Phase 2 PR 6 — close the 2 low findings carried forward from PR 4. Low finding #1 ($Heartbeat probe handle leak in MonitorLoopAsync): the probe calls _proxy.AddItem(connectionHandle, "$Heartbeat") on every monitor tick that observes the connection is past StaleThreshold, but previously discarded the returned item handle — so every probe (one per MonitorInterval, default 5s) leaked one item handle into the MXAccess proxy's internal handle table. Fix: capture the item handle, call RemoveItem(connectionHandle, probeHandle) in the InvokeAsync's finally block so it runs on the same pump turn as the AddItem, best-effort RemoveItem swallow so a dying proxy doesn't throw secondary exceptions out of the probe path. Probe ok becomes probeHandle > 0 so any AddItem that returns 0 (MXAccess's "could not create") counts as a failed probe, matching v1 behavior. Low finding #2 (subscription replay silently swallowed per-tag failures): after a reconnect, the replay loop iterates the pre-reconnect subscription snapshot and calls SubscribeOnPumpAsync for each; previously those failures went into a bare catch { /* skip */ } so an operator had no signal when specific tags failed to re-subscribe — the first indication downstream was a quality drop on OPC UA clients. Fix: new SubscriptionReplayFailedEventArgs (TagReference + Exception) + SubscriptionReplayFailed event on MxAccessClient that fires once per tag that fails to re-subscribe, Log.Warning per failure with the reconnect counter + tag reference, and a summary log line at the end of the replay loop ("{failed} of {total} failed" or "{total} re-subscribed cleanly"). Serilog using + ILogger Log = Serilog.Log.ForContext<MxAccessClient>() added. Tests — MxAccessClientMonitorLoopTests (new file, 2 cases): Heartbeat_probe_calls_RemoveItem_for_every_AddItem constructs a CountingProxy IMxProxy that tracks AddItem/RemoveItem pair counts scoped to the "$Heartbeat" address, runs the client with MonitorInterval=150ms + StaleThreshold=50ms for 700ms, asserts HeartbeatAddCount > 1, HeartbeatAddCount == HeartbeatRemoveCount, OutstandingHeartbeatHandles == 0 after dispose; SubscriptionReplayFailed_fires_for_each_tag_that_fails_to_replay uses a ReplayFailingProxy that throws on the next $Heartbeat probe (to trigger the reconnect path) and throws on the replay-time AddItem for specified tag names ("BadTag.A", "BadTag.B"), subscribes GoodTag.X + BadTag.A + BadTag.B before triggering probe failure, collects SubscriptionReplayFailed args into a ConcurrentBag, asserts exactly 2 events fired and both bad tags are represented — GoodTag.X replays cleanly so it does not fire. Host.Tests csproj gains a Reference to lib/ArchestrA.MxAccess.dll because IMxProxy's MxDataChangeHandler delegate signature mentions MXSTATUS_PROXY and the compiler resolves all delegate parameter types when a test class implements the interface, even if the test code never names the type. No regressions: full Galaxy.Host.Tests Unit suite 26 pass / 0 fail (2 new monitor-loop tests + 9 PR5 historian + 15 pre-existing PostMortemMmf/RecyclePolicy/StaPump/MemoryWatchdog/EndToEndIpc/Handshake). Galaxy.Host builds clean (0 errors, 0 warnings) — the new Serilog.Log.ForContext usage picks up the existing Serilog package ref that PR 4 pulled in for the monitor-loop infrastructure. Both findings were flagged as non-blocking for PR 4 merge and are now resolved alongside whichever merge order the reviewer picks; this PR branches off phase-2-pr4-findings so it can rebase cleanly if PR 4 lands first or be re-based onto master after PR 4 merges.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host/Backend/MxAccess/SubscriptionReplayFailedEventArgs.cs

@@ -0,0 +1,20 @@
+using System;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.MxAccess;
+
+/// <summary>
+///     Fired by <see cref="MxAccessClient.SubscriptionReplayFailed"/> when a previously-active
+///     subscription fails to be restored after a reconnect. The backend should treat the tag as
+///     unhealthy until the next successful resubscribe.
+/// </summary>
+public sealed class SubscriptionReplayFailedEventArgs : EventArgs
+{
+    public SubscriptionReplayFailedEventArgs(string tagReference, Exception exception)
+    {
+        TagReference = tagReference;
+        Exception = exception;
+    }
+
+    public string TagReference { get; }
+    public Exception Exception { get; }
+}