fix(tests): single shared bootstrap; idempotent SerializationThreadWorker.Exit (#2473)

## Problem

Running the full `UOContent.Tests` suite, the test host **hangs ~2.5 minutes at shutdown and then crashes** (`Test host process crashed` / run aborted). The tests themselves are fine — they complete in ~1s — but the process can't exit.

Captured via `--blame-hang` dump. The blocking thread:

```
System.Threading.WaitHandle.WaitOne()
Server.SerializationThreadWorker.Sleep()        SerializationThreadWorker.cs:54  (_stopEvent.WaitOne())
Server.SerializationThreadWorker.Exit()         SerializationThreadWorker.cs:61
Server.World.ExitSerializationThreads()         World.cs:429
Server.Tests.UOContentFixture..ctor()
```

### Root cause

Both collection fixtures (`UOContentFixture` and `PathfindingTestFixture`) each run the **full process-global ModernUO bootstrap**. `World.Load()` is guarded to run once per process, so the **second** fixture's `World.Load()` is a no-op and does **not** respawn the serialization workers — but `World.ExitSerializationThreads()` is **not** guarded, so the second fixture calls `Exit()` on workers whose threads have already terminated. `Exit()` → `Sleep()` → `_stopEvent.WaitOne()` then blocks forever (a dead thread never sets the event). The first collection's tests run; the second collection's fixture deadlocks in its constructor; the host eventually gets killed.

This is why single-collection (filtered) runs were fine — only one fixture ever bootstraps — but the full suite hangs. It's not a parallelization race: even strictly sequential, the second fixture deadlocks.

## Fix

**(a) Engine — idempotent `SerializationThreadWorker.Exit()`**
A second `Exit()` is now a safe no-op instead of a permanent block. Only the owning (main) thread calls `Exit()`, so no synchronization is needed, and the single-call production shutdown path is unchanged.

**(b) Tests — one shared bootstrap, strictly sequential collections**
- New `TestServerBootstrap.EnsureInitialized()` runs the superset global init **exactly once per process** (lock + once-flag).
- `UOContentFixture` / `PathfindingTestFixture` slim down to delegate to it and no longer tear down global state (which the single-bootstrap model owns for the host's lifetime).
- `[assembly: CollectionBehavior(DisableTestParallelization = true)]` so collections never overlap.

## Result

| | Before | After |
|---|---|---|
| Tests run (full suite) | 258 (UOContent collection deadlocked) | **418** |
| Outcome | 2.5-min hang → host crash | **418 passed, clean exit** |
| Wall time | killed | **~7s** |

Projects/Server.Tests/Fixtures/ServerFixture.cs

@@ -1,4 +1,3 @@
-using System;
 using Xunit;
 
 namespace Server.Tests;
@@ -8,7 +7,7 @@ namespace Server.Tests;
 /// Configure client path via MODERNUO_CLIENT_PATH environment variable or place files at C:\Ultima Online Classic.
 /// </summary>
 [CollectionDefinition("Sequential Server Tests", DisableParallelization = true)]
-public class ServerFixture : ICollectionFixture<ServerFixture>, IDisposable
+public class ServerFixture : ICollectionFixture<ServerFixture>
 {
     /// <summary>
     /// True if TileData was successfully loaded from client files.
@@ -35,13 +34,8 @@ public class ServerFixture : ICollectionFixture<ServerFixture>, IDisposable
     /// </summary>
     public static ushort SurfaceImpassableTileId => TestServerInitializer.SurfaceImpassableTileId;
 
-    public ServerFixture()
+    // Global init runs exactly once via the shared, guarded TestServerInitializer. The single
-    {
+    // bootstrap owns global state for the lifetime of the test host, so there is no
-        TestServerInitializer.Initialize(loadTileData: true);
+    // per-collection teardown — matching UOContent.Tests' TestServerInitializer pattern.
-    }
+    public ServerFixture() => TestServerInitializer.Initialize(loadTileData: true);
-
-    public void Dispose()
-    {
-        Timer.Init(0);
-    }
 }

