Fix deserialization of empty grid chunks (#4565)

Robust.Server/GameObjects/EntitySystems/MapLoaderSystem.cs

@@ -290,6 +290,9 @@ public sealed class MapLoaderSystem : EntitySystem
 
         ReadGrids(data);
 
+        // grids prior to engine v175 might've been serialized with empty chunks which now throw debug asserts.
+        RemoveEmptyChunks(data);
+
         // Then, go hierarchically in order and do the entity things.
         StartupEntities(data);
 
@@ -305,6 +308,25 @@ public sealed class MapLoaderSystem : EntitySystem
         return true;
     }
 
+    private void RemoveEmptyChunks(MapData data)
+    {
+        var gridQuery = _serverEntityManager.GetEntityQuery<MapGridComponent>();
+        foreach (var uid in data.EntitiesToDeserialize.Keys)
+        {
+            if (!gridQuery.TryGetComponent(uid, out var gridComp))
+                continue;
+
+            foreach (var (index, chunk) in gridComp.Chunks)
+            {
+                if (chunk.FilledTiles > 0)
+                    continue;
+
+                Log.Warning($"Encountered empty chunk while deserializing map. Grid: {ToPrettyString(uid)}. Chunk index: {index}");
+                gridComp.Chunks.Remove(index);
+            }
+        }
+    }
+
     private bool VerifyEntitiesExist(MapData data, BeforeEntityReadEvent ev)
     {
         _stopwatch.Restart();

Robust.Server/Maps/MapChunkSerializer.cs

@@ -12,6 +12,7 @@ using Robust.Shared.Serialization.Markdown.Mapping;
 using Robust.Shared.Serialization.Markdown.Validation;
 using Robust.Shared.Serialization.Markdown.Value;
 using Robust.Shared.Serialization.TypeSerializers.Interfaces;
+using Robust.Shared.Utility;
 
 namespace Robust.Server.Maps;
 
@@ -93,6 +94,7 @@ internal sealed class MapChunkSerializer : ITypeSerializer<MapChunk, MappingData
         IDependencyCollection dependencies, bool alwaysWrite = false,
         ISerializationContext? context = null)
     {
+        DebugTools.Assert(value.FilledTiles > 0, "Attempting to write an empty chunk");
         var root = new MappingDataNode();
         var ind = new ValueDataNode($"{value.X},{value.Y}");
         root.Add("ind", ind);

Robust.Shared/Map/Enumerators/GridTileEnumerator.cs

@@ -2,6 +2,7 @@ using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
 using Robust.Shared.GameObjects;
 using Robust.Shared.Maths;
+using Robust.Shared.Utility;
 
 namespace Robust.Shared.Map.Enumerators;
 
@@ -26,33 +27,34 @@ public struct GridTileEnumerator
     }
 
     public bool MoveNext([NotNullWhen(true)] out TileRef? tileRef)
+    {
+        while (true)
         {
-            while (true)
+            if (_index == _chunkSize * _chunkSize)
             {
-                if (_index == _chunkSize * _chunkSize)
+                if (!_chunkEnumerator.MoveNext())
                 {
-                    if (!_chunkEnumerator.MoveNext())
-                    {
-                        tileRef = null;
-                        return false;
-                    }
-
-                    _index = 0;
+                    tileRef = null;
+                    return false;
                 }
 
-                var (chunkOrigin, chunk) = _chunkEnumerator.Current;
-                var x = (ushort) (_index / _chunkSize);
-                var y = (ushort) (_index % _chunkSize);
-                var tile = chunk.GetTile(x, y);
-                _index++;
-
-                if (_ignoreEmpty && tile.IsEmpty)
-                    continue;
-
-                var gridX = x + chunkOrigin.X * _chunkSize;
-                var gridY = y + chunkOrigin.Y * _chunkSize;
-                tileRef = new TileRef(_gridUid, gridX, gridY, tile);
-                return true;
+                _index = 0;
             }
+
+            var (chunkOrigin, chunk) = _chunkEnumerator.Current;
+            DebugTools.Assert(chunk.FilledTiles > 0, $"Encountered empty chunk while enumerating tiles");
+            var x = (ushort) (_index / _chunkSize);
+            var y = (ushort) (_index % _chunkSize);
+            var tile = chunk.GetTile(x, y);
+            _index++;
+
+            if (_ignoreEmpty && tile.IsEmpty)
+                continue;
+
+            var gridX = x + chunkOrigin.X * _chunkSize;
+            var gridY = y + chunkOrigin.Y * _chunkSize;
+            tileRef = new TileRef(_gridUid, gridX, gridY, tile);
+            return true;
         }
+    }
 }