fix(health-monitoring): resolve HealthMonitoring-003..009 — central offline grace, register unknown-site heartbeats, test coverage

tests/ScadaLink.HealthMonitoring.Tests/CentralHealthAggregatorTests.cs

@@ -219,6 +219,94 @@ public class CentralHealthAggregatorTests
         Assert.True(final.IsOnline);
     }
 
+    /// <summary>
+    /// HealthMonitoring-007 regression: a heartbeat for a site that has not yet
+    /// sent a full report (e.g. immediately after a central restart/failover, when
+    /// the aggregator's in-memory state is empty) must register the site as online
+    /// rather than being silently discarded. Otherwise reachable sites show as
+    /// "unknown" for up to a full report interval during the failover window.
+    /// </summary>
+    [Fact]
+    public void MarkHeartbeat_RegistersUnknownSite_AsOnlineAwaitingReport()
+    {
+        var now = _timeProvider.GetUtcNow();
+
+        _aggregator.MarkHeartbeat("site-new", now);
+
+        var state = _aggregator.GetSiteState("site-new");
+        Assert.NotNull(state);
+        Assert.True(state.IsOnline);
+        Assert.Null(state.LatestReport);
+        Assert.Equal(now, state.LastHeartbeatAt);
+    }
+
+    [Fact]
+    public void MarkHeartbeat_KeepsSiteOnline_BetweenReports()
+    {
+        _aggregator.ProcessReport(MakeReport("site-1", 1));
+
+        // Time advances past the offline timeout, but heartbeats keep arriving.
+        _timeProvider.Advance(TimeSpan.FromSeconds(45));
+        _aggregator.MarkHeartbeat("site-1", _timeProvider.GetUtcNow());
+        _timeProvider.Advance(TimeSpan.FromSeconds(45));
+        _aggregator.MarkHeartbeat("site-1", _timeProvider.GetUtcNow());
+
+        _aggregator.CheckForOfflineSites();
+
+        Assert.True(_aggregator.GetSiteState("site-1")!.IsOnline);
+    }
+
+    [Fact]
+    public void MarkHeartbeat_BringsOfflineSiteBackOnline()
+    {
+        _aggregator.ProcessReport(MakeReport("site-1", 1));
+
+        _timeProvider.Advance(TimeSpan.FromSeconds(61));
+        _aggregator.CheckForOfflineSites();
+        Assert.False(_aggregator.GetSiteState("site-1")!.IsOnline);
+
+        _aggregator.MarkHeartbeat("site-1", _timeProvider.GetUtcNow());
+        Assert.True(_aggregator.GetSiteState("site-1")!.IsOnline);
+    }
+
+    /// <summary>
+    /// HealthMonitoring-005 regression: the synthetic "central" site has no
+    /// heartbeat source — its LastHeartbeatAt is only bumped by the 30s
+    /// CentralHealthReportLoop self-report. A single skipped/late self-report
+    /// (leader GC pause, brief stall, mid-failover) would leave it with no signal
+    /// for &gt;60s and flap it offline even though the central cluster is healthy.
+    /// The "central" keyspace entry must get a longer offline grace than real sites.
+    /// </summary>
+    [Fact]
+    public void OfflineDetection_CentralSite_HasLongerGraceThanRealSites()
+    {
+        _aggregator.ProcessReport(MakeReport(CentralHealthReportLoop.CentralSiteId, 1));
+        _aggregator.ProcessReport(MakeReport("site-1", 1));
+
+        // One missed central self-report (~30s) plus the normal 60s site timeout:
+        // a real site would already be offline here, but central must not be —
+        // it only gets one self-report every 30s, so 60s is barely two reports.
+        _timeProvider.Advance(TimeSpan.FromSeconds(75));
+        _aggregator.CheckForOfflineSites();
+
+        Assert.False(_aggregator.GetSiteState("site-1")!.IsOnline);
+        Assert.True(
+            _aggregator.GetSiteState(CentralHealthReportLoop.CentralSiteId)!.IsOnline,
+            "central must survive a single missed self-report");
+    }
+
+    [Fact]
+    public void OfflineDetection_CentralSite_StillGoesOfflineOnGenuineLoss()
+    {
+        _aggregator.ProcessReport(MakeReport(CentralHealthReportLoop.CentralSiteId, 1));
+
+        // Well beyond even the central grace window — genuine total loss.
+        _timeProvider.Advance(TimeSpan.FromMinutes(10));
+        _aggregator.CheckForOfflineSites();
+
+        Assert.False(_aggregator.GetSiteState(CentralHealthReportLoop.CentralSiteId)!.IsOnline);
+    }
+
     [Fact]
     public void SequenceNumberReset_RejectedUntilExceedsPrevMax()
     {