Projects/Server.Tests/Fixtures/TestServerInitializer.cs

@@ -10,7 +10,7 @@ namespace Server.Tests;
 /// Shared server initialization logic for test fixtures.
 /// Ensures the server is only initialized once across all test collections.
 /// </summary>
-public static class TestServerInitializer
+internal static class TestServerInitializer
 {
     private const string DefaultDataDirectory = @"C:\Ultima Online Classic";
     private static bool _initialized;

Projects/Server/Serialization/SerializationThreadWorker.cs

@@ -29,6 +29,7 @@ public class SerializationThreadWorker
     private readonly AutoResetEvent _stopEvent; // Main thread waits for the worker finish draining
     private bool _pause;
     private bool _exit;
+    private bool _exited;
     private byte[] _heap;
 
     private readonly ConcurrentQueue<IGenericSerializable> _entities;
@@ -56,6 +57,12 @@ public class SerializationThreadWorker
 
     public void Exit()
     {
+        if (_exited)
+        {
+            return;
+        }
+
+        _exited = true;
         _exit = true;
         Wake();
         Sleep();

Projects/UOContent.Tests/Fixtures/PathfindingTestFixture.cs

@@ -1,92 +1,12 @@
-using System;
-using System.IO;
-using System.Reflection;
-using Server.Items;
-using Server.Misc;
-using Server.Movement;
-using Server.Tests.Maps;
 using Xunit;
 
 namespace Server.Tests.Pathfinding;
 
 [CollectionDefinition("Sequential Pathfinding Tests", DisableParallelization = true)]
-public class PathfindingTestFixture : ICollectionFixture<PathfindingTestFixture>, IDisposable
+public class PathfindingTestFixture : ICollectionFixture<PathfindingTestFixture>
 {
-    public PathfindingTestFixture()
+    // Shares the single process-wide bootstrap (TestServerInitializer, in the parent
-    {
+    // Server.Tests namespace). The superset bootstrap already loads the UO client tile data
-        Core.ApplicationAssembly = Assembly.GetExecutingAssembly();
+    // and configures movement/pathfinding, so this collection needs no extra setup.
-        Core.LoopContext = new EventLoopContext();
+    public PathfindingTestFixture() => TestServerInitializer.Initialize();
-        Core.Expansion = Expansion.EJ;
-
-        ServerConfiguration.Load(true);
-        ServerConfiguration.AssemblyDirectories.Add(Core.BaseDirectory);
-
-        var clientFiles = Environment.GetEnvironmentVariable("MODERNUO_TEST_DATA_DIR")
-                          ?? @"C:\Ultima Online Classic";
-        ServerConfiguration.DataDirectories.Add(clientFiles);
-
-        AssemblyHandler.LoadAssemblies(["Server.dll", "UOContent.dll"]);
-
-        SkillsInfo.Configure();
-        Server.Network.NetState.Configure();
-        TestMapDefinitions.ConfigureTestMapDefinitions();
-
-        World.Configure();
-        Timer.Init(0);
-        RaceDefinitions.Configure();
-        MovementImpl.Configure();
-        PathFollower.Configure();
-        World.Load();
-        World.ExitSerializationThreads();
-        DecayScheduler.Configure();
-
-        // TileData's static cctor short-circuits when running under xUnit
-        // (see Server/TileData.cs:295). Force-load via reflection so LandTable/ItemTable
-        // flags are populated; without this, every tile reads as flag=None and
-        // MovementImpl.CheckMovement treats everything as walkable.
-        ForceLoadTileData();
-
-        VerifyTrammelTileDataLoaded();
-    }
-
-    private static void ForceLoadTileData()
-    {
-        var loadMethod = typeof(TileData).GetMethod(
-            "Load",
-            BindingFlags.Static | BindingFlags.NonPublic
-        );
-        if (loadMethod == null)
-        {
-            throw new InvalidOperationException(
-                "TileData.Load not found via reflection — engine may have refactored."
-            );
-        }
-        loadMethod.Invoke(null, null);
-    }
-
-    private static void VerifyTrammelTileDataLoaded()
-    {
-        var trammel = Map.Maps[1];
-        if (trammel == null)
-        {
-            throw new InvalidOperationException(
-                "Trammel (mapId=1) was not registered. Check TestMapDefinitions."
-            );
-        }
-
-        var tile = trammel.Tiles.GetLandTile(1500, 1600);
-        if (tile.ID == 0)
-        {
-            throw new InvalidOperationException(
-                $"Trammel tile data did not load — GetLandTile(1500,1600) returned ID 0. " +
-                $"Verify Distribution/Data/map1*.mul (or map1LegacyMUL.uop) is present at " +
-                $"{Path.Combine(Core.BaseDirectory, "Data")}."
-            );
-        }
-    }
-
-    public void Dispose()
-    {
-        Timer.Init(0);
-    }
 }

