allow path to match if it's just missing the directory separator at the end.
+                if (retPath != baseDir.TrimEnd(Path.DirectorySeparatorChar))
+                    throw new InvalidOperationException($"This branch should never be reached. Path: {path}");
+            }
+
+            return retPath;
+        }
     }
 }

Robust.Shared/ContentPack/ResourceManager.cs

@@ -41,13 +41,13 @@ namespace Robust.Shared.ContentPack
         public IWritableDirProvider UserData { get; private set; } = default!;
 
         /// <inheritdoc />
-        public virtual void Initialize(string? userData)
+        public virtual void Initialize(string? userData, bool hideRootDir)
         {
             Sawmill = _logManager.GetSawmill("res");
 
             if (userData != null)
             {
-                UserData = new WritableDirProvider(Directory.CreateDirectory(userData));
+                UserData = new WritableDirProvider(Directory.CreateDirectory(userData), hideRootDir);
             }
             else
             {
@@ -379,7 +379,13 @@ namespace Robust.Shared.ContentPack
             {
                 if (root is DirLoader loader)
                 {
-                    yield return new ResPath(loader.GetPath(new ResPath(@"/")));
+                    var rootDir = loader.GetPath(new ResPath(@"/"));
+
+                    // TODO: GET RID OF THIS.
+                    // This code shouldn't be passing OS disk paths through ResPath.
+                    rootDir = rootDir.Replace(Path.DirectorySeparatorChar, '/');
+
+                    yield return new ResPath(rootDir);
                 }
             }
         }

Robust.Shared/ContentPack/Sandbox.yml

@@ -17,6 +17,10 @@ WhitelistedNamespaces:
 - Content
 - OpenDreamShared
 
+AllowedAssemblyPrefixes:
+- OpenDream
+- Content
+
 # The type whitelist does NOT care about which assembly types come from.
 # This is because types switch assembly all the time.
 # Just look up stuff like StreamReader on https://apisof.net.
@@ -696,7 +700,7 @@ Types:
       - "bool IsMatch(string, string, System.Text.RegularExpressions.RegexOptions, System.TimeSpan)"
       - "int GroupNumberFromName(string)"
       - "int[] GetGroupNumbers()"
-      - "string Escape()"
+      - "string Escape(string)"
       - "string GroupNameFromNumber(int)"
       - "string Replace(string, string)"
       - "string Replace(string, string, int)"

Robust.Shared/ContentPack/WritableDirProvider.cs

@@ -10,17 +10,22 @@ namespace Robust.Shared.ContentPack
     /// <inheritdoc />
     internal sealed class WritableDirProvider : IWritableDirProvider
     {
-        /// <inheritdoc />
+        private readonly bool _hideRootDir;
+
         public string RootDir { get; }
 
+        string? IWritableDirProvider.RootDir => _hideRootDir ? null : RootDir;
+
         /// <summary>
         /// Constructs an instance of <see cref="WritableDirProvider"/>.
         /// </summary>
         /// <param name="rootDir">Root file system directory to allow writing.</param>
-        public WritableDirProvider(DirectoryInfo rootDir)
+        /// <param name="hideRootDir">If true, <see cref="IWritableDirProvider.RootDir"/> is reported as null.</param>
+        public WritableDirProvider(DirectoryInfo rootDir, bool hideRootDir)
         {
             // FullName does not have a trailing separator, and we MUST have a separator.
             RootDir = rootDir.FullName + Path.DirectorySeparatorChar.ToString();
+            _hideRootDir = hideRootDir;
         }
 
         #region File Access
@@ -119,7 +124,7 @@ namespace Robust.Shared.ContentPack
                 throw new FileNotFoundException();
 
             var dirInfo = new DirectoryInfo(GetFullPath(path));
-            return new WritableDirProvider(dirInfo);
+            return new WritableDirProvider(dirInfo, _hideRootDir);
         }
 
         /// <inheritdoc />
@@ -180,20 +185,7 @@ namespace Robust.Shared.ContentPack
 
             path = path.Clean();
 
-            return GetFullPath(RootDir, path);
-        }
-
-        private static string GetFullPath(string root, ResPath path)
-        {
-            var relPath = path.ToRelativeSystemPath();
-            if (relPath.Contains("\\..") || relPath.Contains("/.."))
-            {
-                // Hard cap on any exploit smuggling a .. in there.
-                // Since that could allow leaving sandbox.
-                throw new InvalidOperationException($"This branch should never be reached. Path: {path}");
-            }
-
-            return Path.GetFullPath(Path.Combine(root, relPath));
+            return PathHelpers.SafeGetResourcePath(RootDir, path);
         }
     }
 }

Robust.Shared/EntitySerialization/Components/LoadedMapComponent.cs

@@ -0,0 +1,12 @@
+using Robust.Shared.EntitySerialization.Systems;
+using Robust.Shared.GameObjects;
+
+namespace Robust.Shared.EntitySerialization.Components;
+
+/// <summary>
+/// Added to Maps that were loaded by <see cref="MapLoaderSystem"/>. If not present then this map was created externally.
+/// </summary>
+[RegisterComponent, UnsavedComponent]
+public sealed partial class LoadedMapComponent : Component
+{
+}

Robust.Shared/EntitySerialization/Components/MapSaveTileMapComponent.cs

@@ -1,7 +1,8 @@
 using System.Collections.Generic;
+using Robust.Shared.EntitySerialization.Systems;
 using Robust.Shared.GameObjects;
 
-namespace Robust.Server.GameObjects;
+namespace Robust.Shared.EntitySerialization.Components;
 
 /// <summary>
 /// Used by <see cref="MapLoaderSystem"/> to track the original tile map from when a map was loaded.

Robust.Shared/EntitySerialization/Components/YamlUidComponent.cs

@@ -0,0 +1,20 @@
+using Robust.Shared.GameObjects;
+using Robust.Shared.ViewVariables;
+
+namespace Robust.Shared.EntitySerialization.Components;
+
+/// <summary>
+/// This component is optionally added to entities that get loaded from yaml files. It stores the UID that the entity
+/// had within the yaml file. This is used when saving the entity back to a yaml file so that it re-uses the same UID.
+/// </summary>
+/// <remarks>
+/// This is primarily intended to reduce the diff sizes when modifying yaml maps. Note that there is no guarantee that
+/// the given uid will be used when writing the entity. E.g., if more than one entity have this component with the
+/// same uid, only one of those entities will be saved with the requested id.
+/// </remarks>
+[RegisterComponent, UnsavedComponent]
+public sealed partial class YamlUidComponent : Component
+{
+    [ViewVariables(VVAccess.ReadWrite)]
+    public int Uid;
+}

