feat(dashboard): mirror events via SessionEventDistributor subscriber (fixes dark feed without gRPC client)

src/ZB.MOM.WW.MxGateway.Server/Sessions/SessionEventDistributor.cs

@@ -240,7 +240,19 @@ public sealed class SessionEventDistributor : IAsyncDisposable
     ///     subscriber and completes its channel without disturbing the pump or other
     ///     subscribers.
     /// </summary>
-    public IEventSubscriberLease Register()
+    /// <param name="isInternal">
+    ///     <see langword="true"/> for a gateway-owned internal subscriber (Task 6: the
+    ///     session's dashboard mirror) that must NOT participate in the single-subscriber
+    ///     overflow accounting. An internal subscriber is excluded from the
+    ///     <c>isOnlySubscriber</c> count, so a lone external gRPC subscriber still reports
+    ///     <c>isOnlySubscriber == true</c> (preserving legacy FailFast session-fault
+    ///     behavior) even while the dashboard subscriber is attached; and an internal
+    ///     subscriber that itself overflows always reports <c>isOnlySubscriber == false</c>,
+    ///     so a slow/broken dashboard can never fault the session — it is merely
+    ///     disconnected from the mirror. Defaults to <see langword="false"/> (external
+    ///     subscriber) so every existing call site is unchanged.
+    /// </param>
+    public IEventSubscriberLease Register(bool isInternal = false)
     {
         // The pump is the single writer for this channel; readers are single-consumer
         // (one gRPC stream / dashboard subscriber). Synchronous continuations are
@@ -265,7 +277,7 @@ public sealed class SessionEventDistributor : IAsyncDisposable
             });
 
         long id = Interlocked.Increment(ref _nextSubscriberId);
-        Subscriber subscriber = new(id, channel);
+        Subscriber subscriber = new(id, channel, isInternal);
 
         // The disposed check AND the map add happen under the same lock with no await
         // in between. DisposeAsync sets _disposed=true under this same lock before it
@@ -411,7 +423,14 @@ public sealed class SessionEventDistributor : IAsyncDisposable
         // race window opens — a concurrent second registration could cause Count to read as 1
         // here even with two subscribers, producing a false FailFast that faults a shared
         // session. Resolve before enabling multi-subscriber.
-        bool isOnlySubscriber = _subscribers.Count == 1;
+        //
+        // Task 6: the gateway-owned internal dashboard subscriber is excluded from this
+        // accounting. (a) An internal subscriber that overflows is NEVER the "only subscriber"
+        // — a slow/broken dashboard must never fault the session, only disconnect its own
+        // mirror. (b) Internal subscribers are excluded from the count, so a lone external
+        // gRPC subscriber still reports isOnlySubscriber==true and preserves the legacy
+        // FailFast session-fault behavior even while the dashboard mirror is attached.
+        bool isOnlySubscriber = !subscriber.IsInternal && CountExternalSubscribers() == 1;
 
         _logger.LogDebug(
             "Event distributor disconnecting subscriber {SubscriberId} in session {SessionId} after queue overflow (worker sequence {WorkerSequence}).",
@@ -446,6 +465,22 @@ public sealed class SessionEventDistributor : IAsyncDisposable
         }
     }
 
+    // Counts external (non-internal) subscribers. Drives the isOnlySubscriber FailFast
+    // decision so the gateway-owned internal dashboard subscriber never inflates the count.
+    private int CountExternalSubscribers()
+    {
+        int count = 0;
+        foreach (Subscriber subscriber in _subscribers.Values)
+        {
+            if (!subscriber.IsInternal)
+            {
+                count++;
+            }
+        }
+
+        return count;
+    }
+
     private void CompleteAllSubscribers(Exception? error)
     {
         foreach (Subscriber subscriber in _subscribers.Values)
@@ -593,11 +628,15 @@ public sealed class SessionEventDistributor : IAsyncDisposable
 
     private readonly record struct ReplayEntry(MxEvent Event, DateTimeOffset RetainedAt);
 
-    private sealed class Subscriber(long id, Channel<MxEvent> channel)
+    private sealed class Subscriber(long id, Channel<MxEvent> channel, bool isInternal)
     {
         public long Id { get; } = id;
 
         public Channel<MxEvent> Channel { get; } = channel;
+
+        // True for the gateway-owned internal dashboard subscriber. Excluded from the
+        // single-subscriber overflow accounting so it cannot fault the session.
+        public bool IsInternal { get; } = isInternal;
     }
 
     private sealed class SubscriberLease(SessionEventDistributor distributor, Subscriber subscriber)