Projects/UOContent.Tests/Fixtures/TestServerInitializer.cs

@@ -0,0 +1,115 @@
+using System;
+using System.IO;
+using System.Reflection;
+using System.Threading;
+using Server.Items;
+using Server.Misc;
+using Server.Movement;
+using Server.Tests.Maps;
+
+namespace Server.Tests;
+
+/// <summary>
+/// Single, process-wide ModernUO bootstrap for the UOContent test host. Mirrors Server.Tests'
+/// TestServerInitializer in name and shape; kept as a separate (non-shared) copy because this
+/// one loads the UOContent assembly and configures the UOContent-specific systems. Both types
+/// are <c>internal</c> so the shared name stays scoped to each assembly.
+///
+/// ModernUO bootstraps its global singletons (Core, ServerConfiguration, AssemblyHandler,
+/// NetState/io-ring, World, Timer, the serialization workers, and TileData) exactly once per
+/// process. <see cref="World.Load"/> is guarded to run once, and
+/// <see cref="World.ExitSerializationThreads"/> must run once against the live workers. Each
+/// xUnit collection gets its own fixture instance, so this guard makes the bootstrap run a
+/// single time regardless of how many collection fixtures are constructed. The two stateful
+/// collections use <c>[CollectionDefinition(DisableParallelization = true)]</c> so they never
+/// overlap; pure tests still run in parallel.
+/// </summary>
+internal static class TestServerInitializer
+{
+    private static bool _initialized;
+    private static readonly Lock _lock = new();
+
+    public static void Initialize()
+    {
+        lock (_lock)
+        {
+            if (_initialized)
+            {
+                return;
+            }
+
+            Core.ApplicationAssembly = Assembly.GetExecutingAssembly();
+            Core.LoopContext = new EventLoopContext();
+            Core.Expansion = Expansion.EJ;
+
+            ServerConfiguration.Load(true);
+            ServerConfiguration.AssemblyDirectories.Add(Core.BaseDirectory);
+
+            // Required for the pathfinding tests (real .mul tile data). Harmless for the rest.
+            var clientFiles = Environment.GetEnvironmentVariable("MODERNUO_TEST_DATA_DIR")
+                              ?? @"C:\Ultima Online Classic";
+            ServerConfiguration.DataDirectories.Add(clientFiles);
+
+            AssemblyHandler.LoadAssemblies(["Server.dll", "UOContent.dll"]);
+
+            SkillsInfo.Configure();
+            Server.Network.NetState.Configure();
+            TestMapDefinitions.ConfigureTestMapDefinitions();
+
+            World.Configure();
+            Timer.Init(0);
+            RaceDefinitions.Configure();
+            MovementImpl.Configure();
+            PathFollower.Configure();
+            World.Load();
+            World.ExitSerializationThreads();
+            DecayScheduler.Configure();
+
+            // TileData's static cctor short-circuits when running under xUnit
+            // (see Server/TileData.cs:295). Force-load via reflection so LandTable/ItemTable
+            // flags are populated; without this, every tile reads as flag=None and
+            // MovementImpl.CheckMovement treats everything as walkable.
+            ForceLoadTileData();
+
+            VerifyTrammelTileDataLoaded();
+
+            _initialized = true;
+        }
+    }
+
+    private static void ForceLoadTileData()
+    {
+        var loadMethod = typeof(TileData).GetMethod(
+            "Load",
+            BindingFlags.Static | BindingFlags.NonPublic
+        );
+        if (loadMethod == null)
+        {
+            throw new InvalidOperationException(
+                "TileData.Load not found via reflection — engine may have refactored."
+            );
+        }
+        loadMethod.Invoke(null, null);
+    }
+
+    private static void VerifyTrammelTileDataLoaded()
+    {
+        var trammel = Map.Maps[1];
+        if (trammel == null)
+        {
+            throw new InvalidOperationException(
+                "Trammel (mapId=1) was not registered. Check TestMapDefinitions."
+            );
+        }
+
+        var tile = trammel.Tiles.GetLandTile(1500, 1600);
+        if (tile.ID == 0)
+        {
+            throw new InvalidOperationException(
+                $"Trammel tile data did not load — GetLandTile(1500,1600) returned ID 0. " +
+                $"Verify Distribution/Data/map1*.mul (or map1LegacyMUL.uop) is present at " +
+                $"{Path.Combine(Core.BaseDirectory, "Data")}."
+            );
+        }
+    }
+}