Robust.Shared/EntitySerialization/EntitySerializer.cs

@@ -0,0 +1,988 @@
+using System;
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+using System.Globalization;
+using System.Linq;
+using Robust.Shared.Configuration;
+using Robust.Shared.EntitySerialization.Components;
+using Robust.Shared.EntitySerialization.Systems;
+using Robust.Shared.GameObjects;
+using Robust.Shared.IoC;
+using Robust.Shared.Log;
+using Robust.Shared.Map;
+using Robust.Shared.Map.Components;
+using Robust.Shared.Prototypes;
+using Robust.Shared.Serialization;
+using Robust.Shared.Serialization.Manager;
+using Robust.Shared.Serialization.Markdown;
+using Robust.Shared.Serialization.Markdown.Mapping;
+using Robust.Shared.Serialization.Markdown.Sequence;
+using Robust.Shared.Serialization.Markdown.Validation;
+using Robust.Shared.Serialization.Markdown.Value;
+using Robust.Shared.Serialization.TypeSerializers.Interfaces;
+using Robust.Shared.Timing;
+using Robust.Shared.Utility;
+
+namespace Robust.Shared.EntitySerialization;
+
+/// <summary>
+/// This class provides methods for serializing entities into yaml. It provides some more control over
+/// serialization than the methods provided by <see cref="MapLoaderSystem"/>.
+/// </summary>
+/// <remarks>
+/// There are several methods (e.g., <see cref="SerializeEntityRecursive"/> that serialize entities into a
+/// per-entity <see cref="MappingDataNode"/> stored in the <see cref="EntityData"/> dictionary, which is indexed by the
+/// entity's assigned yaml id (see <see cref="GetYamlUid"/>. The generated data can then be written to a larger yaml
+/// document using the various "Write" methods. (e.g., <see cref="WriteEntitySection"/>). After a one has finished using
+/// the generated data, the serializer needs to be reset (<see cref="Reset"/>) using it again to serialize other entities.
+/// </remarks>
+public sealed class EntitySerializer : ISerializationContext,
+    ITypeSerializer<EntityUid, ValueDataNode>,
+    ITypeSerializer<NetEntity, ValueDataNode>
+{
+    public const int MapFormatVersion = 7;
+    // v6->v7: PR #5572 - Added more metadata, List maps/grids/orphans, include some life-stage information
+    // v5->v6: PR #4307 - Converted Tile.TypeId from ushort to int
+    // v4->v5: PR #3992 - Removed name & author fields
+    // v3->v4: PR #3913 - Grouped entities by prototype
+    // v2->v3: PR #3468
+
+    public SerializationManager.SerializerProvider SerializerProvider { get; } = new();
+
+    [Dependency] public readonly EntityManager EntMan = default!;
+    [Dependency] public readonly IGameTiming Timing = default!;
+    [Dependency] private readonly IComponentFactory _factory = default!;
+    [Dependency] private readonly ISerializationManager _serialization = default!;
+    [Dependency] private readonly ITileDefinitionManager _tileDef = default!;
+    [Dependency] private readonly IConfigurationManager _conf = default!;
+    [Dependency] private readonly ILogManager _logMan = default!;
+
+    private readonly ISawmill _log;
+    public readonly Dictionary<EntityUid, int> YamlUidMap = new();
+    public readonly HashSet<int> YamlIds = new();
+
+
+    public string? CurrentComponent { get; private set; }
+    public Entity<MetaDataComponent>? CurrentEntity { get; private set; }
+    public int CurrentEntityYamlUid { get; private set; }
+
+    /// <summary>
+    /// Tile ID -> yaml tile ID mapping.
+    /// </summary>
+    private readonly Dictionary<int, int> _tileMap = new();
+    private readonly HashSet<int> _yamlTileIds = new();
+
+    /// <inheritdoc/>
+    public bool WritingReadingPrototypes { get; private set; }
+
+    /// <summary>
+    /// If set, the serializer will refuse to serialize the given entity and will orphan any entity that is parented to
+    /// it. This is useful for serializing things like a grid (or multiple grids & entities) that are parented to a map
+    /// without actually serializing the map itself.
+    /// </summary>
+    public EntityUid Truncate { get; private set; }
+
+    /// <summary>
+    /// List of all entities that have previously been ignored via <see cref="Truncate"/>.
+    /// </summary>
+    /// <remarks>
+    /// This is tracked in case somebody does something weird, like trying to save a grid w/o its map, and then later on
+    /// including the map in the file. AFAIK, that should work in principle, though it would lead to a weird file where
+    /// the grid is orphaned and not on the map where it should be.
+    /// </remarks>
+    public readonly HashSet<EntityUid> Truncated = new();
+
+    public readonly SerializationOptions Options;
+
+    /// <summary>
+    /// Cached prototype data. This is used to avoid writing redundant data that is already specified in an entity's
+    /// prototype.
+    /// </summary>
+    public readonly Dictionary<string, Dictionary<string, MappingDataNode>> PrototypeCache = new();
+
+    /// <summary>
+    /// The serialized entity data.
+    /// </summary>
+    public readonly Dictionary<int, (EntityUid Uid, MappingDataNode Node)> EntityData = new();
+
+    /// <summary>
+    /// <see cref="EntityData"/> indices grouped by their entity prototype ids.
+    /// </summary>
+    public readonly Dictionary<string, List<int>> Prototypes = new();
+
+    /// <summary>
+    /// Yaml ids of all serialized map entities.
+    /// </summary>
+    public readonly List<int> Maps = new();
+
+    /// <summary>
+    /// Yaml ids of all serialized null-space entities.
+    /// This only includes entities that were initially in null-space, it does not include entities that were
+    /// serialized without their parents. Those are in <see cref="Orphans"/>.
+    /// </summary>
+    public readonly List<int> Nullspace = new();
+
+    /// <summary>
+    /// Yaml ids of all serialized grid entities.
+    /// </summary>
+    public readonly List<int> Grids = new();
+
+    /// <summary>
+    /// Yaml ids of all serialized entities in the file whose parents were not serialized. This does not include
+    /// entities that did not have a parent (e.g., maps or null-space entities). I.e., these are the entities that
+    /// need to be attached to a new parent when loading the file, unless you want to load them into null-space.
+    /// </summary>
+    public readonly List<int> Orphans = new();
+
+    private readonly string _metaName;
+    private readonly string _xformName;
+    private readonly MappingDataNode _emptyMetaNode;
+    private readonly MappingDataNode _emptyXformNode;
+    private int _nextYamlUid = 1;
+    private int _nextYamlTileId;
+
+    private readonly List<EntityUid> _autoInclude = new();
+    private readonly EntityQuery<YamlUidComponent> _yamlQuery;
+    private readonly EntityQuery<MapGridComponent> _gridQuery;
+    private readonly EntityQuery<MapComponent> _mapQuery;
+    private readonly EntityQuery<MetaDataComponent> _metaQuery;
+    private readonly EntityQuery<TransformComponent> _xformQuery;
+
+    /// <summary>
+    /// C# event for checking whether an entity is serializable. Can be used by content to prevent specific entities
+    /// from getting serialized.
+    /// </summary>
+    public event IsSerializableDelegate? OnIsSerializeable;
+    public delegate void IsSerializableDelegate(Entity<MetaDataComponent> ent, ref bool serializable);
+
+    public EntitySerializer(IDependencyCollection _dependency, SerializationOptions options)
+    {
+        _dependency.InjectDependencies(this);
+
+        _log = _logMan.GetSawmill("entity_serializer");
+        SerializerProvider.RegisterSerializer(this);
+
+        _metaName = _factory.GetComponentName(typeof(MetaDataComponent));
+        _xformName = _factory.GetComponentName(typeof(TransformComponent));
+        _emptyMetaNode = _serialization.WriteValueAs<MappingDataNode>(typeof(MetaDataComponent), new MetaDataComponent(), alwaysWrite: true, context: this);
+
+        CurrentComponent = _xformName;
+        _emptyXformNode = _serialization.WriteValueAs<MappingDataNode>(typeof(TransformComponent), new TransformComponent(), alwaysWrite: true, context: this);
+        CurrentComponent = null;
+
+        _yamlQuery = EntMan.GetEntityQuery<YamlUidComponent>();
+        _gridQuery = EntMan.GetEntityQuery<MapGridComponent>();
+        _mapQuery = EntMan.GetEntityQuery<MapComponent>();
+        _metaQuery = EntMan.GetEntityQuery<MetaDataComponent>();
+        _xformQuery = EntMan.GetEntityQuery<TransformComponent>();
+        Options = options;
+    }
+
+    public bool IsSerializable(Entity<MetaDataComponent?> ent)
+    {
+        if (ent.Comp == null && !EntMan.TryGetComponent(ent.Owner, out ent.Comp))
+            return false;
+
+        if (ent.Comp.EntityPrototype?.MapSavable == false)
+            return false;
+
+        bool serializable = true;
+        OnIsSerializeable?.Invoke(ent!, ref serializable);
+        return serializable;
+    }
+
+    #region Serialize API
+
+    /// <summary>
+    /// Serialize a single entity. This does not automatically include
+    /// children, though depending on the setting of <see cref="SerializationOptions.MissingEntityBehaviour"/> it may
+    /// auto-include additional entities aside from the one provided.
+    /// </summary>
+    public void SerializeEntity(EntityUid uid)
+    {
+        if (!IsSerializable(uid))
+            throw new Exception($"{EntMan.ToPrettyString(uid)} is not serializable");
+
+        DebugTools.AssertNull(CurrentEntity);
+        ReserveYamlId(uid);
+        SerializeEntityInternal(uid);
+        DebugTools.AssertNull(CurrentEntity);
+        if (_autoInclude.Count != 0)
+            ProcessAutoInclude();
+    }
+
+    /// <summary>
+    /// Serialize a set of entities. This does not automatically include children or parents, though depending on the
+    /// setting of <see cref="SerializationOptions.MissingEntityBehaviour"/> it may auto-include additional entities
+    /// aside from the one provided.
+    /// </summary>
+    public void SerializeEntities(HashSet<EntityUid> entities)
+    {
+        foreach (var uid in entities)
+        {
+            if (!IsSerializable(uid))
+                throw new Exception($"{EntMan.ToPrettyString(uid)} is not serializable");
+        }
+
+        ReserveYamlIds(entities);
+        SerializeEntitiesInternal(entities);
+    }
+
+    /// <summary>
+    /// Serializes an entity and all of its serializable children. Note that this will not automatically serialize the
+    /// entity's parents.
+    /// </summary>
+    public void SerializeEntityRecursive(EntityUid root)
+    {
+        if (!IsSerializable(root))
+            throw new Exception($"{EntMan.ToPrettyString(root)} is not serializable");
+
+        Truncate = _xformQuery.GetComponent(root).ParentUid;
+        Truncated.Add(Truncate);
+        InitializeTileMap(root);
+        HashSet<EntityUid> entities = new();
+        RecursivelyIncludeChildren(root, entities);
+        ReserveYamlIds(entities);
+        SerializeEntitiesInternal(entities);
+        Truncate = EntityUid.Invalid;
+    }
+
+    #endregion
+
+    /// <summary>
+    /// Initialize the <see cref="_tileMap"/> that is used to serialize grid chunks using
+    /// <see cref="MapChunkSerializer"/>. This initialization just involves checking to see if any of the entities being
+    /// serialized were previously deserialized. If they were, it will re-use the old tile map. This is not actually required,
+    /// and is just meant to prevent large map file diffs when the internal tile ids change. I.e., you can serialize entities
+    /// without initializing the tile map.
+    /// </summary>
+    private void InitializeTileMap(EntityUid root)
+    {
+        if (!FindSavedTileMap(root, out var savedMap))
+            return;
+
+        // Note: some old maps were saved with duplicate id strings.
+        // I.e, multiple integers that correspond to the same prototype id.
+        // Hence the TryAdd()
+        //
+        // Though now we also need to use TryAdd in case InitializeTileMap() is called multiple times.
+        // E.g., if different grids get added separately to a single save file, in which case the
+        // tile map may already be partially populated.
+        foreach (var (origId, prototypeId) in savedMap)
+        {
+            if (_tileDef.TryGetDefinition(prototypeId, out var definition))
+            {
+                _tileMap.TryAdd(definition.TileId, origId);
+                _yamlTileIds.Add(origId); // Make sure we record the IDs we're using so when we need to reserve new ones we can
+            }
+        }
+    }
+
+    private bool FindSavedTileMap(EntityUid root, [NotNullWhen(true)] out Dictionary<int, string>? map)
+    {
+        // Try and fetch the mapping directly
+        if (EntMan.TryGetComponent(root, out MapSaveTileMapComponent? comp))
+        {
+            map = comp.TileMap;
+            return true;
+        }
+
+        // iterate over all of its children and grab the first grid with a mapping
+        var xform = _xformQuery.GetComponent(root);
+        foreach (var child in xform._children)
+        {
+            if (!EntMan.TryGetComponent(child, out MapSaveTileMapComponent? cComp))
+                continue;
+            map = cComp.TileMap;
+            return true;
+        }
+
+        map = null;
+        return false;
+    }
+
+    #region AutoInclude
+
+    private void ProcessAutoInclude()
+    {
+        DebugTools.AssertEqual(_autoInclude.ToHashSet().Count, _autoInclude.Count);
+
+        var ents = new HashSet<EntityUid>();
+
+        switch (Options.MissingEntityBehaviour)
+        {
+            case MissingEntityBehaviour.PartialInclude:
+                // Include the entity and any of its direct parents
+                foreach (var uid in _autoInclude)
+                {
+                    RecursivelyIncludeParents(uid, ents);
+                }
+                break;
+            case MissingEntityBehaviour.IncludeNullspace:
+            case MissingEntityBehaviour.AutoInclude:
+                // Find the root transform of all the included entities
+                var roots = new HashSet<EntityUid>();
+                foreach (var uid in _autoInclude)
+                {
+                    GetRootNode(uid, roots);
+                }
+
+                // Recursively include all children of these root nodes.
+                foreach (var root in roots)
+                {
+                    RecursivelyIncludeChildren(root, ents);
+                }
+                break;
+
+            default:
+                throw new ArgumentOutOfRangeException();
+        }
+
+        _autoInclude.Clear();
+        SerializeEntitiesInternal(ents);
+    }
+
+    private void RecursivelyIncludeChildren(EntityUid uid, HashSet<EntityUid> ents)
+    {
+        if (!IsSerializable(uid))
+            return;
+
+        ents.Add(uid);
+        var xform = _xformQuery.GetComponent(uid);
+        foreach (var child in xform._children)
+        {
+            RecursivelyIncludeChildren(child, ents);
+        }
+    }
+
+    private void GetRootNode(EntityUid uid, HashSet<EntityUid> ents)
+    {
+        if (!IsSerializable(uid))
+            throw new NotSupportedException($"Attempted to auto-include an unserializable entity: {EntMan.ToPrettyString(uid)}");
+
+        var xform = _xformQuery.GetComponent(uid);
+        while (xform.ParentUid.IsValid() && xform.ParentUid != Truncate)
+        {
+            uid = xform.ParentUid;
+            xform = _xformQuery.GetComponent(uid);
+
+            if (!IsSerializable(uid))
+                throw new NotSupportedException($"Encountered an un-serializable parent entity: {EntMan.ToPrettyString(uid)}");
+        }
+
+        ents.Add(uid);
+    }
+
+    private void RecursivelyIncludeParents(EntityUid uid, HashSet<EntityUid> ents)
+    {
+        while (uid.IsValid() && uid != Truncate)
+        {
+            if (!ents.Add(uid))
+                break;
+
+            if (!IsSerializable(uid))
+                throw new NotSupportedException($"Encountered an un-serializable parent entity: {EntMan.ToPrettyString(uid)}");
+
+            uid = _xformQuery.GetComponent(uid).ParentUid;
+        }
+    }
+
+    #endregion
+
+    private void SerializeEntitiesInternal(HashSet<EntityUid> entities)
+    {
+        foreach (var uid in entities)
+        {
+            DebugTools.AssertNull(CurrentEntity);
+            SerializeEntityInternal(uid);
+        }
+
+        DebugTools.AssertNull(CurrentEntity);
+        if (_autoInclude.Count != 0)
+            ProcessAutoInclude();
+    }
+
+    /// <summary>
+    /// Serialize a single entity, and store the results in <see cref="EntityData"/>.
+    /// </summary>
+    private void SerializeEntityInternal(EntityUid uid)
+    {
+        var saveId = GetYamlUid(uid);
+        DebugTools.Assert(!EntityData.ContainsKey(saveId));
+
+        // It might be possible that something could cause an entity to be included twice.
+        // E.g., if someone serializes a grid w/o its map, and then tries to separately include the map and all its children.
+        // In that case, the grid would already have been serialized as a orphan.
+        // uhhh.... I guess its fine?
+        if (EntityData.ContainsKey(saveId))
+            return;
+
+        var meta = _metaQuery.GetComponent(uid);
+        var protoId = meta.EntityPrototype?.ID ?? string.Empty;
+
+        switch (meta.EntityLifeStage)
+        {
+            case <= EntityLifeStage.Initializing:
+                _log.Error($"Encountered an uninitialized entity: {EntMan.ToPrettyString(uid)}");
+                break;
+            case >= EntityLifeStage.Terminating:
+                _log.Error($"Encountered terminating or deleted entity: {EntMan.ToPrettyString(uid)}");
+                break;
+        }
+
+        CurrentEntityYamlUid = saveId;
+        CurrentEntity = (uid, meta);
+
+        Prototypes.GetOrNew(protoId).Add(saveId);
+        var xform = _xformQuery.GetComponent(uid);
+
+        if (_mapQuery.HasComp(uid))
+            Maps.Add(saveId);
+        else if (xform.ParentUid == EntityUid.Invalid)
+            Nullspace.Add(saveId);
+
+        if (_gridQuery.HasComp(uid))
+        {
+            // The current assumption is that grids cannot be in null-space, because the rest of the code
+            // (broadphase, etc) don't support grids without maps.
+            DebugTools.Assert(xform.ParentUid != EntityUid.Invalid || _mapQuery.HasComp(uid));
+            Grids.Add(saveId);
+        }
+
+        var entData = new MappingDataNode
+        {
+            {"uid", saveId.ToString(CultureInfo.InvariantCulture)}
+        };
+
+        EntityData[saveId] = (uid, entData);
+        var cache = GetProtoCache(meta.EntityPrototype);
+
+        // Store information about whether a given entity has been map-initialized.
+        // In principle, if a map has been map-initialized, then all entities on that map should also be map-initialized.
+        // But technically there is nothing that prevents someone from moving a post-init entity onto a pre-init map and vice-versa.
+        // Also, we need to record this information even if the map is not being serialized.
+        // In 99% of cases, this data is probably redundant and just bloats the file, but I can't think of a better way of handling it.
+        // At least it should only bloat post-init maps, which aren't really getting used so far.
+        if (meta.EntityLifeStage == EntityLifeStage.MapInitialized)
+        {
+            if (Options.ExpectPreInit)
+                _log.Error($"Expected all entities to be pre-mapinit, but encountered post-init entity: {EntMan.ToPrettyString(uid)}");
+            entData.Add("mapInit", "true");
+
+            // If an entity has been map-initialized, we assume it is un-paused.
+            // If it is paused, we have to specify it.
+            if (meta.EntityPaused)
+                entData.Add("paused", "true");
+        }
+        else
+        {
+            // If an entity has not yet been map-initialized, we assume it is paused.
+            // I don't know in what situations it wouldn't be, but might as well future proof this.
+            if (!meta.EntityPaused)
+                entData.Add("paused", "false");
+        }
+
+        var components = new SequenceDataNode();
+        if (xform.NoLocalRotation && xform.LocalRotation != 0)
+        {
+            _log.Error($"Encountered a no-rotation entity with non-zero local rotation: {EntMan.ToPrettyString(uid)}");
+            xform._localRotation = 0;
+        }
+
+        foreach (var component in EntMan.GetComponentsInternal(uid))
+        {
+            var compType = component.GetType();
+
+            var reg = _factory.GetRegistration(compType);
+            if (reg.Unsaved)
+                continue;
+
+            CurrentComponent = reg.Name;
+            MappingDataNode? compMapping;
+            MappingDataNode? protoMapping = null;
+            if (cache != null && cache.TryGetValue(reg.Name, out protoMapping))
+            {
+                // If this has a prototype, we need to use alwaysWrite: true.
+                // E.g., an anchored prototype might have anchored: true. If we we are saving an un-anchored
+                // instance of this entity, and if we have alwaysWrite: false, then compMapping would not include
+                // the anchored data-field (as false is the default for this bool data field), so the entity would
+                // implicitly be saved as anchored.
+                compMapping = _serialization.WriteValueAs<MappingDataNode>(compType, component, alwaysWrite: true, context: this);
+
+                // This will not recursively call Except() on the values of the mapping. It will only remove
+                // key-value pairs if both the keys and values are equal.
+                compMapping = compMapping.Except(protoMapping);
+                if(compMapping == null)
+                    continue;
+            }
+            else
+            {
+                compMapping = _serialization.WriteValueAs<MappingDataNode>(compType, component, alwaysWrite: false, context: this);
+            }
+
+            // Don't need to write it if nothing was written! Note that if this entity has no associated
+            // prototype, we ALWAYS want to write the component, because merely the fact that it exists is
+            // information that needs to be written.
+            if (compMapping.Children.Count != 0 || protoMapping == null)
+            {
+                compMapping.InsertAt(0, "type", new ValueDataNode(reg.Name));
+                components.Add(compMapping);
+            }
+        }
+
+        CurrentComponent = null;
+        if (components.Count != 0)
+            entData.Add("components", components);
+
+        // TODO ENTITY SERIALIZATION
+        // Consider adding a Action<EntityUid, MappingDataNode>? OnEntitySerialized
+        // I.e., allow content to modify the per-entity data? I don't know if that would actually be useful, as content
+        // could just as easily append a separate entity dictionary to the output that has the extra per-entity data they
+        // want to serialize.
+
+        if (meta.EntityPrototype == null)
+        {
+            CurrentEntityYamlUid = 0;
+            CurrentEntity = null;
+            return;
+        }
+
+        // an entity may have less components than the original prototype, so we need to check if any are missing.
+        SequenceDataNode? missingComponents = null;
+        foreach (var (name, comp) in meta.EntityPrototype.Components)
+        {
+            // try comp instead of has-comp as it checks whether the component is supposed to have been
+            // deleted.
+            if (EntMan.TryGetComponent(uid, comp.Component.GetType(), out _))
+                continue;
+
+            missingComponents ??= new();
+            missingComponents.Add(new ValueDataNode(name));
+        }
+
+        if (missingComponents != null)
+            entData.Add("missingComponents", missingComponents);
+
+        CurrentEntityYamlUid = 0;
+        CurrentEntity = null;
+    }
+
+    private Dictionary<string, MappingDataNode>? GetProtoCache(EntityPrototype? proto)
+    {
+        if (proto == null)
+            return null;
+
+        if (PrototypeCache.TryGetValue(proto.ID, out var cache))
+            return cache;
+
+        PrototypeCache[proto.ID] = cache = new(proto.Components.Count);
+        WritingReadingPrototypes = true;
+
+        foreach (var (compName, comp) in proto.Components)
+        {
+            CurrentComponent = compName;
+            cache.Add(compName, _serialization.WriteValueAs<MappingDataNode>(comp.Component.GetType(), comp.Component, alwaysWrite: true, context: this));
+        }
+
+        CurrentComponent = null;
+        WritingReadingPrototypes = false;
+        cache.TryAdd(_metaName, _emptyMetaNode);
+        cache.TryAdd(_xformName, _emptyXformNode);
+        return cache;
+    }
+
+    #region Write
+
+    public MappingDataNode Write()
+    {
+        DebugTools.AssertEqual(Maps.ToHashSet().Count, Maps.Count, "Duplicate maps?");
+        DebugTools.AssertEqual(Grids.ToHashSet().Count, Grids.Count, "Duplicate grids?");
+        DebugTools.AssertEqual(Orphans.ToHashSet().Count, Orphans.Count, "Duplicate orphans?");
+        DebugTools.AssertEqual(Nullspace.ToHashSet().Count, Nullspace.Count, "Duplicate nullspace?");
+
+        return new MappingDataNode
+        {
+            {"meta", WriteMetadata()},
+            {"maps", WriteIds(Maps)},
+            {"grids", WriteIds(Grids)},
+            {"orphans", WriteIds(Orphans)},
+            {"nullspace", WriteIds(Nullspace)},
+            {"tilemap", WriteTileMap()},
+            {"entities", WriteEntitySection()},
+        };
+    }
+
+    public MappingDataNode WriteMetadata()
+    {
+        return new MappingDataNode
+        {
+            {"format", MapFormatVersion.ToString(CultureInfo.InvariantCulture)},
+            {"category", GetCategory().ToString()},
+            {"engineVersion", _conf.GetCVar(CVars.BuildEngineVersion) },
+            {"forkId", _conf.GetCVar(CVars.BuildForkId)},
+            {"forkVersion", _conf.GetCVar(CVars.BuildVersion)},
+            {"time", DateTime.UtcNow.ToString(CultureInfo.InvariantCulture)},
+            {"entityCount", EntityData.Count.ToString(CultureInfo.InvariantCulture)}
+        };
+    }
+
+    public SequenceDataNode WriteIds(List<int> ids)
+    {
+        var result = new SequenceDataNode();
+        foreach (var id in ids)
+        {
+            result.Add(new ValueDataNode(id.ToString(CultureInfo.InvariantCulture)));
+        }
+        return result;
+    }
+
+    /// <summary>
+    /// Serialize the <see cref="_tileMap"/> to yaml. This data is required to deserialize any serialized grid chunks using <see cref="MapChunkSerializer"/>.
+    /// </summary>
+    public MappingDataNode WriteTileMap()
+    {
+        var map = new MappingDataNode();
+        foreach (var (tileId, yamlTileId) in _tileMap.OrderBy(x => x.Key))
+        {
+            // This can come up if tests try to serialize test maps with custom / placeholder tile ids without registering them with the tile def manager..
+            if (!_tileDef.TryGetDefinition(tileId, out var def))
+                throw new Exception($"Attempting to serialize a tile {tileId} with no valid tile definition.");
+
+            var yamlId = yamlTileId.ToString(CultureInfo.InvariantCulture);
+            map.Add(yamlId, def.ID);
+        }
+        return map;
+    }
+
+    public SequenceDataNode WriteEntitySection()
+    {
+        if (YamlIds.Count != YamlUidMap.Count || YamlIds.Count != EntityData.Count)
+        {
+            // Maybe someone reserved a yaml id with ReserveYamlId() or implicitly with GetId() without actually
+            // ever serializing the entity, This can lead to references to non-existent entities.
+            throw new Exception($"Entity count mismatch");
+        }
+
+        var prototypes = new SequenceDataNode();
+        var protos = Prototypes.Keys.ToList();
+        protos.Sort(StringComparer.InvariantCulture);
+
+        foreach (var protoId in protos)
+        {
+            var entities = new SequenceDataNode();
+            var node = new MappingDataNode
+            {
+                { "proto", protoId },
+                { "entities", entities},
+            };
+
+            prototypes.Add(node);
+
+            var saveIds = Prototypes[protoId];
+            saveIds.Sort();
+            foreach (var saveId in saveIds)
+            {
+                var entData = EntityData[saveId].Node;
+                entities.Add(entData);
+            }
+        }
+
+        return prototypes;
+    }
+
+    /// <summary>
+    /// Get the category that the serialized data belongs to. If one was specified in the
+    /// <see cref="SerializationOptions"/> it will use that after validating it, otherwise it will attempt to infer a
+    /// category.
+    /// </summary>
+    public FileCategory GetCategory()
+    {
+        switch (Options.Category)
+        {
+            case FileCategory.Save:
+                return FileCategory.Save;
+
+            case FileCategory.Map:
+                return Maps.Count == 1 ? FileCategory.Map : FileCategory.Unknown;
+
+            case FileCategory.Grid:
+                if (Maps.Count > 0 || Grids.Count != 1)
+                    return FileCategory.Unknown;
+                return FileCategory.Grid;
+
+            case FileCategory.Entity:
+                if (Maps.Count > 0 || Grids.Count > 0 || Orphans.Count != 1)
+                    return FileCategory.Unknown;
+                return FileCategory.Entity;
+
+            default:
+                if (Maps.Count == 1)
+                {
+                    // Contains a single map, and no orphaned entities that need reparenting.
+                    if (Orphans.Count == 0)
+                        return FileCategory.Map;
+                }
+                else if (Grids.Count == 1)
+                {
+                    // Contains a single orphaned grid.
+                    if (Orphans.Count == 1 && Grids[0] == Orphans[0])
+                        return FileCategory.Grid;
+                }
+                else if (Orphans.Count == 1)
+                {
+                    // A lone orphaned entity.
+                    return FileCategory.Entity;
+                }
+
+                return FileCategory.Unknown;
+        }
+    }
+
+    #endregion
+
+    #region YamlIds
+
+    /// <summary>
+    /// Get (or allocate) the integer id that will be used in the serialized file to refer to the given entity.
+    /// </summary>
+    public int GetYamlUid(EntityUid uid)
+    {
+        return !YamlUidMap.TryGetValue(uid, out var id) ? AllocateYamlUid(uid) : id;
+    }
+
+    private int AllocateYamlUid(EntityUid uid)
+    {
+        if (Truncated.Contains(uid))
+        {
+            _log.Error(
+                "Including a previously truncated entity within the serialization process? Something probably wrong");
+        }
+
+        DebugTools.Assert(!YamlUidMap.ContainsKey(uid));
+        while (!YamlIds.Add(_nextYamlUid))
+        {
+            _nextYamlUid++;
+        }
+
+        YamlUidMap.Add(uid, _nextYamlUid);
+        return _nextYamlUid++;
+    }
+
+    /// <summary>
+    /// Get (or allocate) the integer id that will be used in the serialized file to refer to the given grid tile id.
+    /// </summary>
+    public int GetYamlTileId(int tileId)
+    {
+        if (_tileMap.TryGetValue(tileId, out var yamlId))
+            return yamlId;
+
+        return AllocateYamlTileId(tileId);
+    }
+
+    private int AllocateYamlTileId(int tileId)
+    {
+        while (!_yamlTileIds.Add(_nextYamlTileId))
+        {
+            _nextYamlTileId++;
+        }
+
+        _tileMap[tileId] = _nextYamlTileId;
+        return _nextYamlTileId++;
+    }
+
+    /// <summary>
+    /// This method ensures that the given entities have a yaml ids assigned. If the entities have a
+    /// <see cref="YamlUidComponent"/>, they will attempt to use that id, which exists to prevent large map file diffs
+    /// due to changing yaml ids.
+    /// </summary>
+    public void ReserveYamlIds(HashSet<EntityUid> entities)
+    {
+        List<EntityUid> needIds = new();
+        foreach (var uid in entities)
+        {
+            if (YamlUidMap.ContainsKey(uid))
+                continue;
+
+            if (_yamlQuery.TryGetComponent(uid, out var comp) && comp.Uid > 0 && YamlIds.Add(comp.Uid))
+            {
+                if (Truncated.Contains(uid))
+                {
+                    _log.Error(
+                        "Including a previously truncated entity within the serialization process? Something probably wrong");
+                }
+
+                YamlUidMap.Add(uid, comp.Uid);
+            }
+            else
+            {
+                needIds.Add(uid);
+            }
+        }
+
+        foreach (var uid in needIds)
+        {
+            AllocateYamlUid(uid);
+        }
+    }
+
+    /// <summary>
+    /// This method ensures that the given entity has a yaml id assigned to it. If the entity has a
+    /// <see cref="YamlUidComponent"/>, it will attempt to use that id, which exists to prevent large map file diffs due
+    /// to changing yaml ids.
+    /// </summary>
+    public void ReserveYamlId(EntityUid uid)
+    {
+        if (YamlUidMap.ContainsKey(uid))
+            return;
+
+        if (_yamlQuery.TryGetComponent(uid, out var comp) && comp.Uid > 0 && YamlIds.Add(comp.Uid))
+        {
+            if (Truncated.Contains(uid))
+            {
+                _log.Error(
+                    "Including a previously truncated entity within the serialization process? Something probably wrong");
+            }
+
+            YamlUidMap.Add(uid, comp.Uid);
+        }
+        else
+            AllocateYamlUid(uid);
+    }
+
+    #endregion
+
+    #region ITypeSerializer
+
+    ValidationNode ITypeValidator<EntityUid, ValueDataNode>.Validate(
+        ISerializationManager serializationManager,
+        ValueDataNode node,
+        IDependencyCollection dependencies,
+        ISerializationContext? context)
+    {
+        if (node.Value == "invalid")
+            return new ValidatedValueNode(node);
+
+        if (!int.TryParse(node.Value, out _))
+            return new ErrorNode(node, "Invalid EntityUid");
+
+        return new ValidatedValueNode(node);
+    }
+
+    public DataNode Write(
+        ISerializationManager serializationManager,
+        EntityUid value,
+        IDependencyCollection dependencies,
+        bool alwaysWrite = false,
+        ISerializationContext? context = null)
+    {
+        if (YamlUidMap.TryGetValue(value, out var yamlId))
+            return new ValueDataNode(yamlId.ToString(CultureInfo.InvariantCulture));
+
+        if (CurrentComponent == _xformName)
+        {
+            if (value == EntityUid.Invalid)
+                return new ValueDataNode("invalid");
+
+            DebugTools.Assert(!Orphans.Contains(CurrentEntityYamlUid));
+            Orphans.Add(CurrentEntityYamlUid);
+
+            if (Options.ErrorOnOrphan && CurrentEntity != null && value != Truncate)
+                _log.Error($"Serializing entity {EntMan.ToPrettyString(CurrentEntity)} without including its parent {EntMan.ToPrettyString(value)}");
+
+            return new ValueDataNode("invalid");
+        }
+
+        if (value == EntityUid.Invalid)
+        {
+            if (Options.MissingEntityBehaviour != MissingEntityBehaviour.Ignore)
+                _log.Error($"Encountered an invalid entityUid reference.");
+
+            return new ValueDataNode("invalid");
+        }
+
+        if (value == Truncate)
+        {
+            _log.Error(
+                $"{EntMan.ToPrettyString(CurrentEntity)}:{CurrentComponent} is attempting to serialize references to a truncated entity {EntMan.ToPrettyString(Truncate)}.");
+        }
+
+        switch (Options.MissingEntityBehaviour)
+        {
+            case MissingEntityBehaviour.Error:
+                _log.Error(EntMan.Deleted(value)
+                    ? $"Encountered a reference to a deleted entity {value} while serializing {EntMan.ToPrettyString(CurrentEntity)}."
+                    : $"Encountered a reference to a missing entity: {value} while serializing {EntMan.ToPrettyString(CurrentEntity)}.");
+                return new ValueDataNode("invalid");
+            case MissingEntityBehaviour.Ignore:
+                return new ValueDataNode("invalid");
+            case MissingEntityBehaviour.IncludeNullspace:
+                if (!EntMan.TryGetComponent(value, out TransformComponent? xform)
+                    || xform.ParentUid != EntityUid.Invalid
+                    || _gridQuery.HasComp(value)
+                    || _mapQuery.HasComp(value))
+                {
+                    goto case MissingEntityBehaviour.Error;
+                }
+                goto case MissingEntityBehaviour.AutoInclude;
+            case MissingEntityBehaviour.PartialInclude:
+            case MissingEntityBehaviour.AutoInclude:
+                if (Options.LogAutoInclude is {} level)
+                    _log.Log(level, $"Auto-including entity {EntMan.ToPrettyString(value)} referenced by {EntMan.ToPrettyString(CurrentEntity)}");
+                _autoInclude.Add(value);
+                var id = GetYamlUid(value);
+                return new ValueDataNode(id.ToString(CultureInfo.InvariantCulture));
+            default:
+                throw new ArgumentOutOfRangeException();
+        }
+    }
+
+    EntityUid ITypeReader<EntityUid, ValueDataNode>.Read(
+        ISerializationManager serializationManager,
+        ValueDataNode node,
+        IDependencyCollection dependencies,
+        SerializationHookContext hookCtx,
+        ISerializationContext? context,
+        ISerializationManager.InstantiationDelegate<EntityUid>? _)
+    {
+        return node.Value == "invalid" ? EntityUid.Invalid : EntityUid.Parse(node.Value);
+    }
+
+    public ValidationNode Validate(
+        ISerializationManager serializationManager,
+        ValueDataNode node,
+        IDependencyCollection dependencies,
+        ISerializationContext? context = null)
+    {
+        if (node.Value == "invalid")
+            return new ValidatedValueNode(node);
+
+        if (!int.TryParse(node.Value, out _))
+            return new ErrorNode(node, "Invalid NetEntity");
+
+        return new ValidatedValueNode(node);
+    }
+
+    public NetEntity Read(
+        ISerializationManager serializationManager,
+        ValueDataNode node,
+        IDependencyCollection dependencies,
+        SerializationHookContext hookCtx,
+        ISerializationContext? context = null,
+        ISerializationManager.InstantiationDelegate<NetEntity>? instanceProvider = null)
+    {
+        return node.Value == "invalid" ? NetEntity.Invalid : NetEntity.Parse(node.Value);
+    }
+
+    public DataNode Write(
+        ISerializationManager serializationManager,
+        NetEntity value,
+        IDependencyCollection dependencies,
+        bool alwaysWrite = false,
+        ISerializationContext? context = null)
+    {
+        var uid = EntMan.GetEntity(value);
+        return serializationManager.WriteValue(uid, alwaysWrite, context);
+    }
+
+    #endregion
+}

