worker(alarms): net48 index fix; enforce ProbeIntervalSeconds; OOM-safe catch; reset-on-failure test

src/ZB.MOM.WW.MxGateway.Worker.Tests/MxAccess/FailoverAlarmConsumerTests.cs

@@ -159,7 +159,7 @@ public sealed class FailoverAlarmConsumerTests
 
         sut.Subscribe(@"\\HOST\Galaxy!Area"); // threshold=1 → Subtag (mode change 1)
         Assert.Single(changes);
-        Assert.Equal(AlarmProviderMode.Subtag, changes[^1].Mode);
+        Assert.Equal(AlarmProviderMode.Subtag, changes[changes.Count - 1].Mode);
 
         // Primary heals: PollOnce stops throwing. ProbeOnce should call only
         // PollOnce (not Subscribe) to detect recovery.
@@ -172,9 +172,9 @@ public sealed class FailoverAlarmConsumerTests
         sut.ProbeOnce(); // cleanProbes=2 → failback to Alarmmgr (mode change 2)
 
         Assert.Equal(2, changes.Count);
-        Assert.Equal(AlarmProviderMode.Alarmmgr, changes[^1].Mode);
+        Assert.Equal(AlarmProviderMode.Alarmmgr, changes[changes.Count - 1].Mode);
         Assert.Equal(AlarmProviderMode.Alarmmgr, sut.Mode);
-        Assert.Equal(0, changes[^1].HResult);
+        Assert.Equal(0, changes[changes.Count - 1].HResult);
 
         // ProbeOnce must not have called Subscribe at all during probing.
         Assert.Equal(subscribeCountAfterFailover, primary.SubscribeCount);
@@ -263,4 +263,32 @@ public sealed class FailoverAlarmConsumerTests
         Assert.Equal(22, sut.AcknowledgeByGuid(Guid.NewGuid(), "c", "n", "node", "dom", "full"));
         Assert.Equal(22, sut.AcknowledgeByName("a", "p", "g", "c", "n", "node", "dom", "full"));
     }
+
+    /// <summary>
+    ///     Proves that an intermittent failure during failback probing resets the
+    ///     clean-probe counter to zero, requiring a fresh unbroken run of
+    ///     <see cref="FailoverSettings.StableProbes"/> before failing back.
+    /// </summary>
+    [Fact]
+    public void FailbackProbe_IntermittentFailure_ResetsCleanCount()
+    {
+        var primary = new FlakyPrimary { ThrowOnPoll = true };
+        var standby = new StubStandby();
+        using var sut = new FailoverAlarmConsumer(primary, standby, new FailoverSettings(threshold: 1, probeIntervalSeconds: 0, stableProbes: 3));
+        sut.Subscribe(@"\\HOST\Galaxy!Area");           // threshold=1 → switch to Subtag
+        Assert.Equal(AlarmProviderMode.Subtag, sut.Mode);
+
+        primary.ThrowOnPoll = false;
+        sut.ProbeOnce();  // clean 1
+        sut.ProbeOnce();  // clean 2
+        primary.ThrowOnPoll = true;
+        sut.ProbeOnce();  // fails → reset to 0
+        Assert.Equal(AlarmProviderMode.Subtag, sut.Mode);
+
+        primary.ThrowOnPoll = false;
+        sut.ProbeOnce();  // clean 1
+        sut.ProbeOnce();  // clean 2
+        sut.ProbeOnce();  // clean 3 → failback
+        Assert.Equal(AlarmProviderMode.Alarmmgr, sut.Mode);
+    }
 }