Projects/UOContent.Tests/Fixtures/UOContentFixture.cs

@@ -1,63 +1,13 @@
-using System;
-using System.Reflection;
-using Server.Items;
-using Server.Misc;
-using Server.Tests.Maps;
 using Xunit;
 
 namespace Server.Tests;
 
 [CollectionDefinition("Sequential UOContent Tests", DisableParallelization = true)]
-public class UOContentFixture : ICollectionFixture<UOContentFixture>, IDisposable
+public class UOContentFixture : ICollectionFixture<UOContentFixture>
 {
-    public UOContentFixture()
+    // All process-global initialization lives in TestServerInitializer and runs exactly once,
-    {
+    // shared across every collection fixture. Tearing down global state here is intentionally
-        Core.ApplicationAssembly = Assembly.GetExecutingAssembly();
+    // omitted: the world/serialization workers are initialized once and reused for the whole
-        Core.LoopContext = new EventLoopContext();
+    // test host, so there is nothing per-collection to dispose.
-        Core.Expansion = Expansion.EJ;
+    public UOContentFixture() => TestServerInitializer.Initialize();
-
-        // Load Configurations
-        ServerConfiguration.Load(true);
-
-        // Load UOContent.dll into the type resolver
-        ServerConfiguration.AssemblyDirectories.Add(Core.BaseDirectory);
-        AssemblyHandler.LoadAssemblies(["Server.dll", "UOContent.dll"]);
-
-        // Load Skills
-        SkillsInfo.Configure();
-
-        // Configure networking (initializes RingSocketManager for tests)
-        Server.Network.NetState.Configure();
-
-        // Configure / Initialize
-        TestMapDefinitions.ConfigureTestMapDefinitions();
-
-        // Configure the world
-        World.Configure();
-
-        Timer.Init(0);
-
-        // Configure Races
-        RaceDefinitions.Configure();
-
-        // Load the world
-        World.Load();
-
-        World.ExitSerializationThreads();
-
-        DecayScheduler.Configure();
-    }
-
-    private static int _counter;
-
-    public void Dispose()
-    {
-        _counter++;
-
-        if (_counter > 1)
-        {
-            throw new Exception("NO!");
-        }
-        Timer.Init(0);
-    }
 }