fix(GWC-33): unregister terminal worker queue gauge

src/ZB.MOM.WW.MxGateway.Server/Workers/WorkerClient.cs

@@ -444,11 +444,10 @@ public sealed class WorkerClient : IWorkerClient
             {
                 _state = WorkerClientState.Closed;
             }
+
+            UnregisterEventQueueDepthSource();
         }
 
-        // Drop out of the worker queue-depth gauge before teardown: whatever this client still holds
-        // is about to be discarded, and the sum must not keep counting a client that is going away.
-        _eventQueueDepthRegistration?.Dispose();
         KillOwnedProcess("Dispose");
         _stopCts.Cancel();
         _outboundEnvelopes.Writer.TryComplete();
@@ -1092,6 +1091,7 @@ public sealed class WorkerClient : IWorkerClient
             }
 
             _state = WorkerClientState.Closed;
+            UnregisterEventQueueDepthSource();
         }
 
         _stopCts.Cancel();
@@ -1126,6 +1126,7 @@ public sealed class WorkerClient : IWorkerClient
             }
 
             _state = WorkerClientState.Faulted;
+            UnregisterEventQueueDepthSource();
         }
 
         _stopCts.Cancel();
@@ -1139,6 +1140,16 @@ public sealed class WorkerClient : IWorkerClient
         _logger.LogWarning(exception, "Worker client faulted for session {SessionId}: {Message}", SessionId, message);
     }
 
+    /// <summary>
+    ///     Removes this terminal client's abandoned backlog from the gateway-wide queue-depth
+    ///     gauge. The registration handle is idempotent, so terminal transitions and later disposal
+    ///     may all call this safely while the faulted client remains retained for diagnostics.
+    /// </summary>
+    private void UnregisterEventQueueDepthSource()
+    {
+        _eventQueueDepthRegistration?.Dispose();
+    }
+
     private void KillOwnedProcess(string reason)
     {
         WorkerProcessHandle? processHandle = _connection.ProcessHandle;

src/ZB.MOM.WW.MxGateway.Tests/Gateway/Workers/WorkerClientTests.cs

@@ -1147,6 +1147,41 @@ public sealed class WorkerClientTests
         Assert.Equal(0, metrics.GetSnapshot().WorkerEventQueueDepth);
     }
 
+    /// <summary>
+    ///     GWC-33: a faulted client can remain retained for diagnostics, but its abandoned event
+    ///     backlog must stop contributing to the gateway-wide queue-depth gauge as soon as the
+    ///     client enters a terminal state rather than waiting for disposal.
+    /// </summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    [Fact]
+    public async Task WorkerEventQueueDepthGauge_WhenClientFaults_StopsCountingRetainedClient()
+    {
+        using GatewayMetrics metrics = new();
+        await using PipePair pipePair = await PipePair.CreateAsync();
+        await using WorkerClient client = CreateClient(
+            pipePair,
+            new WorkerClientOptions
+            {
+                EventChannelCapacity = 4,
+                EventChannelFullModeTimeout = TimeSpan.FromMinutes(5),
+                HeartbeatGrace = TimeSpan.FromSeconds(30),
+                HeartbeatCheckInterval = TimeSpan.FromSeconds(30),
+            },
+            metrics: metrics);
+        await CompleteHandshakeAsync(client, pipePair);
+
+        await pipePair.WriteAsync(CreateEventEnvelope(sequence: 1, MxEventFamily.OnDataChange));
+        await WaitUntilAsync(
+            () => metrics.GetSnapshot().WorkerEventQueueDepth == 1,
+            TestTimeout);
+
+        await pipePair.WriteAsync(CreateWorkerFaultEnvelope("scripted retained-client fault"));
+        await WaitUntilAsync(() => client.State == WorkerClientState.Faulted, TestTimeout);
+
+        Assert.Equal(WorkerClientState.Faulted, client.State);
+        Assert.Equal(0, metrics.GetSnapshot().WorkerEventQueueDepth);
+    }
+
     private static WorkerClient CreateClient(
         PipePair pipePair,
         WorkerClientOptions? options = null,