Robust.Shared/EntitySerialization/LoadResult.cs

@@ -0,0 +1,71 @@
+using System;
+using System.Collections.Generic;
+using Robust.Shared.GameObjects;
+using Robust.Shared.Map.Components;
+
+namespace Robust.Shared.EntitySerialization;
+
+/// <summary>
+/// Class containing information about entities that were loaded from a yaml file.
+/// </summary>
+public sealed class LoadResult
+{
+    /// <summary>
+    /// The file format version.
+    /// </summary>
+    public int Version;
+
+    /// <summary>
+    /// The category of the file that was loaded in.
+    /// This might not match the actual final result. E.g., when loading in a grid file, a map may automatically gets
+    /// generated for it via <see cref="EntityDeserializer.AdoptGrids"/>.
+    /// </summary>
+    public FileCategory Category = FileCategory.Unknown;
+
+    /// <summary>
+    /// The engine version that was used to write the file. See <see cref="CVars.BuildEngineVersion"/>.
+    /// </summary>
+    public string? EngineVersion;
+
+    /// <summary>
+    /// The fork that was used to write the file. See <see cref="CVars.BuildForkId"/>.
+    /// </summary>
+    public string? ForkId;
+
+    /// <summary>
+    /// The fork version that was used to write the file. See <see cref="CVars.BuildVersion"/>.
+    /// </summary>
+    public string? ForkVersion;
+
+    /// <summary>
+    /// The <see cref="DateTime.UtcNow"/> when the file was created.
+    /// </summary>
+    public DateTime? Time;
+
+    /// <summary>
+    /// Set of all entities that were created while the file was being loaded.
+    /// </summary>
+    public readonly HashSet<EntityUid> Entities = new();
+
+    /// <summary>
+    /// Set of entities that are not parented to other entities. This will be a combination of <see cref="Maps"/>,
+    /// <see cref="Orphans"/>, and <see cref="NullspaceEntities"/>.
+    /// </summary>
+    public readonly HashSet<EntityUid> RootNodes = new();
+
+    public readonly HashSet<Entity<MapComponent>> Maps = new();
+
+    public readonly HashSet<Entity<MapGridComponent>> Grids = new();
+
+    /// <summary>
+    /// Deserialized entities that need to be assigned a new parent. These differ from "true" null-space entities.
+    /// E,g, saving a grid without saving the map would make the grid an "orphan".
+    /// </summary>
+    public readonly HashSet<EntityUid> Orphans = new();
+
+    /// <summary>
+    /// List of null-space entities. This contains all entities without a parent that don't have a
+    /// <see cref="MapComponent"/>, and were not listed as orphans
+    /// </summary>
+    public readonly HashSet<EntityUid> NullspaceEntities = new();
+}

Robust.Shared/EntitySerialization/MapChunkSerializer.cs

@@ -14,19 +14,25 @@ using Robust.Shared.Serialization.Markdown.Value;
 using Robust.Shared.Serialization.TypeSerializers.Interfaces;
 using Robust.Shared.Utility;
 
-namespace Robust.Server.Maps;
+namespace Robust.Shared.EntitySerialization;
 
 [TypeSerializer]
 internal sealed class MapChunkSerializer : ITypeSerializer<MapChunk, MappingDataNode>, ITypeCopyCreator<MapChunk>
 {
-    public ValidationNode Validate(ISerializationManager serializationManager, MappingDataNode node,
-        IDependencyCollection dependencies, ISerializationContext? context = null)
+    public ValidationNode Validate(
+        ISerializationManager serializationManager,
+        MappingDataNode node,
+        IDependencyCollection dependencies,
+        ISerializationContext? context = null)
     {
         throw new NotImplementedException();
     }
 
     public MapChunk Read(ISerializationManager serializationManager, MappingDataNode node,
-        IDependencyCollection dependencies, SerializationHookContext hookCtx, ISerializationContext? context = null, ISerializationManager.InstantiationDelegate<MapChunk>? instantiationDelegate = null)
+        IDependencyCollection dependencies,
+        SerializationHookContext hookCtx,
+        ISerializationContext? context = null,
+        ISerializationManager.InstantiationDelegate<MapChunk>? instantiationDelegate = null)
     {
         var ind = (Vector2i) serializationManager.Read(typeof(Vector2i), node["ind"], hookCtx, context)!;
         var tileNode = (ValueDataNode)node["tiles"];
@@ -50,10 +56,8 @@ internal sealed class MapChunkSerializer : ITypeSerializer<MapChunk, MappingData
 
         IReadOnlyDictionary<int, string>? tileMap = null;
 
-        if (context is MapSerializationContext serContext)
-        {
+        if (context is EntityDeserializer serContext)
             tileMap = serContext.TileMap;
-        }
 
         if (tileMap == null)
         {
@@ -104,16 +108,12 @@ internal sealed class MapChunkSerializer : ITypeSerializer<MapChunk, MappingData
 
         root.Add("version", new ValueDataNode("6"));
 
-        Dictionary<int, int>? tileWriteMap = null;
-        if (context is MapSerializationContext mapContext)
-            tileWriteMap = mapContext.TileWriteMap;
-
-        gridNode.Value = SerializeTiles(value, tileWriteMap);
+        gridNode.Value = SerializeTiles(value, context as EntitySerializer);
 
         return root;
     }
 
-    private static string SerializeTiles(MapChunk chunk, Dictionary<int, int>? tileWriteMap)
+    private static string SerializeTiles(MapChunk chunk, EntitySerializer? serializer)
     {
         // number of bytes written per tile, because sizeof(Tile) is useless.
         const int structSize = 6;
@@ -124,17 +124,34 @@ internal sealed class MapChunkSerializer : ITypeSerializer<MapChunk, MappingData
         using (var stream = new MemoryStream(barr))
         using (var writer = new BinaryWriter(stream))
         {
+            if (serializer == null)
+            {
+                for (ushort y = 0; y < chunk.ChunkSize; y++)
+                {
+                    for (ushort x = 0; x < chunk.ChunkSize; x++)
+                    {
+                        var tile = chunk.GetTile(x, y);
+                        writer.Write(tile.TypeId);
+                        writer.Write((byte) tile.Flags);
+                        writer.Write(tile.Variant);
+                    }
+                }
+                return Convert.ToBase64String(barr);
+            }
+
+            var lastTile = -1;
+            var yamlId = -1;
             for (ushort y = 0; y < chunk.ChunkSize; y++)
             {
                 for (ushort x = 0; x < chunk.ChunkSize; x++)
                 {
                     var tile = chunk.GetTile(x, y);
-                    var typeId = tile.TypeId;
-                    if (tileWriteMap != null)
-                        typeId = tileWriteMap[typeId];
+                    if (tile.TypeId != lastTile)
+                        yamlId = serializer.GetYamlTileId(tile.TypeId);
 
-                    writer.Write(typeId);
-                    writer.Write((byte)tile.Flags);
+                    lastTile = tile.TypeId;
+                    writer.Write(yamlId);
+                    writer.Write((byte) tile.Flags);
                     writer.Write(tile.Variant);
                 }
             }
@@ -143,8 +160,12 @@ internal sealed class MapChunkSerializer : ITypeSerializer<MapChunk, MappingData
         return Convert.ToBase64String(barr);
     }
 
-    public MapChunk CreateCopy(ISerializationManager serializationManager, MapChunk source,
-        IDependencyCollection dependencies, SerializationHookContext hookCtx, ISerializationContext? context = null)
+    public MapChunk CreateCopy(
+        ISerializationManager serializationManager,
+        MapChunk source,
+        IDependencyCollection dependencies,
+        SerializationHookContext hookCtx,
+        ISerializationContext? context = null)
     {
         var mapManager = dependencies.Resolve<IMapManager>();
         mapManager.SuppressOnTileChanged = true;

Robust.Shared/EntitySerialization/Options.cs

@@ -0,0 +1,139 @@
+using System.Numerics;
+using JetBrains.Annotations;
+using Robust.Shared.EntitySerialization.Components;
+using Robust.Shared.GameObjects;
+using Robust.Shared.Log;
+using Robust.Shared.Map;
+using Robust.Shared.Maths;
+
+namespace Robust.Shared.EntitySerialization;
+
+public record struct SerializationOptions
+{
+    public static readonly SerializationOptions Default = new();
+
+    /// <summary>
+    /// What to do when serializing the EntityUid of an entity that is not one of entities currently being serialized.
+    /// I.e., What should happen when serializing a map that has entities with components that store references to a
+    /// null-space entity? Note that this does not affect the treatment of <see cref="TransformComponent.ParentUid"/>,
+    /// which will never auto-include parents.
+    /// </summary>
+    public MissingEntityBehaviour MissingEntityBehaviour = MissingEntityBehaviour.IncludeNullspace;
+
+    /// <summary>
+    /// Whether or not to log an error when serializing an entity without its parent.
+    /// </summary>
+    public bool ErrorOnOrphan = true;
+
+    /// <summary>
+    /// Log level to use when auto-including entities while serializing. Null implies no logs.
+    /// See <see cref="MissingEntityBehaviour"/>.
+    /// </summary>
+    public LogLevel? LogAutoInclude = LogLevel.Info;
+
+    /// <summary>
+    /// If true, the serializer will log an error if it encounters a post map-init entity.
+    /// </summary>
+    public bool ExpectPreInit;
+
+    public FileCategory Category;
+
+    public SerializationOptions()
+    {
+    }
+}
+
+public record struct DeserializationOptions()
+{
+    public static readonly DeserializationOptions Default = new();
+
+    /// <summary>
+    /// If true, each loaded entity will get a <see cref="YamlUidComponent"/> that stores the uid that the entity
+    /// had in the yaml file. This is used to maintain consistent entity labelling on subsequent saves.
+    /// </summary>
+    public bool StoreYamlUids = false;
+
+    /// <summary>
+    /// If true, all maps that get created while loading this file will get map-initialized.
+    /// </summary>
+    public bool InitializeMaps = false;
+
+    /// <summary>
+    /// If true, all maps that get created while loading this file will get paused.
+    /// Note that the converse is not true, paused maps will not get un-paused if this is false.
+    /// Pre-mapinit maps are assumed to be paused.
+    /// </summary>
+    public bool PauseMaps = false;
+
+    /// <summary>
+    /// Whether or not to log an error when starting up a grid entity that has no map.
+    /// This usually indicates that someone is attempting to load an incorrect file type (e.g. loading a grid as a map).
+    /// </summary>
+    public bool LogOrphanedGrids = true;
+
+    /// <summary>
+    /// Whether or not to log an error when encountering an yaml entity id.
+    /// <see cref="TransformComponent.ParentUid"/> is exempt from this.
+    /// </summary>
+    public bool LogInvalidEntities = true;
+
+    /// <summary>
+    /// Whether or not to automatically assign map ids to any deserialized map entities.
+    /// If false, maps need to be manually given ids before entities are initialized.
+    /// </summary>
+    public bool AssignMapids = true;
+}
+
+/// <summary>
+/// Superset of <see cref="EntitySerialization.DeserializationOptions"/> that contain information relevant to loading
+/// maps & grids, potentially onto other existing maps.
+/// </summary>
+public struct MapLoadOptions()
+{
+    public static readonly MapLoadOptions Default = new();
+
+    /// <summary>
+    /// If specified, all orphaned entities and the children of all loaded maps will be re-parented onto this map.
+    /// I.e., this will merge map contents onto an existing map. This will also cause any maps that get loaded to
+    /// delete themselves after their children have been moved.
+    /// </summary>
+    /// <remarks>
+    /// Note that this option effectively causes <see cref="DeserializationOptions.InitializeMaps"/> and
+    /// <see cref="DeserializationOptions.PauseMaps"/> to have no effect, as the target map is not a map that was
+    /// created by the deserialization.
+    /// </remarks>
+    public MapId? MergeMap = null;
+
+    /// <summary>
+    /// Offset to apply to the position of any loaded entities that are directly parented to a map.
+    /// </summary>
+    public Vector2 Offset;
+
+    /// <summary>
+    /// Rotation to apply to the position & local rotation of any loaded entities that are directly parented to a map.
+    /// </summary>
+    public Angle Rotation;
+
+    /// <summary>
+    /// Options to use when deserializing entities.
+    /// </summary>
+    public DeserializationOptions DeserializationOptions = DeserializationOptions.Default;
+
+    /// <summary>
+    /// When loading a single map, this will attempt to force the map to use the given map id. Generally, it is better
+    /// to allow the map system to auto-allocate a map id, to avoid accidentally re-using an old id.
+    /// </summary>
+    public MapId? ForceMapId;
+
+    /// <summary>
+    /// The expected <see cref="LoadResult.Category"/> for the file currently being read in, at the end of the entity
+    /// creation step. Will log errors if the category doesn't match the expected one (e.g., trying to load a "map" from a file
+    /// that doesn't contain any map entities).
+    /// </summary>
+    /// <remarks>
+    /// Note that the effective final category may change by the time the file has fully loaded. E.g., when loading a
+    /// file containing an orphaned grid, a map may be automatically created for the grid, but the category will still
+    /// be <see cref="FileCategory.Grid"/>
+    /// </remarks>
+    public FileCategory? ExpectedCategory;
+}

