server(alarms): provider-mode gauge startup baseline; reconcile-lock comment; de-flake monitor test

src/ZB.MOM.WW.MxGateway.Server/Alarms/GatewayAlarmMonitor.cs

@@ -167,6 +167,10 @@ public sealed class GatewayAlarmMonitor : BackgroundService, IGatewayAlarmServic
             _providerSince = DateTimeOffset.UtcNow;
         }
 
+        // Align the observable gauge with the Alarmmgr baseline without recording
+        // a switch — the gauge was 0 (unknown) from construction until now.
+        _metrics.SetAlarmProviderMode(ModeToInt(AlarmProviderMode.Alarmmgr));
+
         GatewaySession session = await _sessionManager.OpenSessionAsync(
                 new SessionOpenRequest(BackendName, MonitorClientName, Guid.NewGuid().ToString("N"), CommandTimeout: null),
                 MonitorClientName,
@@ -405,6 +409,10 @@ public sealed class GatewayAlarmMonitor : BackgroundService, IGatewayAlarmServic
 
         try
         {
+            // Intentionally awaited OUTSIDE _sync: ReconcileAsync acquires _sync itself,
+            // so holding it across the await here would deadlock. Subscribers therefore
+            // see the ProviderStatus push (above) slightly before the cache is re-seeded
+            // by the reconcile — an accepted brief inconsistency.
             await ReconcileAsync(sessionId, cancellationToken).ConfigureAwait(false);
         }
         catch (OperationCanceledException)

src/ZB.MOM.WW.MxGateway.Server/Metrics/GatewayMetrics.cs

@@ -402,6 +402,12 @@ public sealed class GatewayMetrics : IDisposable
             new KeyValuePair<string, object?>("reason", reason));
     }
 
+    /// <summary>Sets the current alarm provider-mode gauge without recording a switch (e.g. startup baseline).</summary>
+    public void SetAlarmProviderMode(int mode)
+    {
+        lock (_syncRoot) { _alarmProviderMode = mode; }
+    }
+
     /// <summary>
     /// Returns a snapshot of all current metric values.
     /// </summary>

src/ZB.MOM.WW.MxGateway.Tests/Alarms/GatewayAlarmMonitorProviderModeTests.cs

@@ -56,8 +56,11 @@ public sealed class GatewayAlarmMonitorProviderModeTests
         await monitor.StartAsync(cts.Token);
         await sessions.WaitForSubscribeAsync(WaitTimeout);
 
-        // Subscribe a live feed reader, drain its first (provider status) message.
+        // Subscribe a live feed reader. Gate emitting the mode-change event until the
+        // reader has consumed its baseline ProviderStatus message, avoiding a race where
+        // the event arrives before the subscriber is registered and draining its snapshot.
         List<AlarmFeedMessage> received = [];
+        TaskCompletionSource baselineReceived = new(TaskCreationOptions.RunContinuationsAsynchronously);
         using CancellationTokenSource streamCts = new();
         Task reader = Task.Run(async () =>
         {
@@ -65,7 +68,15 @@ public sealed class GatewayAlarmMonitorProviderModeTests
             {
                 await foreach (AlarmFeedMessage message in monitor.StreamAsync(null, streamCts.Token))
                 {
-                    lock (received) { received.Add(message); }
+                    lock (received)
+                    {
+                        received.Add(message);
+                        // Signal once the first message (baseline ProviderStatus) has arrived.
+                        if (received.Count == 1)
+                        {
+                            baselineReceived.TrySetResult();
+                        }
+                    }
                 }
             }
             catch (OperationCanceledException)
@@ -74,10 +85,13 @@ public sealed class GatewayAlarmMonitorProviderModeTests
             }
         });
 
+        // Wait for the baseline ProviderStatus to arrive before emitting the mode change,
+        // so the subscriber is registered and the event is not dropped.
+        await baselineReceived.Task.WaitAsync(WaitTimeout);
+
         // Emit the worker event that flips the provider into subtag mode.
         sessions.EmitEvent(new MxEvent
         {
-            Family = MxEventFamily.OnAlarmProviderModeChanged,
             OnAlarmProviderModeChanged = new OnAlarmProviderModeChangedEvent
             {
                 Mode = AlarmProviderMode.Subtag,