Fix reliability findings

src/MxGateway.Tests/Gateway/Sessions/SessionManagerTests.cs

@@ -179,6 +179,48 @@ public sealed class SessionManagerTests
         Assert.Equal(1, workerClient.KillCount);
     }
 
+    [Fact]
+    public async Task CloseSessionAsync_WhenWorkerShutdownFails_RemovesSessionAndReleasesSlot()
+    {
+        FakeWorkerClient failingWorkerClient = new()
+        {
+            ShutdownException = new WorkerClientException(
+                WorkerClientErrorCode.ShutdownTimeout,
+                "Worker shutdown timed out."),
+        };
+        FakeWorkerClient replacementWorkerClient = new();
+        SessionRegistry registry = new();
+        using GatewayMetrics metrics = new();
+        SessionManager manager = CreateManager(
+            new QueueingSessionWorkerClientFactory(failingWorkerClient, replacementWorkerClient),
+            registry,
+            metrics,
+            CreateOptions(maxSessions: 1));
+        GatewaySession firstSession = await manager.OpenSessionAsync(
+            CreateOpenRequest(),
+            "client-1",
+            CancellationToken.None);
+        metrics.EventReceived(firstSession.SessionId, MxEventFamily.OnDataChange.ToString());
+
+        SessionManagerException exception = await Assert.ThrowsAsync<SessionManagerException>(
+            async () => await manager.CloseSessionAsync(firstSession.SessionId, CancellationToken.None));
+        GatewaySession secondSession = await manager.OpenSessionAsync(
+            CreateOpenRequest(),
+            "client-2",
+            CancellationToken.None);
+
+        Assert.Equal(SessionManagerErrorCode.CloseFailed, exception.ErrorCode);
+        Assert.False(manager.TryGetSession(firstSession.SessionId, out _));
+        Assert.True(manager.TryGetSession(secondSession.SessionId, out _));
+        Assert.Equal(1, registry.Count);
+        Assert.Equal(1, failingWorkerClient.KillCount);
+        Assert.Equal(1, failingWorkerClient.DisposeCount);
+        GatewayMetricsSnapshot snapshot = metrics.GetSnapshot();
+        Assert.Equal(0, snapshot.SessionsClosed);
+        Assert.False(snapshot.EventsBySession.ContainsKey(firstSession.SessionId));
+        Assert.Equal(1, snapshot.OpenSessions);
+    }
+
     [Fact]
     public async Task OpenSessionAsync_WhenWorkerCreationFails_RemovesSessionFromRegistry()
     {
@@ -254,14 +296,14 @@ public sealed class SessionManagerTests
             metrics ?? new GatewayMetrics());
     }
 
-    private static GatewayOptions CreateOptions()
+    private static GatewayOptions CreateOptions(int maxSessions = 64)
     {
         return new GatewayOptions
         {
             Sessions = new SessionOptions
             {
                 DefaultCommandTimeoutSeconds = 30,
-                MaxSessions = 64,
+                MaxSessions = maxSessions,
             },
             Worker = new WorkerOptions
             {
@@ -359,6 +401,8 @@ public sealed class SessionManagerTests
 
         public int KillCount { get; private set; }
 
+        public int DisposeCount { get; private set; }
+
         public Exception? ShutdownException { get; init; }
 
         public WorkerCommand? LastCommand { get; private set; }
@@ -424,6 +468,7 @@ public sealed class SessionManagerTests
 
         public ValueTask DisposeAsync()
         {
+            DisposeCount++;
             return ValueTask.CompletedTask;
         }
     }