Robust.Shared/EntitySerialization/SerializationEnums.cs

@@ -0,0 +1,88 @@
+using Robust.Shared.GameObjects;
+using Robust.Shared.Upload;
+
+namespace Robust.Shared.EntitySerialization;
+
+/// <summary>
+/// This enum is used to indicate the type of entity data that was written to a file. The actual format of the file does
+/// not change, but it helps avoid mistakes like accidentally using a map file when trying to load a single grid.
+/// </summary>
+public enum FileCategory : byte
+{
+    Unknown,
+
+    /// <summary>
+    /// File should contain a single orphaned entity, its children, and maybe some null-space entities.
+    /// </summary>
+    Entity,
+
+    /// <summary>
+    /// File should contain a single grid, its children, and maybe some null-space entities.
+    /// </summary>
+    Grid,
+
+    /// <summary>
+    /// File should contain a single map, its children, and maybe some null-space entities.
+    /// </summary>
+    Map,
+
+    /// <summary>
+    /// File is a full game save, and will likely contain at least one map and a few null-space entities.
+    /// </summary>
+    /// <remarks>
+    /// The file might also contain additional yaml entries for things like prototypes uploaded via
+    /// <see cref="IGamePrototypeLoadManager"/>, and might contain references to additional resources that need to be
+    /// loaded (e.g., files uploaded using <see cref="SharedNetworkResourceManager"/>).
+    /// </remarks>
+    Save,
+}
+
+public enum MissingEntityBehaviour
+{
+    /// <summary>
+    /// Log an error and replace the reference with <see cref="EntityUid.Invalid"/>
+    /// </summary>
+    Error,
+
+    /// <summary>
+    /// Ignore the reference, replace it with <see cref="EntityUid.Invalid"/>
+    /// </summary>
+    Ignore,
+
+    /// <summary>
+    /// Automatically include & serialize any referenced null-space entities and their children.
+    /// I.e., entities that are not attached to any parent and are not maps. Any non-nullspace entities will result in
+    /// an error.
+    /// </summary>
+    /// <remarks>
+    /// This is primarily intended to make it easy to auto-include information carrying null-space entities. E.g., the
+    /// "minds" of players, or entities that represent power or gas networks on a grid. Note that a full game save
+    /// should still try to explicitly include all relevant entities, as this could still easily fail to auto-include
+    /// relevant entities if they are not explicitly referenced in a data-field by some other entity.
+    /// </remarks>
+    IncludeNullspace,
+
+    /// <summary>
+    /// Automatically include & serialize any referenced entity. Note that this means that the missing entity's
+    /// parents will (generally) also be included, however this will not include other children. E.g., if serializing a
+    /// grid that references an entity on the map, this will also cause the map to get serialized, but will not necessarily
+    /// serialize everything on the map.
+    /// </summary>
+    /// <remarks>
+    /// If trying to serialize an entity without its parent (i.e., its parent is truncated via
+    /// <see cref="EntitySerializer.Truncate"/>), this will try to respect that. E.g., if a referenced entity is on the
+    /// same map as a grid that is getting serialized, it should include the entity without including the map.
+    /// </remarks>
+    /// <remarks>
+    /// Note that this might unexpectedly change the <see cref="FileCategory"/>. I.e., trying to serialize a grid might
+    /// accidentally lead to serializing a (partial?) map file.
+    /// </remarks>
+    PartialInclude,
+
+    /// <summary>
+    /// Variant of <see cref="PartialInclude"/> that will also automatically include the children of any entities that
+    /// that are automatically included. Note that because auto-inclusion generally needs to include an entity's
+    /// parents, this will include more than just the missing entity's direct children.
+    /// </summary>
+    AutoInclude,
+}

Robust.Shared/EntitySerialization/Systems/MapLoaderSystem.Load.cs

@@ -0,0 +1,240 @@
+using System;
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+using System.IO;
+using System.Linq;
+using System.Numerics;
+using Robust.Shared.ContentPack;
+using Robust.Shared.GameObjects;
+using Robust.Shared.Map;
+using Robust.Shared.Map.Components;
+using Robust.Shared.Map.Events;
+using Robust.Shared.Maths;
+using Robust.Shared.Serialization.Markdown;
+using Robust.Shared.Serialization.Markdown.Mapping;
+using Robust.Shared.Utility;
+using Vector2 = System.Numerics.Vector2;
+
+namespace Robust.Shared.EntitySerialization.Systems;
+
+// This partial class file contains methods for loading generic entities and grids. Map specific methods are in another
+// file
+public sealed partial class MapLoaderSystem
+{
+    /// <summary>
+    /// Tries to load entities from a yaml file. Whenever possible, you should try to use <see cref="TryLoadMap"/>,
+    /// <see cref="TryLoadGrid"/>, or <see cref="TryLoadEntity"/> instead.
+    /// </summary>
+    public bool TryLoadGeneric(
+        ResPath file,
+        [NotNullWhen(true)] out HashSet<Entity<MapComponent>>? maps,
+        [NotNullWhen(true)] out HashSet<Entity<MapGridComponent>>? grids,
+        MapLoadOptions? options = null)
+    {
+        grids = null;
+        maps = null;
+        if (!TryLoadGeneric(file, out var data, options))
+            return false;
+
+        maps = data.Maps;
+        grids = data.Grids;
+        return true;
+    }
+
+    /// <summary>
+    /// Tries to load entities from a yaml file. Whenever possible, you should try to use <see cref="TryLoadMap"/>,
+    /// <see cref="TryLoadGrid"/>, or <see cref="TryLoadEntity"/> instead.
+    /// </summary>
+    /// <param name="file">The file to load.</param>
+    /// <param name="result">Data class containing information about the loaded entities</param>
+    /// <param name="options">Optional Options for configuring loading behaviour.</param>
+    public bool TryLoadGeneric(ResPath file, [NotNullWhen(true)] out LoadResult? result, MapLoadOptions? options = null)
+    {
+        result = null;
+
+        if (!TryReadFile(file, out var data))
+            return false;
+
+        _stopwatch.Restart();
+        var ev = new BeforeEntityReadEvent();
+        RaiseLocalEvent(ev);
+
+        var opts = options ?? MapLoadOptions.Default;
+
+        // If we are forcing a map id, we cannot auto-assign ids.
+        opts.DeserializationOptions.AssignMapids = opts.ForceMapId == null;
+
+        if (opts.MergeMap is { } targetId && !_mapSystem.MapExists(targetId))
+            throw new Exception($"Target map {targetId} does not exist");
+
+        if (opts.MergeMap != null && opts.ForceMapId != null)
+            throw new Exception($"Invalid combination of MapLoadOptions");
+
+        if (_mapSystem.MapExists(opts.ForceMapId))
+            throw new Exception($"Target map already exists");
+
+        // Using a local deserializer instead of a cached value, both to ensure that we don't accidentally carry over
+        // data from a previous serializations, and because some entities cause other maps/grids to be loaded during
+        // during mapinit.
+        var deserializer = new EntityDeserializer(
+            _dependency,
+            data,
+            opts.DeserializationOptions,
+            ev.RenamedPrototypes,
+            ev.DeletedPrototypes);
+
+        if (!deserializer.TryProcessData())
+        {
+            Log.Debug($"Failed to process entity data in {file}");
+            return false;
+        }
+
+        try
+        {
+            deserializer.CreateEntities();
+        }
+        catch (Exception e)
+        {
+            Log.Error($"Caught exception while creating entities: {e}");
+            Delete(deserializer.Result);
+            throw;
+        }
+
+        if (opts.ExpectedCategory is { } exp && exp != deserializer.Result.Category)
+        {
+            // Did someone try to load a map file as a grid or vice versa?
+            Log.Error($"File does not contain the expected data. Expected {exp} but got {deserializer.Result.Category}");
+            Delete(deserializer.Result);
+            return false;
+        }
+
+        // Reparent entities if loading entities onto an existing map.
+        var merged = new HashSet<EntityUid>();
+        MergeMaps(deserializer, opts, merged);
+
+        if (!SetMapId(deserializer, opts))
+            return false;
+
+        // Apply any offsets & rotations specified by the load options
+        ApplyTransform(deserializer, opts);
+
+        try
+        {
+            deserializer.StartEntities();
+        }
+        catch (Exception e)
+        {
+            Log.Error($"Caught exception while starting entities: {e}");
+            Delete(deserializer.Result);
+            throw;
+        }
+
+        if (opts.MergeMap is {} map)
+            MapInitalizeMerged(merged, map);
+
+        result = deserializer.Result;
+        Log.Debug($"Loaded map in {_stopwatch.Elapsed}");
+        return true;
+    }
+
+    /// <summary>
+    /// Tries to load a regular (non-map, non-grid) entity from a file.
+    /// The loaded entity will initially be in null-space.
+    /// If the file does not contain exactly one orphaned entity, this will return false and delete loaded entities.
+    /// </summary>
+    public bool TryLoadEntity(
+        ResPath path,
+        [NotNullWhen(true)] out Entity<TransformComponent>? entity,
+        DeserializationOptions? options = null)
+    {
+        var opts = new MapLoadOptions
+        {
+            DeserializationOptions = options ?? DeserializationOptions.Default,
+            ExpectedCategory = FileCategory.Entity
+        };
+
+        entity = null;
+        if (!TryLoadGeneric(path, out var result, opts))
+            return false;
+
+        if (result.Orphans.Count == 1)
+        {
+            var uid = result.Orphans.Single();
+            entity = (uid, Transform(uid));
+            return true;
+        }
+
+        Delete(result);
+        return false;
+    }
+
+    /// <summary>
+    /// Tries to load a grid entity from a file and parent it to the given map.
+    /// If the file does not contain exactly one grid, this will return false and delete loaded entities.
+    /// </summary>
+    public bool TryLoadGrid(
+        MapId map,
+        ResPath path,
+        [NotNullWhen(true)] out Entity<MapGridComponent>? grid,
+        DeserializationOptions? options = null,
+        Vector2 offset = default,
+        Angle rot = default)
+    {
+        var opts = new MapLoadOptions
+        {
+            MergeMap = map,
+            Offset = offset,
+            Rotation = rot,
+            DeserializationOptions = options ?? DeserializationOptions.Default,
+            ExpectedCategory = FileCategory.Grid
+        };
+
+        grid = null;
+        if (!TryLoadGeneric(path, out var result, opts))
+            return false;
+
+        if (result.Grids.Count == 1)
+        {
+            grid = result.Grids.Single();
+            return true;
+        }
+
+        Delete(result);
+        return false;
+    }
+
+    private void ApplyTransform(EntityDeserializer deserializer, MapLoadOptions opts)
+    {
+        if (opts.Rotation == Angle.Zero && opts.Offset == Vector2.Zero)
+            return;
+
+        // If merging onto a single map, the transformation was already applied by SwapRootNode()
+        if (opts.MergeMap != null)
+            return;
+
+        var matrix = Matrix3Helpers.CreateTransform(opts.Offset, opts.Rotation);
+
+        // We want to apply the transforms to all children of any loaded maps. However, we can't just iterate over the
+        // children of loaded maps, as transform component has not yet been initialized. I.e. xform.Children is empty.
+        // Hence we iterate over all entities and check which ones are attached to maps.
+        foreach (var uid in deserializer.Result.Entities)
+        {
+            var xform = Transform(uid);
+
+            if (!_mapQuery.HasComp(xform.ParentUid))
+                continue;
+
+            // The original comment around this bit of logic was just:
+            // > Smelly
+            // I don't know what sloth meant by that, but I guess applying transforms to grid-maps is a no-no?
+            // Or more generally, loading a mapgrid onto another (potentially non-mapgrid) map is just generally kind of weird.
+            if (_gridQuery.HasComponent(xform.ParentUid))
+                continue;
+
+            var rot = xform.LocalRotation + opts.Rotation;
+            var pos = Vector2.Transform(xform.LocalPosition, matrix);
+            _xform.SetLocalPositionRotation(uid, pos, rot, xform);
+            DebugTools.Assert(!xform.NoLocalRotation || xform.LocalRotation == 0);
+        }
+    }
+}

Robust.Shared/EntitySerialization/Systems/MapLoaderSystem.LoadMap.cs

@@ -0,0 +1,239 @@
+using System;
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+using System.Linq;
+using System.Numerics;
+using Robust.Shared.GameObjects;
+using Robust.Shared.Map;
+using Robust.Shared.Map.Components;
+using Robust.Shared.Maths;
+using Robust.Shared.Utility;
+using Vector2 = System.Numerics.Vector2;
+
+namespace Robust.Shared.EntitySerialization.Systems;
+
+// This partial class file contains methods specific to loading maps
+public sealed partial class MapLoaderSystem
+{
+    /// <summary>
+    /// Attempts to load a file containing a single map.
+    /// If the file does not contain exactly one map, this will return false and delete all loaded entities.
+    /// </summary>
+    /// <remarks>
+    /// Note that this will not automatically initialize the map, unless specified via the <see cref="DeserializationOptions"/>.
+    /// </remarks>
+    public bool TryLoadMap(
+        ResPath path,
+        [NotNullWhen(true)] out Entity<MapComponent>? map,
+        [NotNullWhen(true)] out HashSet<Entity<MapGridComponent>>? grids,
+        DeserializationOptions? options = null,
+        Vector2 offset = default,
+        Angle rot = default)
+    {
+        var opts = new MapLoadOptions
+        {
+            Offset = offset,
+            Rotation = rot,
+            DeserializationOptions = options ?? DeserializationOptions.Default,
+            ExpectedCategory = FileCategory.Map
+        };
+
+        map = null;
+        grids = null;
+        if (!TryLoadGeneric(path, out var result, opts))
+            return false;
+
+        if (result.Maps.Count == 1)
+        {
+            map = result.Maps.First();
+            grids = result.Grids;
+            return true;
+        }
+
+        Delete(result);
+        return false;
+    }
+
+    /// <summary>
+    /// Attempts to load a file containing a single map, assign it the given map id.
+    /// </summary>
+    /// <remarks>
+    /// If possible, it is better to use <see cref="TryLoadMap"/> which automatically assigns a <see cref="MapId"/>.
+    /// </remarks>
+    /// <remarks>
+    /// Note that this will not automatically initialize the map, unless specified via the <see cref="DeserializationOptions"/>.
+    /// </remarks>
+    public bool TryLoadMapWithId(
+        MapId mapId,
+        ResPath path,
+        [NotNullWhen(true)] out Entity<MapComponent>? map,
+        [NotNullWhen(true)] out HashSet<Entity<MapGridComponent>>? grids,
+        DeserializationOptions? options = null,
+        Vector2 offset = default,
+        Angle rot = default)
+    {
+        map = null;
+        grids = null;
+
+        var opts = new MapLoadOptions
+        {
+            Offset = offset,
+            Rotation = rot,
+            DeserializationOptions = options ?? DeserializationOptions.Default,
+            ExpectedCategory = FileCategory.Map
+        };
+
+        if (_mapSystem.MapExists(mapId))
+            throw new Exception($"Target map already exists");
+
+        opts.ForceMapId = mapId;
+        if (!TryLoadGeneric(path, out var result, opts))
+            return false;
+
+        if (!_mapSystem.TryGetMap(mapId, out var uid) || !TryComp(uid, out MapComponent? comp))
+            return false;
+
+        map = new(uid.Value, comp);
+        grids = result.Grids;
+        return true;
+    }
+
+    /// <summary>
+    /// Attempts to load a file containing a single map, and merge its children onto another map. After which the
+    /// loaded map gets deleted.
+    /// </summary>
+    public bool TryMergeMap(
+        MapId mapId,
+        ResPath path,
+        [NotNullWhen(true)] out HashSet<Entity<MapGridComponent>>? grids,
+        DeserializationOptions? options = null,
+        Vector2 offset = default,
+        Angle rot = default)
+    {
+        grids = null;
+
+        var opts = new MapLoadOptions
+        {
+            Offset = offset,
+            Rotation = rot,
+            DeserializationOptions = options ?? DeserializationOptions.Default,
+            ExpectedCategory = FileCategory.Map
+        };
+
+        if (!_mapSystem.MapExists(mapId))
+            throw new Exception($"Target map {mapId} does not exist");
+
+        opts.MergeMap = mapId;
+        if (!TryLoadGeneric(path, out var result, opts))
+            return false;
+
+        if (!_mapSystem.TryGetMap(mapId, out var uid) || !TryComp(uid, out MapComponent? comp))
+            return false;
+
+        grids = result.Grids;
+        return true;
+    }
+
+    private void MergeMaps(EntityDeserializer deserializer, MapLoadOptions opts, HashSet<EntityUid> merged)
+    {
+        if (opts.MergeMap is not {} targetId)
+            return;
+
+        if (!_mapSystem.TryGetMap(targetId, out var targetUid))
+            throw new Exception($"Target map {targetId} does not exist");
+
+        deserializer.Result.Category = FileCategory.Unknown;
+        var rotation = opts.Rotation;
+        var matrix = Matrix3Helpers.CreateTransform(opts.Offset, rotation);
+        var target = new Entity<TransformComponent>(targetUid.Value, Transform(targetUid.Value));
+
+        // We want to apply the transforms to all children of any loaded maps. However, we can't just iterate over the
+        // children of loaded maps, as transform component has not yet been initialized. I.e. xform.Children is empty.
+        // Hence we iterate over all entities and check which ones are attached to maps.
+        HashSet<EntityUid> maps = new();
+        HashSet<EntityUid> logged = new();
+        foreach (var uid in deserializer.Result.Entities)
+        {
+            var xform = Transform(uid);
+            if (!_mapQuery.HasComp(xform.ParentUid))
+                continue;
+
+            if (_gridQuery.HasComponent(xform.ParentUid) && logged.Add(xform.ParentUid))
+            {
+                Log.Error($"Merging a grid-map onto another map is not supported.");
+                continue;
+            }
+
+            maps.Add(xform.ParentUid);
+            Merge(merged, uid, target, matrix, rotation);
+        }
+
+        deserializer.ToDelete.UnionWith(maps);
+        deserializer.Result.Maps.RemoveWhere(x => maps.Contains(x.Owner));
+
+        foreach (var uid in deserializer.Result.Orphans)
+        {
+            Merge(merged, uid, target, matrix, rotation);
+        }
+
+        deserializer.Result.Orphans.Clear();
+    }
+
+    private void Merge(
+        HashSet<EntityUid> merged,
+        EntityUid uid,
+        Entity<TransformComponent> target,
+        in Matrix3x2 matrix,
+        Angle rotation)
+    {
+        merged.Add(uid);
+        var xform = Transform(uid);
+        var angle = xform.LocalRotation + rotation;
+        var pos = Vector2.Transform(xform.LocalPosition, matrix);
+        var coords = new EntityCoordinates(target.Owner, pos);
+        _xform.SetCoordinates((uid, xform, MetaData(uid)), coords, rotation: angle, newParent: target.Comp);
+    }
+
+    private void MapInitalizeMerged(HashSet<EntityUid> merged, MapId targetId)
+    {
+        // fuck me I hate this map merging bullshit.
+        // loading a map "onto" another map shouldn't need to be supported by the generic map loading methods.
+        // If something needs to do that, it should implement it itself.
+        // AFAIK this only exists for the loadgamemap command?
+
+        if (!_mapSystem.TryGetMap(targetId, out var targetUid))
+            throw new Exception($"Target map {targetId} does not exist");
+
+        if (_mapSystem.IsInitialized(targetUid.Value))
+        {
+            foreach (var uid in merged)
+            {
+                _mapSystem.RecursiveMapInit(uid);
+            }
+        }
+
+        var paused = _mapSystem.IsPaused(targetUid.Value);
+        foreach (var uid in merged)
+        {
+            _mapSystem.RecursiveSetPaused(uid, paused);
+        }
+    }
+
+    private bool SetMapId(EntityDeserializer deserializer, MapLoadOptions opts)
+    {
+        if (opts.ForceMapId is not { } id)
+            return true;
+
+        if (deserializer.Result.Maps.Count != 1)
+        {
+            Log.Error(
+                $"The {nameof(MapLoadOptions.ForceMapId)} option is only supported when loading a file containing a single map.");
+            Delete(deserializer.Result);
+            return false;
+        }
+
+        var map = deserializer.Result.Maps.Single();
+        _mapSystem.AssignMapId(map, id);
+        return true;
+    }
+}

