fix(dashboard): generation-scoped idle gate in UnsubscribeAsync — no zero-subscriber pump survives

src/ZB.MOM.WW.MxGateway.Server/Dashboard/DashboardSnapshotFeed.cs

@@ -14,8 +14,9 @@ namespace ZB.MOM.WW.MxGateway.Server.Dashboard;
 /// <remarks>
 /// <para>
 /// The pump is idle-gated: it starts when the first subscriber arrives and is cancelled and
-/// awaited when the last one leaves, so an unwatched gateway runs no timer and builds no
-/// snapshots. Successive pumps are chained through <c>_pumpTask</c>, so a rapid
+/// awaited when the last subscriber <em>of the live generation</em> leaves, so an unwatched
+/// gateway runs no timer and builds no snapshots. Successive pumps are chained through
+/// <c>_pumpTask</c>, so a rapid
 /// unsubscribe/resubscribe restarts a fresh pump without ever running two enumerations at
 /// once.
 /// </para>
@@ -116,10 +117,25 @@ public sealed class DashboardSnapshotFeed : IDashboardSnapshotFeed
         Task pump;
         lock (_gate)
         {
-            if (!_subscribers.Remove(subscription) || _subscribers.Count != 0)
+            if (!_subscribers.Remove(subscription))
             {
-                // Either the pump already detached this subscription (it completed or
-                // faulted), or other viewers are still watching.
+                // The pump already detached this subscription (it completed or faulted).
+                return;
+            }
+
+            if (subscription.Generation != _generation)
+            {
+                // This viewer belonged to a generation that has already ended. The live
+                // pump — if there is one — serves other viewers and must not be cancelled
+                // on their behalf; the dying pump is stopping under its own steam.
+                return;
+            }
+
+            if (HasSubscribersLocked(_generation))
+            {
+                // Other viewers are still watching the live generation. Counting the whole
+                // list here would be wrong: subscribers of an ending generation linger in it
+                // until that pump's Reset runs, and they must not hold the idle gate open.
                 return;
             }
 
@@ -149,6 +165,22 @@ public sealed class DashboardSnapshotFeed : IDashboardSnapshotFeed
         }
     }
 
+    /// <summary>Reports whether any subscriber is still being served by a generation.</summary>
+    /// <param name="generation">The generation to look for.</param>
+    /// <returns>True when at least one subscriber carries that generation.</returns>
+    private bool HasSubscribersLocked(long generation)
+    {
+        foreach (Subscription subscriber in _subscribers)
+        {
+            if (subscriber.Generation == generation)
+            {
+                return true;
+            }
+        }
+
+        return false;
+    }
+
     /// <summary>
     /// Starts a pump generation. Must be called while holding <c>_gate</c>; the caller adds
     /// the subscribers that belong to the returned generation.