Fix MapLoaderSystem.SerializeEntitiesRecursive (#6246)

* Fix MapLoaderSystem.SerializeEntitiesRecursive

* a

* oops

RELEASE-NOTES.md

@@ -45,6 +45,7 @@ END TEMPLATE-->
 
 ### Bugfixes
 
+* Fixed `MapLoaderSystem.SerializeEntitiesRecursive()` not properly serialising when given multiple root entities (e.g., multiple maps)
 * Fixed yaml hot reloading throwing invalid path exceptions.
 * The `EntityManager.CreateEntityUninitialized` overload that uses MapCoordinates now actually attaches entities to a grid if one is present at those coordinates, as was stated in it's documentation.
 * Fixed physics joint relays not being properly updated when an entity is removed from a container.

Robust.Shared/EntitySerialization/EntitySerializer.cs

@@ -253,6 +253,42 @@ public sealed class EntitySerializer : ISerializationContext,
         Truncate = EntityUid.Invalid;
     }
 
+    /// <summary>
+    /// Serializes several entities and all of their children. Note that this will not automatically serialize the
+    /// entity's parents.
+    /// </summary>
+    public void SerializeEntityRecursive(HashSet<EntityUid> roots)
+    {
+        if (roots.Count == 0)
+            return;
+
+        InitializeTileMap(roots.First());
+
+        HashSet<EntityUid> allEntities = new();
+        List<(EntityUid Root, HashSet<EntityUid> Children)> entities = new();
+
+        foreach(var root in roots)
+        {
+            if (!IsSerializable(root))
+                throw new Exception($"{EntMan.ToPrettyString(root)} is not serializable");
+
+            var ents = new HashSet<EntityUid>();
+            RecursivelyIncludeChildren(root, ents);
+            entities.Add((root, ents));
+            allEntities.UnionWith(ents);
+        }
+
+        ReserveYamlIds(allEntities);
+
+        foreach (var (root, children) in entities)
+        {
+            Truncate = _xformQuery.GetComponent(root).ParentUid;
+            Truncated.Add(Truncate);
+            SerializeEntitiesInternal(children);
+            Truncate = EntityUid.Invalid;
+        }
+    }
+
     #endregion
 
     /// <summary>

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

@@ -16,7 +16,7 @@ public sealed partial class MapLoaderSystem
     public event EntitySerializer.IsSerializableDelegate? OnIsSerializable;
 
     /// <summary>
-    /// Recursively serialize the given entity and its children.
+    /// Recursively serialize the given entities and all of their children.
     /// </summary>
     public (MappingDataNode Node, FileCategory Category) SerializeEntitiesRecursive(
         HashSet<EntityUid> entities,
@@ -41,12 +41,7 @@ public sealed partial class MapLoaderSystem
 
         var serializer = new EntitySerializer(_dependency, opts);
         serializer.OnIsSerializeable += OnIsSerializable;
-
-        foreach (var ent in entities)
-        {
-            serializer.SerializeEntityRecursive(ent);
-        }
-
+        serializer.SerializeEntityRecursive(entities);
         var data = serializer.Write();
         var cat = serializer.GetCategory();