Robust.Shared/EntitySerialization/Systems/MapLoaderSystem.Save.cs

@@ -0,0 +1,238 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using Robust.Shared.GameObjects;
+using Robust.Shared.Map;
+using Robust.Shared.Map.Events;
+using Robust.Shared.Serialization.Markdown.Mapping;
+using Robust.Shared.Utility;
+
+namespace Robust.Shared.EntitySerialization.Systems;
+
+// This partial class file contains methods for serializing and saving entities, grids, and maps.
+public sealed partial class MapLoaderSystem
+{
+    /// <inheritdoc cref="EntitySerializer.OnIsSerializeable"/>
+    public event EntitySerializer.IsSerializableDelegate? OnIsSerializable;
+
+    /// <summary>
+    /// Recursively serialize the given entity and its children.
+    /// </summary>
+    public (MappingDataNode Node, FileCategory Category) SerializeEntitiesRecursive(
+        HashSet<EntityUid> entities,
+        SerializationOptions? options = null)
+    {
+        _stopwatch.Restart();
+        if (!entities.All(Exists))
+            throw new Exception($"Cannot serialize deleted entities");
+
+        Log.Info($"Serializing entities: {string.Join(", ", entities.Select(x => ToPrettyString(x).ToString()))}");
+
+        var maps = entities.Select(x => Transform(x).MapID).ToHashSet();
+        var ev = new BeforeSerializationEvent(entities, maps);
+        RaiseLocalEvent(ev);
+
+        // In case no options were provided, we assume that if all of the starting entities are pre-init, we should
+        // expect that **all** entities that get serialized should be pre-init.
+        var opts = options ?? SerializationOptions.Default with
+        {
+            ExpectPreInit = (entities.All(x => LifeStage(x) < EntityLifeStage.MapInitialized))
+        };
+
+        var serializer = new EntitySerializer(_dependency, opts);
+        serializer.OnIsSerializeable += OnIsSerializable;
+
+        foreach (var ent in entities)
+        {
+            serializer.SerializeEntityRecursive(ent);
+        }
+
+        var data = serializer.Write();
+        var cat = serializer.GetCategory();
+
+        var ev2 = new AfterSerializationEvent(entities, data, cat);
+        RaiseLocalEvent(ev2);
+
+        Log.Debug($"Serialized {serializer.EntityData.Count} entities in {_stopwatch.Elapsed}");
+        return (data, cat);
+    }
+
+    /// <summary>
+    /// Serialize a standard (non-grid, non-map) entity and all of its children and write the result to a
+    /// yaml file.
+    /// </summary>
+    public bool TrySaveEntity(EntityUid entity, ResPath path, SerializationOptions? options = null)
+    {
+        if (_mapQuery.HasComp(entity))
+        {
+            Log.Error($"{ToPrettyString(entity)} is a map. Use {nameof(TrySaveMap)}.");
+            return false;
+        }
+
+        if (_gridQuery.HasComp(entity))
+        {
+            Log.Error($"{ToPrettyString(entity)} is a grid. Use {nameof(TrySaveGrid)}.");
+            return false;
+        }
+
+        var opts = options ?? SerializationOptions.Default;
+        opts.Category = FileCategory.Entity;
+
+        MappingDataNode data;
+        FileCategory cat;
+        try
+        {
+            (data, cat) = SerializeEntitiesRecursive([entity], opts);
+        }
+        catch (Exception e)
+        {
+            Log.Error($"Caught exception while trying to serialize entity {ToPrettyString(entity)}:\n{e}");
+            return false;
+        }
+
+        if (cat != FileCategory.Entity)
+        {
+            Log.Error($"Failed to save {ToPrettyString(entity)} as a singular entity. Output: {cat}");
+            return false;
+        }
+
+        Write(path, data);
+        return true;
+    }
+
+    /// <summary>
+    /// Serialize a map and all of its children and write the result to a yaml file.
+    /// </summary>
+    public bool TrySaveMap(MapId mapId, ResPath path, SerializationOptions? options = null)
+    {
+        if (_mapSystem.TryGetMap(mapId, out var mapUid))
+            return TrySaveMap(mapUid.Value, path, options);
+
+        Log.Error($"Unable to find map {mapId}");
+        return false;
+    }
+
+    /// <summary>
+    /// Serialize a map and all of its children and write the result to a yaml file.
+    /// </summary>
+    public bool TrySaveMap(EntityUid map, ResPath path, SerializationOptions? options = null)
+    {
+        if (!_mapQuery.HasComp(map))
+        {
+            Log.Error($"{ToPrettyString(map)} is not a map.");
+            return false;
+        }
+
+        var opts = options ?? SerializationOptions.Default;
+        opts.Category = FileCategory.Map;
+
+        MappingDataNode data;
+        FileCategory cat;
+        try
+        {
+            (data, cat) = SerializeEntitiesRecursive([map], opts);
+        }
+        catch (Exception e)
+        {
+            Log.Error($"Caught exception while trying to serialize map {ToPrettyString(map)}:\n{e}");
+            return false;
+        }
+
+        if (cat != FileCategory.Map)
+        {
+            Log.Error($"Failed to save {ToPrettyString(map)} as a map. Output: {cat}");
+            return false;
+        }
+
+        Write(path, data);
+        return true;
+    }
+
+    /// <summary>
+    /// Serialize a grid and all of its children and write the result to a yaml file.
+    /// </summary>
+    public bool TrySaveGrid(EntityUid grid, ResPath path, SerializationOptions? options = null)
+    {
+        if (!_gridQuery.HasComp(grid))
+        {
+            Log.Error($"{ToPrettyString(grid)} is not a grid.");
+            return false;
+        }
+
+        if (_mapQuery.HasComp(grid))
+        {
+            Log.Error($"{ToPrettyString(grid)} is a map, not (just) a grid. Use {nameof(TrySaveMap)}");
+            return false;
+        }
+
+        var opts = options ?? SerializationOptions.Default;
+        opts.Category = FileCategory.Grid;
+
+        MappingDataNode data;
+        FileCategory cat;
+        try
+        {
+            (data, cat) = SerializeEntitiesRecursive([grid], opts);
+        }
+        catch (Exception e)
+        {
+            Log.Error($"Caught exception while trying to serialize grid {ToPrettyString(grid)}:\n{e}");
+            return false;
+        }
+
+        if (cat != FileCategory.Grid)
+        {
+            Log.Error($"Failed to save {ToPrettyString(grid)} as a grid. Output: {cat}");
+            return false;
+        }
+
+        Write(path, data);
+        return true;
+    }
+
+    /// <summary>
+    /// Serialize an entities and all of their children to a yaml file.
+    /// This makes no assumptions about the expected entity or resulting file category.
+    /// If possible, use the map/grid specific variants instead.
+    /// </summary>
+    public bool TrySaveGeneric(
+        EntityUid uid,
+        ResPath path,
+        out FileCategory category,
+        SerializationOptions? options = null)
+    {
+        return TrySaveGeneric([uid], path, out category, options);
+    }
+
+    /// <summary>
+    /// Serialize one or more entities and all of their children to a yaml file.
+    /// This makes no assumptions about the expected entity or resulting file category.
+    /// If possible, use the map/grid specific variants instead.
+    /// </summary>
+    public bool TrySaveGeneric(
+        HashSet<EntityUid> entities,
+        ResPath path,
+        out FileCategory category,
+        SerializationOptions? options = null)
+    {
+        category = FileCategory.Unknown;
+        if (entities.Count == 0)
+            return false;
+
+        var opts = options ?? SerializationOptions.Default;
+
+        MappingDataNode data;
+        try
+        {
+            (data, category) = SerializeEntitiesRecursive(entities, opts);
+        }
+        catch (Exception e)
+        {
+            Log.Error($"Caught exception while trying to serialize entities:\n{e}");
+            return false;
+        }
+
+        Write(path, data);
+        return true;
+    }
+}

Robust.Shared/EntitySerialization/Systems/MapLoaderSystem.cs

@@ -0,0 +1,132 @@
+using System.Diagnostics.CodeAnalysis;
+using System.IO;
+using System.Linq;
+using Robust.Shared.ContentPack;
+using Robust.Shared.GameObjects;
+using Robust.Shared.IoC;
+using Robust.Shared.Map.Components;
+using Robust.Shared.Serialization;
+using Robust.Shared.Serialization.Markdown;
+using Robust.Shared.Serialization.Markdown.Mapping;
+using Robust.Shared.Timing;
+using Robust.Shared.Utility;
+using YamlDotNet.Core;
+using YamlDotNet.RepresentationModel;
+
+namespace Robust.Shared.EntitySerialization.Systems;
+
+/// <summary>
+/// This class provides methods for saving and loading maps and grids.
+/// </summary>
+/// <remarks>
+/// The save & load methods are basically wrappers around <see cref="EntitySerializer"/> and
+/// <see cref="EntityDeserializer"/>, which can be used for more control over serialization.
+/// </remarks>
+public sealed partial class MapLoaderSystem : EntitySystem
+{
+    [Dependency] private readonly IResourceManager _resourceManager = default!;
+    [Dependency] private readonly SharedMapSystem _mapSystem = default!;
+    [Dependency] private readonly SharedTransformSystem _xform = default!;
+    [Dependency] private readonly IDependencyCollection _dependency = default!;
+
+    private Stopwatch _stopwatch = new();
+
+    private EntityQuery<MapComponent> _mapQuery;
+    private EntityQuery<MapGridComponent> _gridQuery;
+
+    public override void Initialize()
+    {
+        base.Initialize();
+        _gridQuery = GetEntityQuery<MapGridComponent>();
+        _mapQuery = GetEntityQuery<MapComponent>();
+        _gridQuery = GetEntityQuery<MapGridComponent>();
+    }
+
+    private void Write(ResPath path, MappingDataNode data)
+    {
+        Log.Info($"Saving serialized results to {path}");
+        path = path.ToRootedPath();
+        var document = new YamlDocument(data.ToYaml());
+        using var writer = _resourceManager.UserData.OpenWriteText(path);
+        {
+            var stream = new YamlStream {document};
+            stream.Save(new YamlMappingFix(new Emitter(writer)), false);
+        }
+    }
+
+    public bool TryReadFile(ResPath file, [NotNullWhen(true)] out MappingDataNode? data)
+    {
+        var resPath = file.ToRootedPath();
+        data = null;
+
+        if (!TryGetReader(resPath, out var reader))
+            return false;
+
+        Log.Info($"Loading file: {resPath}");
+        _stopwatch.Restart();
+
+        using var textReader = reader;
+        var documents = DataNodeParser.ParseYamlStream(reader).ToArray();
+        Log.Debug($"Loaded yml stream in {_stopwatch.Elapsed}");
+
+        // Yes, logging errors in a "try" method is kinda shit, but it was throwing exceptions when I found it and it does
+        // make sense to at least provide some kind of feedback for why it failed.
+        switch (documents.Length)
+        {
+            case < 1:
+                Log.Error("Stream has no YAML documents.");
+                return false;
+            case > 1:
+                Log.Error("Stream too many YAML documents. Map files store exactly one.");
+                return false;
+            default:
+                data = (MappingDataNode) documents[0].Root;
+                return true;
+        }
+    }
+
+    private bool TryGetReader(ResPath resPath, [NotNullWhen(true)] out TextReader? reader)
+    {
+        if (_resourceManager.UserData.Exists(resPath))
+        {
+            // Log warning if file exists in both user and content data.
+            if (_resourceManager.ContentFileExists(resPath))
+                Log.Warning("Reading map user data instead of content");
+
+            reader = _resourceManager.UserData.OpenText(resPath);
+            return true;
+        }
+
+        if (_resourceManager.TryContentFileRead(resPath, out var contentReader))
+        {
+            reader = new StreamReader(contentReader);
+            return true;
+        }
+
+        Log.Error($"File not found: {resPath}");
+        reader = null;
+        return false;
+    }
+
+    /// <summary>
+    /// Helper method for deleting all loaded entities.
+    /// </summary>
+    public void Delete(LoadResult result)
+    {
+        foreach (var uid in result.Maps)
+        {
+            Del(uid);
+        }
+
+        foreach (var uid in result.Orphans)
+        {
+            Del(uid);
+        }
+
+        foreach (var uid in result.Entities)
+        {
+            Del(uid);
+        }
+    }
+
+}

Robust.Shared/GameObjects/Components/Eye/VisibilityComponent.cs

@@ -1,14 +1,13 @@
-using Robust.Shared.GameObjects;
 using Robust.Shared.Serialization.Manager.Attributes;
 
-namespace Robust.Server.GameObjects
+namespace Robust.Shared.GameObjects
 {
     /// <summary>
     /// Controls PVS visibility of entities. THIS COMPONENT CONTROLS WHETHER ENTITIES ARE NETWORKED TO PLAYERS
     /// AND SHOULD NOT BE USED AS THE SOLE WAY TO HIDE AN ENTITY FROM A PLAYER.
     /// </summary>
     [RegisterComponent]
-    [Access(typeof(VisibilitySystem))]
+    [Access(typeof(SharedVisibilitySystem))]
     public sealed partial class VisibilityComponent : Component
     {
         /// <summary>

Robust.Shared/GameObjects/EntityManager.Components.cs

@@ -1075,6 +1075,97 @@ namespace Robust.Shared.GameObjects
             return TryGetComponent(uid.Value, netId, out component, meta);
         }
 
+        /// <inheritdoc/>
+        public bool TryCopyComponent<T>(EntityUid source, EntityUid target, ref T? sourceComponent, [NotNullWhen(true)] out T? targetComp, MetaDataComponent? meta = null) where T : IComponent
+        {
+            if (!MetaQuery.Resolve(target, ref meta))
+            {
+                targetComp = default;
+                return false;
+            }
+
+            if (sourceComponent == null && !TryGetComponent(source, out sourceComponent))
+            {
+                targetComp = default;
+                return false;
+            }
+
+            targetComp = CopyComponentInternal(source, target, sourceComponent, meta);
+            return true;
+        }
+
+        /// <inheritdoc/>
+        public bool TryCopyComponents(
+            EntityUid source,
+            EntityUid target,
+            MetaDataComponent? meta = null,
+            params Type[] sourceComponents)
+        {
+            if (!MetaQuery.TryGetComponent(source, out meta))
+                return false;
+
+            var allCopied = true;
+
+            foreach (var type in sourceComponents)
+            {
+                if (!TryGetComponent(source, type, out var srcComp))
+                {
+                    allCopied = false;
+                    continue;
+                }
+
+                CopyComponent(source, target, srcComp, meta: meta);
+            }
+
+            return allCopied;
+        }
+
+        /// <inheritdoc/>
+        public IComponent CopyComponent(EntityUid source, EntityUid target, IComponent sourceComponent, MetaDataComponent? meta = null)
+        {
+            if (!MetaQuery.Resolve(target, ref meta))
+            {
+                throw new InvalidOperationException();
+            }
+
+            return CopyComponentInternal(source, target, sourceComponent, meta);
+        }
+
+        /// <inheritdoc/>
+        public T CopyComponent<T>(EntityUid source, EntityUid target, T sourceComponent,MetaDataComponent? meta = null) where T : IComponent
+        {
+            if (!MetaQuery.Resolve(target, ref meta))
+            {
+                throw new InvalidOperationException();
+            }
+
+            return CopyComponentInternal(source, target, sourceComponent, meta);
+        }
+
+        /// <inheritdoc/>
+        public void CopyComponents(EntityUid source, EntityUid target, MetaDataComponent? meta = null, params IComponent[] sourceComponents)
+        {
+            if (!MetaQuery.Resolve(target, ref meta))
+                return;
+
+            foreach (var comp in sourceComponents)
+            {
+                CopyComponentInternal(source, target, comp, meta);
+            }
+        }
+
+        private T CopyComponentInternal<T>(EntityUid source, EntityUid target, T sourceComponent, MetaDataComponent meta) where T : IComponent
+        {
+            var compReg = ComponentFactory.GetRegistration(sourceComponent.GetType());
+            var component = (T)ComponentFactory.GetComponent(compReg);
+
+            _serManager.CopyTo(sourceComponent, ref component, notNullableOverride: true);
+            component.Owner = target;
+
+            AddComponentInternal(target, component, compReg, true, false, meta);
+            return component;
+        }
+
         public EntityQuery<TComp1> GetEntityQuery<TComp1>() where TComp1 : IComponent
         {
             var comps = _entTraitArray[CompIdx.ArrayIndex<TComp1>()];

Robust.Shared/GameObjects/EntityManager.cs

@@ -311,11 +311,16 @@ namespace Robust.Shared.GameObjects
         }
 
         /// <inheritdoc />
-        public virtual EntityUid CreateEntityUninitialized(string? prototypeName, ComponentRegistry? overrides = null)
+        public EntityUid CreateEntityUninitialized(string? prototypeName, ComponentRegistry? overrides = null)
         {
             return CreateEntity(prototypeName, out _, overrides);
         }
 
+        public EntityUid CreateEntityUninitialized(string? prototypeName, out MetaDataComponent meta, ComponentRegistry? overrides = null)
+        {
+            return CreateEntity(prototypeName, out meta, overrides);
+        }
+
         /// <inheritdoc />
         public virtual EntityUid CreateEntityUninitialized(string? prototypeName, EntityCoordinates coordinates, ComponentRegistry? overrides = null, Angle rotation = default)
         {
@@ -548,7 +553,28 @@ namespace Robust.Shared.GameObjects
 
             TransformComponent? parentXform = null;
             if (xform.ParentUid.IsValid())
-                TransformQuery.Resolve(xform.ParentUid, ref parentXform);
+            {
+                if (xform.LifeStage < ComponentLifeStage.Initialized)
+                {
+                    // Entity is being deleted before initialization ever finished.
+                    // The entity will not yet have been added to the parent's transform component.
+                    // This is seemingly pretty error prone ATM, and I'm not even sure if it should be supported?
+
+                    // Just in case it HAS somehow been added, make sure we remove it.
+                    if (TransformQuery.TryComp(xform.ParentUid, out parentXform) && parentXform._children.Remove(e))
+                        DebugTools.Assert($"Child entity {ToPrettyString(e)} was added to the parent's child set prior to being initialized?");
+
+                    parentXform = null;
+                    xform._parent = EntityUid.Invalid;
+                    xform._anchored = false;
+                }
+                else
+                {
+                    // Use resolve for automatic error logging.
+                    // ReSharper disable once ReturnValueOfPureMethodIsNotUsed
+                    TransformQuery.Resolve(xform.ParentUid, ref parentXform);
+                }
+            }
 
             // Then actually delete them
             RecursiveDeleteEntity(e, meta, xform, parentXform);
@@ -783,7 +809,7 @@ namespace Robust.Shared.GameObjects
         /// <summary>
         ///     Allocates an entity and stores it but does not load components or do initialization.
         /// </summary>
-        private protected EntityUid AllocEntity(
+        protected internal EntityUid AllocEntity(
             EntityPrototype? prototype,
             out MetaDataComponent metadata)
         {
@@ -793,6 +819,9 @@ namespace Robust.Shared.GameObjects
             return entity;
         }
 
+        /// <inheritdoc cref="AllocEntity(Robust.Shared.Prototypes.EntityPrototype?,out Robust.Shared.GameObjects.MetaDataComponent)"/>
+        internal EntityUid AllocEntity(EntityPrototype? prototype) => AllocEntity(prototype, out _);
+
         /// <summary>
         ///     Allocates an entity and stores it but does not load components or do initialization.
         /// </summary>
@@ -872,21 +901,9 @@ namespace Robust.Shared.GameObjects
             }
         }
 
-        private protected void LoadEntity(EntityUid entity, IEntityLoadContext? context)
-        {
-            EntityPrototype.LoadEntity((entity, MetaQuery.GetComponent(entity)), ComponentFactory, this, _serManager, context);
-        }
-
-        private protected void LoadEntity(EntityUid entity, IEntityLoadContext? context, EntityPrototype? prototype)
-        {
-            var meta = MetaQuery.GetComponent(entity);
-            DebugTools.Assert(meta.EntityPrototype == prototype);
-            EntityPrototype.LoadEntity((entity, meta), ComponentFactory, this, _serManager, context);
-        }
-
         public void InitializeAndStartEntity(EntityUid entity, MapId? mapId = null)
         {
-            var doMapInit = _mapManager.IsMapInitialized(mapId ?? TransformQuery.GetComponent(entity).MapID);
+            var doMapInit = _mapSystem.IsInitialized(mapId ?? TransformQuery.GetComponent(entity).MapID);
             InitializeAndStartEntity(entity, doMapInit);
         }
 

Robust.Shared/GameObjects/EntitySystem.Proxy.cs

@@ -175,7 +175,7 @@ public partial class EntitySystem
     protected void DirtyFields<T>(EntityUid uid, T comp, MetaDataComponent? meta, params ReadOnlySpan<string> fields)
         where T : IComponentDelta
     {
-        EntityManager.DirtyFields(uid, comp, meta);
+        EntityManager.DirtyFields(uid, comp, meta, fields);
     }
 
     /// <summary>
@@ -565,6 +565,52 @@ public partial class EntitySystem
 
     #endregion
 
+    #region Component Copy
+
+    /// <inheritdoc cref="IEntityManager.TryCopyComponent"/>
+    protected bool TryCopyComponent<T>(
+        EntityUid source,
+        EntityUid target,
+        ref T? sourceComponent,
+        [NotNullWhen(true)] out T? targetComp,
+        MetaDataComponent? meta = null) where T : IComponent
+    {
+        return EntityManager.TryCopyComponent(source, target, ref sourceComponent, out targetComp, meta);
+    }
+
+    /// <inheritdoc cref="IEntityManager.TryCopyComponents"/>
+    protected bool TryCopyComponents(
+        EntityUid source,
+        EntityUid target,
+        MetaDataComponent? meta = null,
+        params Type[] sourceComponents)
+    {
+        return EntityManager.TryCopyComponents(source, target, meta, sourceComponents);
+    }
+
+    /// <inheritdoc cref="IEntityManager.CopyComponent"/>
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    protected IComponent CopyComp(EntityUid source, EntityUid target, IComponent sourceComponent, MetaDataComponent? meta = null)
+    {
+        return EntityManager.CopyComponent(source, target, sourceComponent, meta);
+    }
+
+    /// <inheritdoc cref="IEntityManager.CopyComponent{T}"/>
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    protected T CopyComp<T>(EntityUid source, EntityUid target, T sourceComponent, MetaDataComponent? meta = null) where T : IComponent
+    {
+        return EntityManager.CopyComponent(source, target, sourceComponent, meta);
+    }
+
+    /// <inheritdoc cref="IEntityManager.CopyComponents"/>
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    protected void CopyComps(EntityUid source, EntityUid target, MetaDataComponent? meta = null, params IComponent[] sourceComponents)
+    {
+        EntityManager.CopyComponents(source, target, meta, sourceComponents);
+    }
+
+    #endregion
+
     #region Component Has
 
     /// <summary>

Robust.Shared/GameObjects/EntitySystemManager.cs

@@ -22,11 +22,11 @@ namespace Robust.Shared.GameObjects
 {
     public sealed class EntitySystemManager : IEntitySystemManager, IPostInjectInit
     {
-        [IoC.Dependency] private readonly IReflectionManager _reflectionManager = default!;
-        [IoC.Dependency] private readonly IEntityManager _entityManager = default!;
-        [IoC.Dependency] private readonly ProfManager _profManager = default!;
-        [IoC.Dependency] private readonly IDependencyCollection _dependencyCollection = default!;
-        [IoC.Dependency] private readonly ILogManager _logManager = default!;
+        [Dependency] private readonly IReflectionManager _reflectionManager = default!;
+        [Dependency] private readonly IEntityManager _entityManager = default!;
+        [Dependency] private readonly ProfManager _profManager = default!;
+        [Dependency] private readonly IDependencyCollection _dependencyCollection = default!;
+        [Dependency] private readonly ILogManager _logManager = default!;
 
 #if EXCEPTION_TOLERANCE
         [Dependency] private readonly IRuntimeLog _runtimeLog = default!;
@@ -35,6 +35,18 @@ namespace Robust.Shared.GameObjects
         private ISawmill _sawmill = default!;
 
         internal DependencyCollection SystemDependencyCollection = default!;
+
+        public IDependencyCollection DependencyCollection
+        {
+            get
+            {
+                if (_initialized)
+                    return SystemDependencyCollection;
+
+                throw new InvalidOperationException($"{nameof(EntitySystemManager)} has not been initialized.");
+            }
+        }
+
         private readonly List<Type> _systemTypes = new();
 
         private static readonly Histogram _tickUsageHistogram = Metrics.CreateHistogram("robust_entity_systems_update_usage",

Robust.Shared/GameObjects/IEntityManager.Components.cs

@@ -358,6 +358,51 @@ namespace Robust.Shared.GameObjects
         /// <returns>If the component existed in the entity.</returns>
         bool TryGetComponent([NotNullWhen(true)] EntityUid? uid, ushort netId, [NotNullWhen(true)] out IComponent? component, MetaDataComponent? meta = null);
 
+        /// <summary>
+        /// Tries to run <see cref="CopyComponents"/> without throwing if the component doesn't exist.
+        /// </summary>
+        bool TryCopyComponent<T>(
+            EntityUid source,
+            EntityUid target,
+            ref T? sourceComponent,
+            [NotNullWhen(true)] out T? targetComp,
+            MetaDataComponent? meta = null) where T : IComponent;
+
+        /// <summary>
+        /// Tries to run <see cref="CopyComponents"/> without throwing if the components don't exist.
+        /// </summary>
+        bool TryCopyComponents(EntityUid source, EntityUid target, MetaDataComponent? meta = null, params Type[] sourceComponents);
+
+        /// <summary>
+        ///     Copy a single component from source to target entity.
+        /// </summary>
+        /// <param name="source">The source entity to copy from.</param>
+        /// <param name="target">The target entity to copy to.</param>
+        /// <param name="sourceComponent">The source component instance to copy.</param>
+        /// <param name="component">The copied component if successful.</param>
+        /// <param name="meta">Optional metadata of the target entity.</param>
+        IComponent CopyComponent(EntityUid source, EntityUid target, IComponent sourceComponent, MetaDataComponent? meta = null);
+
+        /// <summary>
+        ///     Copy a single component from source to target entity.
+        /// </summary>
+        /// <typeparam name="T">The type of component to copy.</typeparam>
+        /// <param name="source">The source entity to copy from.</param>
+        /// <param name="target">The target entity to copy to.</param>
+        /// <param name="sourceComponent">The source component instance to copy.</param>
+        /// <param name="component">The copied component if successful.</param>
+        /// <param name="meta">Optional metadata of the target entity.</param>
+        T CopyComponent<T>(EntityUid source, EntityUid target, T sourceComponent, MetaDataComponent? meta = null) where T : IComponent;
+
+        /// <summary>
+        /// Copy multiple components from source to target entity using existing component instances.
+        /// </summary>
+        /// <param name="source">The source entity to copy from.</param>
+        /// <param name="target">The target entity to copy to.</param>
+        /// <param name="meta">Optional metadata of the target entity.</param>
+        /// <param name="sourceComponents">Array of component instances to copy.</param>
+        void CopyComponents(EntityUid source, EntityUid target, MetaDataComponent? meta = null, params IComponent[] sourceComponents);
+
         /// <summary>
         /// Returns a cached struct enumerator with the specified component.
         /// </summary>

Robust.Shared/GameObjects/IEntitySystemManager.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
+using Robust.Shared.IoC;
 using Robust.Shared.IoC.Exceptions;
 
 namespace Robust.Shared.GameObjects
@@ -131,5 +132,10 @@ namespace Robust.Shared.GameObjects
         IEnumerable<Type> GetEntitySystemTypes();
         bool TryGetEntitySystem(Type sysType, [NotNullWhen(true)] out object? system);
         object GetEntitySystem(Type sysType);
+
+        /// <summary>
+        /// Dependency collection that contains all the loaded systems.
+        /// </summary>
+        public IDependencyCollection DependencyCollection { get; }
     }
 }

Robust.Shared/GameObjects/NetEntity.cs

@@ -3,6 +3,7 @@ using JetBrains.Annotations;
 using Robust.Shared.IoC;
 using Robust.Shared.Prototypes;
 using Robust.Shared.Serialization;
+using Robust.Shared.Serialization.Manager.Attributes;
 using Robust.Shared.Timing;
 using Robust.Shared.Utility;
 using Robust.Shared.ViewVariables;
@@ -12,7 +13,7 @@ namespace Robust.Shared.GameObjects;
 /// <summary>
 /// Network identifier for entities; used by client and server to refer to the same entity where their local <see cref="EntityUid"/> may differ.
 /// </summary>
-[Serializable, NetSerializable]
+[Serializable, NetSerializable, CopyByRef]
 public readonly struct NetEntity : IEquatable<NetEntity>, IComparable<NetEntity>, ISpanFormattable
 {
     public readonly int Id;

Robust.Shared/GameObjects/Systems/EntityLookupSystem.cs

@@ -125,7 +125,7 @@ public sealed partial class EntityLookupSystem : EntitySystem
         SubscribeLocalEvent<BroadphaseComponent, ComponentAdd>(OnBroadphaseAdd);
         SubscribeLocalEvent<BroadphaseComponent, ComponentInit>(OnBroadphaseInit);
         SubscribeLocalEvent<GridAddEvent>(OnGridAdd);
-        SubscribeLocalEvent<MapChangedEvent>(OnMapChange);
+        SubscribeLocalEvent<MapCreatedEvent>(OnMapChange);
 
         _transform.OnBeforeMoveEvent += OnMove;
         EntityManager.EntityInitialized += OnEntityInit;
@@ -194,9 +194,9 @@ public sealed partial class EntityLookupSystem : EntitySystem
         }
     }
 
-    private void OnMapChange(MapChangedEvent ev)
+    private void OnMapChange(MapCreatedEvent ev)
     {
-        if (ev.Created && ev.Map != MapId.Nullspace)
+        if (ev.MapId != MapId.Nullspace)
         {
             EnsureComp<BroadphaseComponent>(ev.Uid);
         }

Robust.Shared/GameObjects/Systems/PrototypeReloadSystem.cs

@@ -72,7 +72,9 @@ internal sealed class PrototypeReloadSystem : EntitySystem
         {
             var data = newPrototype.Components[name];
             var component = _componentFactory.GetComponent(name);
-            EntityManager.AddComponent(entity, component);
+
+            if (!EntityManager.HasComponent(entity, component.GetType()))
+                EntityManager.AddComponent(entity, component);
         }
 
         // Update entity metadata

Robust.Shared/GameObjects/Systems/SharedMapSystem.Map.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
 using Robust.Shared.GameStates;
 using Robust.Shared.Map;
@@ -70,14 +71,15 @@ public abstract partial class SharedMapSystem
         if (component.MapId == MapId.Nullspace)
         {
             if (state.MapId == MapId.Nullspace)
-                throw new Exception($"Received invalid map state? {ToPrettyString(uid)}");
+                throw new Exception($"Received invalid map state for {ToPrettyString(uid)}");
 
-            component.MapId = state.MapId;
-            Maps.Add(component.MapId, uid);
+            AssignMapId((uid, component), state.MapId);
             RecursiveMapIdUpdate(uid, uid, component.MapId);
         }
 
-        DebugTools.AssertEqual(component.MapId, state.MapId);
+        if (component.MapId != state.MapId)
+            throw new Exception($"Received invalid map state for {ToPrettyString(uid)}");
+
         component.LightingEnabled = state.LightingEnabled;
         component.MapInitialized = state.Initialized;
 
@@ -119,26 +121,75 @@ public abstract partial class SharedMapSystem
         EnsureComp<MovedGridsComponent>(uid);
     }
 
-    private void OnCompInit(EntityUid uid, MapComponent component, ComponentInit args)
+    internal void AssignMapId(Entity<MapComponent> map, MapId? id = null)
     {
-        if (component.MapId == MapId.Nullspace)
-            component.MapId = GetNextMapId();
-
-        DebugTools.AssertEqual(component.MapId.IsClientSide, IsClientSide(uid));
-        if (!Maps.TryAdd(component.MapId, uid))
+        if (map.Comp.MapId != MapId.Nullspace)
         {
-            if (Maps[component.MapId] != uid)
-                throw new Exception($"Attempted to initialize a map {ToPrettyString(uid)} with a duplicate map id {component.MapId}");
+            if (id != null && map.Comp.MapId != id)
+            {
+                QueueDel(map.Owner);
+                throw new Exception($"Map entity {ToPrettyString(map.Owner)} has already been assigned an id");
+            }
+
+            if (!Maps.TryGetValue(map.Comp.MapId, out var existing) || existing != map.Owner)
+            {
+                QueueDel(map.Owner);
+                throw new Exception($"Map entity {ToPrettyString(map.Owner)} was improperly assigned a map id?");
+            }
+
+            DebugTools.Assert(UsedIds.Contains(map.Comp.MapId));
+            return;
         }
 
-        var msg = new MapChangedEvent(uid, component.MapId, true);
-        RaiseLocalEvent(uid, msg, true);
+        map.Comp.MapId = id ?? GetNextMapId();
+
+        if (IsClientSide(map) != map.Comp.MapId.IsClientSide)
+            throw new Exception($"Attempting to assign a client-side map id to a networked entity or vice-versa");
+
+        if (!UsedIds.Add(map.Comp.MapId))
+            Log.Warning($"Re-using a previously used map id ({map.Comp.MapId}) for map entity {ToPrettyString(map)}");
+
+        if (Maps.TryAdd(map.Comp.MapId, map.Owner))
+            return;
+
+        if (Maps[map.Comp.MapId] == map.Owner)
+            return;
+
+        QueueDel(map);
+        throw new Exception(
+            $"Attempted to assign an existing mapId {map.Comp} to a map entity {ToPrettyString(map.Owner)}");
+    }
+
+    private void OnCompInit(Entity<MapComponent> map, ref ComponentInit args)
+    {
+        AssignMapId(map);
+
+#pragma warning disable CS0618 // Type or member is obsolete
+        var msg = new MapChangedEvent(map, map.Comp.MapId, true);
+#pragma warning restore CS0618 // Type or member is obsolete
+        RaiseLocalEvent(map, msg, true);
+        var ev = new MapCreatedEvent(map, map.Comp.MapId);
+        RaiseLocalEvent(map, ev, true);
+    }
+
+    private void OnMapInit(EntityUid uid, MapComponent component, MapInitEvent args)
+    {
+        DebugTools.Assert(!component.MapInitialized);
+        component.MapInitialized = true;
+        Dirty(uid, component);
     }
 
     private void OnCompStartup(EntityUid uid, MapComponent component, ComponentStartup args)
     {
-        if (component.MapPaused)
-            RecursiveSetPaused(uid, true);
+        // un-initialized maps are always paused.
+        component.MapPaused |= !component.MapInitialized;
+
+        if (!component.MapPaused)
+            return;
+
+        // Recursively pause all entities on the map
+        component.MapPaused = false;
+        SetPaused(uid, true);
     }
 
     private void OnMapRemoved(EntityUid uid, MapComponent component, ComponentShutdown args)
@@ -146,8 +197,13 @@ public abstract partial class SharedMapSystem
         DebugTools.Assert(component.MapId != MapId.Nullspace);
         Maps.Remove(component.MapId);
 
+#pragma warning disable CS0618 // Type or member is obsolete
         var msg = new MapChangedEvent(uid, component.MapId, false);
+#pragma warning restore CS0618 // Type or member is obsolete
         RaiseLocalEvent(uid, msg, true);
+
+        var ev = new MapRemovedEvent(uid, component.MapId);
+        RaiseLocalEvent(uid, ev, true);
     }
 
     /// <summary>
@@ -181,19 +237,17 @@ public abstract partial class SharedMapSystem
         if (_netManager.IsClient && _netManager.IsConnected && !mapId.IsClientSide)
             throw new ArgumentException($"Attempted to create a client-side map entity with a non client-side map ID?");
 
-        var uid = EntityManager.CreateEntityUninitialized(null);
-        var map = _factory.GetComponent<MapComponent>();
-        map.MapId = mapId;
-        AddComp(uid, map);
+        if (UsedIds.Contains(mapId))
+            Log.Warning($"Re-using MapId: {mapId}");
 
-        // Give the entity a name, mainly for debugging. Content can always override this with a localized name.
-        var meta = MetaData(uid);
-        _meta.SetEntityName(uid, $"Map Entity", meta);
+        var (uid, map, meta) = CreateUninitializedMap();
+        DebugTools.AssertEqual(map.MapId, MapId.Nullspace);
+        AssignMapId((uid, map), mapId);
 
         // Initialize components. this should add the map id to the collections.
-        EntityManager.InitializeComponents(uid, meta);
-        EntityManager.StartComponents(uid);
-        DebugTools.Assert(Maps[mapId] == uid);
+        EntityManager.InitializeEntity(uid, meta);
+        EntityManager.StartEntity(uid);
+        DebugTools.AssertEqual(Maps[mapId], uid);
 
         if (runMapInit)
             InitializeMap((uid, map));
@@ -202,4 +256,22 @@ public abstract partial class SharedMapSystem
 
         return uid;
     }
+
+    public Entity<MapComponent, MetaDataComponent> CreateUninitializedMap()
+    {
+        var uid = EntityManager.CreateEntityUninitialized(null, out var meta);
+        _meta.SetEntityName(uid, $"Map Entity", meta);
+        return (uid, AddComp<MapComponent>(uid), meta);
+    }
+
+    public void DeleteMap(MapId mapId)
+    {
+        if (TryGetMap(mapId, out var uid))
+            Del(uid);
+    }
+
+    public IEnumerable<MapId> GetAllMapIds()
+    {
+        return Maps.Keys;
+    }
 }

Robust.Shared/GameObjects/Systems/SharedMapSystem.MapInit.cs

@@ -34,13 +34,6 @@ public abstract partial class SharedMapSystem
         return map.Comp.MapInitialized;
     }
 
-    private void OnMapInit(EntityUid uid, MapComponent component, MapInitEvent args)
-    {
-        DebugTools.Assert(!component.MapInitialized);
-        component.MapInitialized = true;
-        EntityManager.Dirty(uid, component);
-    }
-
     public void InitializeMap(MapId mapId, bool unpause = true)
     {
         if(!Maps.TryGetValue(mapId, out var uid))
@@ -63,7 +56,7 @@ public abstract partial class SharedMapSystem
             SetPaused(map, false);
     }
 
-    private void RecursiveMapInit(EntityUid entity)
+    internal void RecursiveMapInit(EntityUid entity)
     {
         var toInitialize = new List<EntityUid> {entity};
         for (var i = 0; i < toInitialize.Count; i++)

Robust.Shared/GameObjects/Systems/SharedMapSystem.Pause.cs

@@ -22,7 +22,7 @@ public abstract partial class SharedMapSystem
         if (!_mapQuery.Resolve(map, ref map.Comp))
             return false;
 
-        return map.Comp.MapPaused;
+        return map.Comp.MapPaused || !map.Comp.MapInitialized;
     }
 
     public void SetPaused(MapId mapId, bool paused)
@@ -49,7 +49,7 @@ public abstract partial class SharedMapSystem
         RecursiveSetPaused(map, paused);
     }
 
-    private void RecursiveSetPaused(EntityUid entity, bool paused)
+    internal void RecursiveSetPaused(EntityUid entity, bool paused)
     {
         _meta.SetEntityPaused(entity, paused);
         foreach (var child in Transform(entity)._children)

Robust.Shared/GameObjects/Systems/SharedMapSystem.cs

@@ -1,3 +1,4 @@
+using System;
 using System.Collections.Generic;
 using Robust.Shared.GameStates;
 using Robust.Shared.IoC;
@@ -32,6 +33,13 @@ namespace Robust.Shared.GameObjects
 
         internal Dictionary<MapId, EntityUid> Maps { get; } = new();
 
+        /// <summary>
+        /// This hashset is used to try prevent MapId re-use. This is mainly for auto-assigned map ids.
+        /// Loading a map with a specific id (e.g., the various mapping commands) may still result in an id being
+        /// reused.
+        /// </summary>
+        protected HashSet<MapId> UsedIds = new();
+
         public override void Initialize()
         {
             base.Initialize();
@@ -53,6 +61,7 @@ namespace Robust.Shared.GameObjects
     /// <summary>
     ///     Arguments for when a map is created or deleted.
     /// </summary>
+    [Obsolete("Use map creation or deletion events")]
     public sealed class MapChangedEvent : EntityEventArgs
     {
         public EntityUid Uid;
@@ -83,6 +92,16 @@ namespace Robust.Shared.GameObjects
         public bool Destroyed => !Created;
     }
 
+    /// <summary>
+    ///     Event raised whenever a map is created.
+    /// </summary>
+    public readonly record struct MapCreatedEvent(EntityUid Uid, MapId MapId);
+
+    /// <summary>
+    ///     Event raised whenever a map is removed.
+    /// </summary>
+    public readonly record struct MapRemovedEvent(EntityUid Uid, MapId MapId);
+
 #pragma warning disable CS0618
     public sealed class GridStartupEvent : EntityEventArgs
     {

Robust.Shared/GameObjects/Systems/SharedTransformSystem.Component.cs

@@ -5,7 +5,7 @@ using Robust.Shared.Maths;
 using Robust.Shared.Physics;
 using Robust.Shared.Utility;
 using System;
-using System.Diagnostics;
+using System.Collections.Generic;
 using System.Linq;
 using System.Runtime.CompilerServices;
 using Robust.Shared.Map.Components;
@@ -214,7 +214,15 @@ public abstract partial class SharedTransformSystem
         }
         else if (_mapQuery.TryComp(uid, out var mapComp))
         {
-            DebugTools.AssertNotEqual(mapComp.MapId, MapId.Nullspace);
+            if (mapComp.MapId == MapId.Nullspace)
+            {
+#if !EXCEPTION_TOLERANCE
+                throw new Exception("Transform is initialising before map ids have been assigned?");
+#endif
+                Log.Error($"Transform is initialising before map ids have been assigned?");
+                _map.AssignMapId((uid, mapComp));
+            }
+
             xform.MapUid = uid;
             xform.MapID = mapComp.MapId;
         }

Robust.Shared/GameObjects/Systems/SharedTransformSystem.cs

@@ -201,7 +201,7 @@ namespace Robust.Shared.GameObjects
             if (xform.GridUid == xform.ParentUid)
                 return (xform.Coordinates, GetWorldRotation(xform, XformQuery));
 
-            DebugTools.Assert(!_mapManager.IsGrid(uid) && !_mapManager.IsMap(uid));
+            DebugTools.Assert(!HasComp<MapComponent>(uid) && !HasComp<MapComponent>(uid));
 
             var (pos, worldRot) = GetWorldPositionRotation(xform, XformQuery);
 

Robust.Shared/GameObjects/Systems/SharedVisibilitySystem.cs

@@ -0,0 +1,26 @@
+namespace Robust.Shared.GameObjects;
+
+public abstract class SharedVisibilitySystem : EntitySystem
+{
+    public virtual void AddLayer(Entity<VisibilityComponent?> ent, ushort layer, bool refresh = true)
+    {
+    }
+
+    public virtual void RemoveLayer(Entity<VisibilityComponent?> ent, ushort layer, bool refresh = true)
+    {
+    }
+
+    public virtual void SetLayer(Entity<VisibilityComponent?> ent, ushort layer, bool refresh = true)
+    {
+    }
+
+    public virtual void RefreshVisibility(EntityUid uid,
+        VisibilityComponent? visibilityComponent = null,
+        MetaDataComponent? meta = null)
+    {
+    }
+
+    public virtual void RefreshVisibility(Entity<VisibilityComponent?, MetaDataComponent?> ent)
+    {
+    }
+}

Robust.Shared/Map/Components/MapComponent.cs

@@ -15,7 +15,7 @@ namespace Robust.Shared.Map.Components
         [DataField]
         public bool LightingEnabled { get; set; } = true;
 
-        [ViewVariables(VVAccess.ReadOnly)]
+        [ViewVariables(VVAccess.ReadOnly), Access(typeof(SharedMapSystem), Other = AccessPermissions.ReadExecute)]
         public MapId MapId { get; internal set; } = MapId.Nullspace;
 
         [DataField, Access(typeof(SharedMapSystem), typeof(MapManager))]

Robust.Shared/Map/Events/MapSerializationEvents.cs

@@ -1,5 +1,7 @@
 using System.Collections.Generic;
+using Robust.Shared.EntitySerialization;
 using Robust.Shared.GameObjects;
+using Robust.Shared.Serialization.Markdown.Mapping;
 
 namespace Robust.Shared.Map.Events;
 
@@ -28,17 +30,13 @@ public sealed class BeforeEntityReadEvent
 }
 
 /// <summary>
-/// This event is broadcast just before an entity gets serialized.
+/// This event is broadcast just before the given entities (and their children) are serialized.
+/// For convenience, the event also contains a set with all the maps that the entities are on. This does not
+/// necessarily mean that the maps are themselves getting serialized.
 /// </summary>
-public sealed class BeforeSaveEvent(EntityUid entity, EntityUid? map)
-{
-    /// <summary>
-    /// The entity that is going to be saved. usually a map or grid.
-    /// </summary>
-    public EntityUid Entity = entity;
+public readonly record struct BeforeSerializationEvent(HashSet<EntityUid> Entities, HashSet<MapId> MapIds);
 
-    /// <summary>
-    /// The map that the <see cref="Entity"/> is on.
-    /// </summary>
-    public EntityUid? Map = map;
-}
+/// <summary>
+/// This event is broadcast just after entities (and their children) have been serialized, but before it gets written to a yaml file.
+/// </summary>
+public readonly record struct AfterSerializationEvent(HashSet<EntityUid> Entities, MappingDataNode Node, FileCategory Category);

Robust.Shared/Map/IMapManager.cs

@@ -47,21 +47,25 @@ namespace Robust.Shared.Map
         /// </summary>
         /// <param name="mapId">The map ID to check existence of.</param>
         /// <returns>True if the map exists, false otherwise.</returns>
+        [Obsolete("Use MapSystem")]
         bool MapExists([NotNullWhen(true)] MapId? mapId);
 
         /// <summary>
         /// Returns the map entity ID for a given map, or an invalid entity Id if the map does not exist.
         /// </summary>
-        [Obsolete("Use TryGetMap")]
+        [Obsolete("Use MapSystem")]
         EntityUid GetMapEntityId(MapId mapId);
 
         /// <summary>
         /// Replaces GetMapEntity()'s throw-on-failure semantics.
         /// </summary>
+        [Obsolete("Use MapSystem")]
         EntityUid GetMapEntityIdOrThrow(MapId mapId);
 
+        [Obsolete("Use MapSystem")]
         IEnumerable<MapId> GetAllMapIds();
 
+        [Obsolete("Use MapSystem")]
         void DeleteMap(MapId mapId);
 
         // ReSharper disable once MethodOverloadWithOptionalParameter
@@ -205,20 +209,22 @@ namespace Robust.Shared.Map
         [Obsolete("Just delete the grid entity")]
         void DeleteGrid(EntityUid euid);
 
+        [Obsolete("Use HasComp")]
         bool IsGrid(EntityUid uid);
+
+        [Obsolete("Use HasComp")]
         bool IsMap(EntityUid uid);
 
         //
         // Pausing functions
         //
 
+        [Obsolete("Use MapSystem")]
         void SetMapPaused(MapId mapId, bool paused);
 
+        [Obsolete("Use MapSystem")]
         void DoMapInitialize(MapId mapId);
 
-        [Obsolete("Use CreateMap's runMapInit argument")]
-        void AddUninitializedMap(MapId mapId);
-
         [Obsolete("Use MapSystem")]
         bool IsMapPaused(MapId mapId);
 

Robust.Shared/Map/ITileDefinitionManager.cs

@@ -67,15 +67,5 @@ namespace Robust.Shared.Map
         /// </summary>
         /// <param name="tileDef">THe definition to register.</param>
         void Register(ITileDefinition tileDef);
-
-        /// <summary>
-        ///     Register a tile alias with this manager.
-        ///     The tile need not exist yet - the alias's creation will be deferred until it exists.
-        ///     Tile aliases do not have IDs of their own and do not show up in enumeration.
-        ///     Their main utility is for easier map migration.
-        /// </summary>
-        /// <param name="src">The source tile (i.e. name of the alias).</param>
-        /// <param name="dst">The destination tile (i.e. the actual concrete tile).</param>
-        void AssignAlias(string src, string dst);
     }
 }

Robust.Shared/Map/MapId.cs

@@ -51,7 +51,7 @@ namespace Robust.Shared.Map
 
         public override string ToString()
         {
-            return Value.ToString();
+            return IsClientSide ? $"c{-Value}" : Value.ToString();
         }
 
         public bool IsClientSide => Value < 0;

Robust.Shared/Map/MapManager.GridCollection.cs

@@ -125,6 +125,9 @@ internal partial class MapManager
         EntityManager.System<MetaDataSystem>().SetEntityName(gridEnt, $"grid", meta);
         EntityManager.InitializeComponents(gridEnt, meta);
         EntityManager.StartComponents(gridEnt);
+        // Note that this does not actually map-initialize the grid entity, even if the map its being spawn on has already been initialized.
+        // I don't know whether that is intentional or not.
+
         return (gridEnt, grid);
     }
 }

Robust.Shared/Map/MapManager.MapCollection.cs

@@ -3,7 +3,6 @@ using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
 using Robust.Shared.GameObjects;
 using Robust.Shared.Map.Components;
-using Robust.Shared.Utility;
 
 namespace Robust.Shared.Map;
 
@@ -28,16 +27,10 @@ public sealed class MapEventArgs : EventArgs
 
 internal partial class MapManager
 {
-    private Dictionary<MapId, EntityUid> _mapEntities => _mapSystem.Maps;
-
     /// <inheritdoc />
     public virtual void DeleteMap(MapId mapId)
     {
-        if (!_mapEntities.TryGetValue(mapId, out var ent) || !ent.IsValid())
-            throw new InvalidOperationException($"Attempted to delete nonexistent map '{mapId}'");
-
-        EntityManager.DeleteEntity(ent);
-        DebugTools.Assert(!_mapEntities.ContainsKey(mapId));
+        _mapSystem.DeleteMap(mapId);
     }
 
     /// <inheritdoc />
@@ -81,7 +74,7 @@ internal partial class MapManager
     /// <inheritdoc />
     public IEnumerable<MapId> GetAllMapIds()
     {
-        return _mapEntities.Keys;
+        return _mapSystem.GetAllMapIds();
     }
 
     /// <inheritdoc />

Robust.Shared/Map/MapManager.Pause.cs

@@ -1,7 +1,5 @@
-using System;
 using System.Globalization;
 using Robust.Shared.GameObjects;
-using Robust.Shared.Map.Components;
 
 namespace Robust.Shared.Map
 {
@@ -27,14 +25,6 @@ namespace Robust.Shared.Map
             return _mapSystem.IsInitialized(mapId);
         }
 
-        public void AddUninitializedMap(MapId mapId)
-        {
-            var ent = GetMapEntityId(mapId);
-            EntityManager.GetComponent<MapComponent>(ent).MapInitialized = false;
-            var meta = EntityManager.GetComponent<MetaDataComponent>(ent);
-            ((EntityManager)EntityManager).SetLifeStage(meta, EntityLifeStage.Initialized);
-        }
-
         /// <inheritdoc />
         public bool IsMapPaused(MapId mapId)
         {
@@ -87,7 +77,7 @@ namespace Robust.Shared.Map
                         return;
                     }
 
-                    shell.WriteLine(IsMapPaused(mapId).ToString());
+                    shell.WriteLine(_mapSystem.IsPaused(mapId).ToString());
                 });
 
             _conhost.RegisterCommand("unpausemap",

Robust.Shared/Map/MapManager.Queries.cs

@@ -45,54 +45,54 @@ internal partial class MapManager
     public void FindGridsIntersecting<T>(MapId mapId, T shape, Transform transform,
         ref List<Entity<MapGridComponent>> grids, bool approx = IMapManager.Approximate, bool includeMap = IMapManager.IncludeMap) where T : IPhysShape
     {
-        if (_mapEntities.TryGetValue(mapId, out var mapEnt))
-            FindGridsIntersecting(mapEnt, shape, transform, ref grids, approx, includeMap);
+        if (_mapSystem.TryGetMap(mapId, out var mapEnt))
+            FindGridsIntersecting(mapEnt.Value, shape, transform, ref grids, approx, includeMap);
     }
 
     public void FindGridsIntersecting<T>(MapId mapId, T shape, Transform transform, GridCallback callback, bool approx = IMapManager.Approximate, bool includeMap = IMapManager.IncludeMap) where T : IPhysShape
     {
-        if (_mapEntities.TryGetValue(mapId, out var mapEnt))
-            FindGridsIntersecting(mapEnt, shape, transform, callback, includeMap, approx);
+        if (_mapSystem.TryGetMap(mapId, out var mapEnt))
+            FindGridsIntersecting(mapEnt.Value, shape, transform, callback, includeMap, approx);
     }
 
     public void FindGridsIntersecting(MapId mapId, Box2 worldAABB, GridCallback callback, bool approx = IMapManager.Approximate, bool includeMap = IMapManager.IncludeMap)
     {
-        if (_mapEntities.TryGetValue(mapId, out var mapEnt))
-            FindGridsIntersecting(mapEnt, worldAABB, callback, approx, includeMap);
+        if (_mapSystem.TryGetMap(mapId, out var mapEnt))
+            FindGridsIntersecting(mapEnt.Value, worldAABB, callback, approx, includeMap);
     }
 
     public void FindGridsIntersecting<TState>(MapId mapId, Box2 worldAABB, ref TState state, GridCallback<TState> callback, bool approx = IMapManager.Approximate, bool includeMap = IMapManager.IncludeMap)
     {
-        if (_mapEntities.TryGetValue(mapId, out var map))
-            FindGridsIntersecting(map, worldAABB, ref state, callback, approx, includeMap);
+        if (_mapSystem.TryGetMap(mapId, out var map))
+            FindGridsIntersecting(map.Value, worldAABB, ref state, callback, approx, includeMap);
     }
 
     public void FindGridsIntersecting(MapId mapId, Box2 worldAABB, ref List<Entity<MapGridComponent>> grids,
         bool approx = IMapManager.Approximate, bool includeMap = IMapManager.IncludeMap)
     {
-        if (_mapEntities.TryGetValue(mapId, out var map))
-            FindGridsIntersecting(map, worldAABB, ref grids, approx, includeMap);
+        if (_mapSystem.TryGetMap(mapId, out var map))
+            FindGridsIntersecting(map.Value, worldAABB, ref grids, approx, includeMap);
     }
 
     public void FindGridsIntersecting(MapId mapId, Box2Rotated worldBounds, GridCallback callback, bool approx = IMapManager.Approximate,
         bool includeMap = IMapManager.IncludeMap)
     {
-        if (_mapEntities.TryGetValue(mapId, out var mapEnt))
-            FindGridsIntersecting(mapEnt, worldBounds, callback, approx, includeMap);
+        if (_mapSystem.TryGetMap(mapId, out var mapEnt))
+            FindGridsIntersecting(mapEnt.Value, worldBounds, callback, approx, includeMap);
     }
 
     public void FindGridsIntersecting<TState>(MapId mapId, Box2Rotated worldBounds, ref TState state, GridCallback<TState> callback,
         bool approx = IMapManager.Approximate, bool includeMap = IMapManager.IncludeMap)
     {
-        if (_mapEntities.TryGetValue(mapId, out var mapEnt))
-            FindGridsIntersecting(mapEnt, worldBounds, ref state, callback, approx, includeMap);
+        if (_mapSystem.TryGetMap(mapId, out var mapEnt))
+            FindGridsIntersecting(mapEnt.Value, worldBounds, ref state, callback, approx, includeMap);
     }
 
     public void FindGridsIntersecting(MapId mapId, Box2Rotated worldBounds, ref List<Entity<MapGridComponent>> grids,
         bool approx = IMapManager.Approximate, bool includeMap = IMapManager.IncludeMap)
     {
-        if (_mapEntities.TryGetValue(mapId, out var mapEnt))
-            FindGridsIntersecting(mapEnt, worldBounds, ref grids, approx, includeMap);
+        if (_mapSystem.TryGetMap(mapId, out var mapEnt))
+            FindGridsIntersecting(mapEnt.Value, worldBounds, ref grids, approx, includeMap);
     }
 
     #endregion
@@ -338,8 +338,8 @@ internal partial class MapManager
     /// </summary>
     public bool TryFindGridAt(MapId mapId, Vector2 worldPos, out EntityUid uid, [NotNullWhen(true)] out MapGridComponent? grid)
     {
-        if (_mapEntities.TryGetValue(mapId, out var map))
-            return TryFindGridAt(map, worldPos, out uid, out grid);
+        if (_mapSystem.TryGetMap(mapId, out var map))
+            return TryFindGridAt(map.Value, worldPos, out uid, out grid);
 
         uid = default;
         grid = null;

Robust.Shared/Map/MapManager.cs

@@ -11,10 +11,10 @@ namespace Robust.Shared.Map;
 
 /// <inheritdoc cref="IMapManager" />
 [Virtual]
-internal partial class MapManager : IMapManagerInternal, IEntityEventSubscriber, IPostInjectInit
+internal partial class MapManager : IMapManagerInternal, IEntityEventSubscriber
 {
-    [field: Dependency] public IGameTiming GameTiming { get; } = default!;
-    [field: Dependency] public IEntityManager EntityManager { get; } = default!;
+    [Dependency] public readonly IGameTiming GameTiming = default!;
+    [Dependency] public readonly IEntityManager EntityManager = default!;
     [Dependency] private readonly IManifoldManager _manifolds = default!;
     [Dependency] private readonly ILogManager _logManager = default!;
     [Dependency] private readonly IConsoleHost _conhost = default!;
@@ -34,6 +34,7 @@ internal partial class MapManager : IMapManagerInternal, IEntityEventSubscriber,
         _gridTreeQuery = EntityManager.GetEntityQuery<GridTreeComponent>();
         _gridQuery = EntityManager.GetEntityQuery<MapGridComponent>();
         InitializeMapPausing();
+        _sawmill = _logManager.GetSawmill("system.map");
     }
 
     /// <inheritdoc />
@@ -74,9 +75,4 @@ internal partial class MapManager : IMapManagerInternal, IEntityEventSubscriber,
             EntityManager.DeleteEntity(uid);
         }
     }
-
-    void IPostInjectInit.PostInject()
-    {
-        _sawmill = _logManager.GetSawmill("system.map");
-    }
 }

Robust.Shared/Map/MapSerializationContext.cs

@@ -1,173 +0,0 @@
-using System;
-using System.Collections.Generic;
-using System.Diagnostics.CodeAnalysis;
-using System.Globalization;
-using JetBrains.Annotations;
-using Robust.Shared.GameObjects;
-using Robust.Shared.IoC;
-using Robust.Shared.Log;
-using Robust.Shared.Serialization;
-using Robust.Shared.Serialization.Manager;
-using Robust.Shared.Serialization.Markdown;
-using Robust.Shared.Serialization.Markdown.Validation;
-using Robust.Shared.Serialization.Markdown.Value;
-using Robust.Shared.Serialization.TypeSerializers.Interfaces;
-using Robust.Shared.Timing;
-
-namespace Robust.Shared.Map;
-
-internal sealed class MapSerializationContext : ISerializationContext, IEntityLoadContext,
-    ITypeSerializer<EntityUid, ValueDataNode>
-{
-    public SerializationManager.SerializerProvider SerializerProvider { get; } = new();
-
-    // Run-specific data
-    public Dictionary<int, string>? TileMap;
-    public readonly Dictionary<string, IComponent> CurrentReadingEntityComponents = new();
-    public HashSet<string> CurrentlyIgnoredComponents = new();
-    public string? CurrentComponent;
-    public EntityUid? CurrentWritingEntity;
-    public IEntityManager EntityManager;
-    public IGameTiming Timing;
-
-    private Dictionary<int, EntityUid> _uidEntityMap = new();
-    private Dictionary<EntityUid, int> _entityUidMap = new();
-
-    // Native tile ID -> map tile ID map for writing maps.
-    public Dictionary<int, int> TileWriteMap = [];
-
-    /// <summary>
-    /// Are we currently iterating prototypes or entities for writing.
-    /// </summary>
-    public bool WritingReadingPrototypes { get; set; }
-
-    /// <summary>
-    /// Whether the map has been MapInitialized or not.
-    /// </summary>
-    public bool MapInitialized;
-
-    /// <summary>
-    /// How long the target map has been paused. Used for time offsets.
-    /// </summary>
-    public TimeSpan PauseTime;
-
-    /// <summary>
-    /// The parent of the entity being saved, This entity is not itself getting saved.
-    /// </summary>
-    private EntityUid? _parentUid;
-
-    public MapSerializationContext(IEntityManager entityManager, IGameTiming timing)
-    {
-        EntityManager = entityManager;
-        Timing = timing;
-        SerializerProvider.RegisterSerializer(this);
-    }
-
-    public void Set(
-        Dictionary<int, EntityUid> uidEntityMap,
-        Dictionary<EntityUid, int> entityUidMap,
-        bool mapPreInit,
-        TimeSpan pauseTime,
-        EntityUid? parentUid)
-    {
-        _uidEntityMap = uidEntityMap;
-        _entityUidMap = entityUidMap;
-        MapInitialized = mapPreInit;
-        PauseTime = pauseTime;
-        if (parentUid != null && parentUid.Value.IsValid())
-            _parentUid = parentUid;
-    }
-
-    public void Clear()
-    {
-        CurrentReadingEntityComponents.Clear();
-        CurrentlyIgnoredComponents.Clear();
-        CurrentComponent = null;
-        CurrentWritingEntity = null;
-        PauseTime = TimeSpan.Zero;
-    }
-
-    // Create custom object serializers that will correctly allow data to be overriden by the map file.
-    bool IEntityLoadContext.TryGetComponent(string componentName, [NotNullWhen(true)] out IComponent? component)
-    {
-        return CurrentReadingEntityComponents.TryGetValue(componentName, out component);
-    }
-
-    public IEnumerable<string> GetExtraComponentTypes()
-    {
-        return CurrentReadingEntityComponents!.Keys;
-    }
-
-    public bool ShouldSkipComponent(string compName)
-    {
-        return CurrentlyIgnoredComponents.Contains(compName);
-    }
-
-    ValidationNode ITypeValidator<EntityUid, ValueDataNode>.Validate(ISerializationManager serializationManager,
-        ValueDataNode node, IDependencyCollection dependencies, ISerializationContext? context)
-    {
-        if (node.Value == "null")
-        {
-            return new ValidatedValueNode(node);
-        }
-
-        if (!int.TryParse(node.Value, out var val) || !_uidEntityMap.ContainsKey(val))
-        {
-            return new ErrorNode(node, "Invalid EntityUid", true);
-        }
-
-        return new ValidatedValueNode(node);
-    }
-
-    public DataNode Write(ISerializationManager serializationManager, EntityUid value,
-        IDependencyCollection dependencies, bool alwaysWrite = false,
-        ISerializationContext? context = null)
-    {
-        if (!_entityUidMap.TryGetValue(value, out var entityUidMapped))
-        {
-            if (CurrentComponent == "Transform")
-            {
-                if (!value.IsValid() || value == _parentUid)
-                    return new ValueDataNode("invalid");
-            }
-
-            dependencies
-                .Resolve<ILogManager>()
-                .GetSawmill("map")
-                .Error("Encountered an invalid entityUid '{0}' while serializing a map.", value);
-
-            return new ValueDataNode("invalid");
-        }
-
-        return new ValueDataNode(entityUidMapped.ToString(CultureInfo.InvariantCulture));
-    }
-
-    EntityUid ITypeReader<EntityUid, ValueDataNode>.Read(ISerializationManager serializationManager,
-        ValueDataNode node,
-        IDependencyCollection dependencies,
-        SerializationHookContext hookCtx,
-        ISerializationContext? context, ISerializationManager.InstantiationDelegate<EntityUid>? _)
-    {
-        if (node.Value == "invalid" && CurrentComponent == "Transform")
-            return EntityUid.Invalid;
-
-        if (int.TryParse(node.Value, out var val) && _uidEntityMap.TryGetValue(val, out var entity))
-            return entity;
-
-        dependencies
-            .Resolve<ILogManager>()
-            .GetSawmill("map")
-            .Error("Error in map file: found local entity UID '{0}' which does not exist.", val);
-
-        return EntityUid.Invalid;
-
-    }
-
-    [MustUseReturnValue]
-    public EntityUid Copy(ISerializationManager serializationManager, EntityUid source, EntityUid target,
-        bool skipHook,
-        ISerializationContext? context = null)
-    {
-        return new((int)source);
-    }
-}

Robust.Shared/Map/TileDefinitionManager.cs

@@ -27,10 +27,6 @@ namespace Robust.Shared.Map
 
         public virtual void Initialize()
         {
-            foreach (var prototype in _prototypeManager.EnumeratePrototypes<TileAliasPrototype>())
-            {
-                AssignAlias(prototype.ID, prototype.Target);
-            }
         }
 
         public virtual void Register(ITileDefinition tileDef)
@@ -45,46 +41,8 @@ namespace Robust.Shared.Map
             tileDef.AssignTileId(id);
             TileDefs.Add(tileDef);
             _tileNames[name] = tileDef;
-
-            AliasingHandleDeferred(name);
         }
 
-        private void AliasingHandleDeferred(string name)
-        {
-            // Aliases may have been held back due to tiles not being registered yet, handle this.
-            if (_awaitingAliases.ContainsKey(name))
-            {
-                var list = _awaitingAliases[name];
-                _awaitingAliases.Remove(name);
-                foreach (var alias in list)
-                {
-                    AssignAlias(alias, name);
-                }
-            }
-        }
-
-
-        public virtual void AssignAlias(string src, string dst)
-        {
-            if (_tileNames.ContainsKey(src))
-            {
-                throw new ArgumentException("Another tile definition or alias with the same name has already been registered.", nameof(src));
-            }
-
-            if (_tileNames.ContainsKey(dst))
-            {
-                // Simple enough, source to destination.
-                _tileNames[src] = _tileNames[dst];
-                AliasingHandleDeferred(src);
-            }
-            else
-            {
-                // Less simple - stash this alias for later so it appears when the target does.
-                if (!_awaitingAliases.ContainsKey(dst))
-                    _awaitingAliases[dst] = new();
-                _awaitingAliases[dst].Add(src);
-            }
-        }
 
         public Tile GetVariantTile(string name, IRobustRandom random)
         {

Robust.Shared/Network/NetManager.ClientConnect.cs

@@ -1,6 +1,5 @@
 using System;
 using System.Collections.Generic;
-using System.Collections.Immutable;
 using System.Linq;
 using System.Net;
 using System.Net.Http;
@@ -11,7 +10,6 @@ using System.Security.Cryptography;
 using System.Threading;
 using System.Threading.Tasks;
 using Lidgren.Network;
-using Robust.Shared.Log;
 using Robust.Shared.Network.Messages.Handshake;
 using Robust.Shared.Utility;
 using SpaceWizards.Sodium;
@@ -98,7 +96,7 @@ namespace Robust.Shared.Network
             {
                 await CCDoHandshake(winningPeer, winningConnection, userNameRequest, mainCancelToken);
             }
-            catch (TaskCanceledException)
+            catch (OperationCanceledException)
             {
                 winningPeer.Peer.Shutdown("Cancelled");
                 _toCleanNetPeers.Add(winningPeer.Peer);
@@ -120,7 +118,10 @@ namespace Robust.Shared.Network
             _logger.Debug("Handshake completed, connection established.");
         }
 
-        private async Task CCDoHandshake(NetPeerData peer, NetConnection connection, string userNameRequest,
+        private async Task CCDoHandshake(
+            NetPeerData peer,
+            NetConnection connection,
+            string userNameRequest,
             CancellationToken cancel)
         {
             var encrypt = _config.GetCVar(CVars.NetEncrypt);
@@ -289,37 +290,51 @@ namespace Robust.Shared.Network
         private async Task<(NetPeerData winningPeer, NetConnection winningConnection)?>
             CCHappyEyeballs(int port, IPAddress first, IPAddress? second, CancellationToken mainCancelToken)
         {
-            NetPeerData CreatePeerForIp(IPAddress address)
+            // Try to establish a connection with an IP address and wait for it to either connect or fail
+            // Returns a disposable wrapper around the peer/connection because ParallelTask
+            async Task<ConnectionAttempt> AttemptConnection(IPAddress address, CancellationToken cancel)
             {
                 var config = _getBaseNetPeerConfig();
-                if (address.AddressFamily == AddressFamily.InterNetworkV6)
-                {
-                    config.LocalAddress = IPAddress.IPv6Any;
-                }
-                else
-                {
-                    config.LocalAddress = IPAddress.Any;
-                }
+                config.LocalAddress = address.AddressFamily == AddressFamily.InterNetworkV6
+                    ? IPAddress.IPv6Any
+                    : IPAddress.Any;
 
                 var peer = new NetPeer(config);
                 peer.Start();
-                var data = new NetPeerData(peer);
-                _netPeers.Add(data);
-                return data;
+                var peerData = new NetPeerData(peer);
+                _netPeers.Add(peerData);
+
+                var connection = peer.Connect(new IPEndPoint(address, port));
+
+                try
+                {
+                    // We need AwaitNonInitStatusChange to properly handle connection state transitions
+                    var reason = await AwaitNonInitStatusChange(connection, cancel);
+
+                    if (connection.Status != NetConnectionStatus.Connected)
+                    {
+                        // Connection failed, clean up and yeet an exception
+                        peer.Shutdown(reason);
+                        _toCleanNetPeers.Add(peer);
+                        throw new Exception($"Connection failed: {reason}");
+                    }
+
+                    return new ConnectionAttempt(peerData, connection, this);
+                }
+                catch (Exception)
+                {
+                    // Something went wrong!
+                    peer.Shutdown("Connection attempt failed");
+                    _toCleanNetPeers.Add(peer);
+                    throw;
+                }
             }
 
-            // Create first peer.
-            var firstPeer = CreatePeerForIp(first);
-            var firstConnection = firstPeer.Peer.Connect(new IPEndPoint(first, port));
-            NetPeerData? secondPeer = null;
-            NetConnection? secondConnection = null;
-            string? secondReason = null;
-
+            // Waits for a connection's status to change from InitiatedConnect to anything else
             async Task<string> AwaitNonInitStatusChange(NetConnection connection, CancellationToken cancellationToken)
             {
-                NetConnectionStatus status;
                 string reason;
-
+                NetConnectionStatus status;
                 do
                 {
                     reason = await AwaitStatusChange(connection, cancellationToken);
@@ -329,124 +344,37 @@ namespace Robust.Shared.Network
                 return reason;
             }
 
-            async Task ConnectSecondDelayed(CancellationToken cancellationToken)
-            {
-                DebugTools.AssertNotNull(second);
-                // Connecting via second peer is delayed by 25ms to give an advantage to IPv6, if it works.
-                var delay = TimeSpan.FromSeconds(_config.GetCVar(CVars.NetHappyEyeballsDelay));
-                await Task.Delay(delay, cancellationToken);
-                if (cancellationToken.IsCancellationRequested)
-                {
-                    return;
-                }
-
-                secondPeer = CreatePeerForIp(second);
-                secondConnection = secondPeer.Peer.Connect(new IPEndPoint(second, port));
-
-                secondReason = await AwaitNonInitStatusChange(secondConnection, cancellationToken);
-            }
-
-            NetPeerData? winningPeer;
-            NetConnection? winningConnection;
-            string? firstReason = null;
             try
             {
-                if (second != null)
-                {
-                    // We have two addresses to try.
-                    var cancellation = CancellationTokenSource.CreateLinkedTokenSource(mainCancelToken);
-                    var firstPeerChanged = AwaitNonInitStatusChange(firstConnection, cancellation.Token);
-                    var secondPeerChanged = ConnectSecondDelayed(cancellation.Token);
+                // Create list of IPs to try
+                var addresses = second != null
+                    ? new[] { first, second }
+                    : new[] { first };
 
-                    var firstChange = await Task.WhenAny(firstPeerChanged, secondPeerChanged);
+                // Use ParallelTask to handle the connection attempts
+                var delay = TimeSpan.FromSeconds(_config.GetCVar(CVars.NetHappyEyeballsDelay));
+                var (result, _) = await HappyEyeballsHttp.ParallelTask(
+                    addresses.Length,
+                    (i, token) => AttemptConnection(addresses[i], token),
+                    delay,
+                    mainCancelToken);
 
-                    if (firstChange == firstPeerChanged)
-                    {
-                        _logger.Debug("First peer status changed.");
-                        // First peer responded first.
-                        if (firstConnection.Status == NetConnectionStatus.Connected)
-                        {
-                            // First peer won!
-                            _logger.Debug("First peer succeeded.");
-                            cancellation.Cancel();
-                            if (secondPeer != null)
-                            {
-                                secondPeer.Peer.Shutdown("First connection attempt won.");
-                                _toCleanNetPeers.Add(secondPeer.Peer);
-                            }
-
-                            winningPeer = firstPeer;
-                            winningConnection = firstConnection;
-                        }
-                        else
-                        {
-                            // First peer failed, try the second one I guess.
-                            _logger.Debug("First peer failed.");
-                            firstPeer.Peer.Shutdown("You failed.");
-                            _toCleanNetPeers.Add(firstPeer.Peer);
-                            firstReason = await firstPeerChanged;
-                            await secondPeerChanged;
-                            winningPeer = secondPeer;
-                            winningConnection = secondConnection;
-                        }
-                    }
-                    else
-                    {
-                        if (secondConnection!.Status == NetConnectionStatus.Connected)
-                        {
-                            // Second peer won!
-                            _logger.Debug("Second peer succeeded.");
-                            cancellation.Cancel();
-                            firstPeer.Peer.Shutdown("Second connection attempt won.");
-                            _toCleanNetPeers.Add(firstPeer.Peer);
-                            winningPeer = secondPeer;
-                            winningConnection = secondConnection;
-                        }
-                        else
-                        {
-                            // First peer failed, try the second one I guess.
-                            _logger.Debug("Second peer failed.");
-                            secondPeer!.Peer.Shutdown("You failed.");
-                            _toCleanNetPeers.Add(secondPeer.Peer);
-                            firstReason = await firstPeerChanged;
-                            winningPeer = firstPeer;
-                            winningConnection = firstConnection;
-                        }
-                    }
-                }
-                else
-                {
-                    // Only one address to try. Pretty straight forward.
-                    firstReason = await AwaitNonInitStatusChange(firstConnection, mainCancelToken);
-                    winningPeer = firstPeer;
-                    winningConnection = firstConnection;
-                }
+                return (result.Peer, result.Connection);
             }
-            catch (TaskCanceledException)
+            catch (OperationCanceledException)
             {
-                firstPeer.Peer.Shutdown("Cancelled");
-                _toCleanNetPeers.Add(firstPeer.Peer);
-                if (secondPeer != null)
-                {
-                    // ReSharper disable once PossibleNullReferenceException
-                    secondPeer.Peer.Shutdown("Cancelled");
-                    _toCleanNetPeers.Add(secondPeer.Peer);
-                }
-
+                // Connection attempt was cancelled, nothing to see here
+                OnConnectFailed("Connection attempt cancelled.");
                 return null;
             }
-
-            // winningPeer can still be failed at this point.
-            // If it is, neither succeeded. RIP.
-            if (winningConnection!.Status != NetConnectionStatus.Connected)
+            catch (AggregateException ae)
             {
-                winningPeer!.Peer.Shutdown("You failed");
-                _toCleanNetPeers.Add(winningPeer.Peer);
-                OnConnectFailed((secondReason ?? firstReason)!);
+                // ParallelTask throws AggregateException with all connection failures
+                // We just take the first one
+                var message = ae.InnerExceptions.First().Message;
+                OnConnectFailed(message);
                 return null;
             }
-
-            return (winningPeer!, winningConnection);
         }
 
         private Task<string> AwaitStatusChange(NetConnection connection, CancellationToken cancellationToken = default)
@@ -471,7 +399,8 @@ namespace Robust.Shared.Network
             return tcs.Task;
         }
 
-        private Task<NetIncomingMessage> AwaitData(NetConnection connection,
+        private Task<NetIncomingMessage> AwaitData(
+            NetConnection connection,
             CancellationToken cancellationToken = default)
         {
             if (_awaitingData.ContainsKey(connection))
@@ -529,5 +458,17 @@ namespace Robust.Shared.Network
         }
 
         private sealed record JoinRequest(string Hash, string? Hwid);
+
+        private sealed class ConnectionAttempt(NetPeerData peer, NetConnection connection, NetManager netManager) : IDisposable
+        {
+            public NetPeerData Peer { get; } = peer;
+            public NetConnection Connection { get; } = connection;
+
+            public void Dispose()
+            {
+                Peer.Peer.Shutdown("Disposing unused connection attempt");
+                netManager._toCleanNetPeers.Add(Peer.Peer);
+            }
+        }
     }
 }

Robust.Shared/Network/NetManager.cs

@@ -587,6 +587,14 @@ namespace Robust.Shared.Network
         public void ClientDisconnect(string reason)
         {
             DebugTools.Assert(IsClient, "Should never be called on the server.");
+
+            // First handle any in-progress connection attempt
+            if (ClientConnectState != ClientConnectionState.NotConnecting)
+            {
+                _cancelConnectTokenSource?.Cancel();
+            }
+
+            // Then handle existing connection if any
             if (ServerChannel != null)
             {
                 Disconnect?.Invoke(this, new NetDisconnectedArgs(ServerChannel, reason));

Robust.Shared/Physics/FixtureSerializer.cs

@@ -1,4 +1,5 @@
 using System.Collections.Generic;
+using Robust.Shared.EntitySerialization;
 using Robust.Shared.GameObjects;
 using Robust.Shared.IoC;
 using Robust.Shared.Map;
@@ -78,13 +79,11 @@ public sealed class FixtureSerializer : ITypeSerializer<Dictionary<string, Fixtu
         if (value.Count == 0)
             return seq;
 
-        if (context is MapSerializationContext mapContext)
+        if (context is EntitySerializer ctx)
         {
             // Don't serialize mapgrid fixtures because it's bloat and we'll just generate them at runtime.
-            if (dependencies.Resolve<IEntityManager>().HasComponent<MapGridComponent>(mapContext.CurrentWritingEntity))
-            {
+            if (ctx.EntMan.HasComponent<MapGridComponent>(ctx.CurrentEntity))
                 return seq;
-            }
         }
 
         foreach (var (id, fixture) in value)