fix(data-connection-layer): resolve DataConnectionLayer-008,013 — O(1) unsubscribe via reverse index, atomic disconnect guard

src/ScadaLink.DataConnectionLayer/Adapters/RealOpcUaClient.cs

@@ -24,7 +24,10 @@ public class RealOpcUaClient : IOpcUaClient
     // Clear() is undefined behaviour, so they must be ConcurrentDictionary.
     private readonly ConcurrentDictionary<string, MonitoredItem> _monitoredItems = new();
     private readonly ConcurrentDictionary<string, Action<string, object?, DateTime, uint>> _callbacks = new();
-    private volatile bool _connectionLostFired;
+    // DataConnectionLayer-013: int flag toggled with Interlocked.Exchange so the
+    // once-only ConnectionLost guard in OnSessionKeepAlive is atomic, not just visible.
+    // 0 = not fired, 1 = fired.
+    private int _connectionLostFired;
     private OpcUaConnectionOptions _options = new();
     private readonly OpcUaGlobalOptions _globalOptions;
     private readonly ILogger<RealOpcUaClient> _logger;
@@ -112,7 +115,7 @@ public class RealOpcUaClient : IOpcUaClient
             "ScadaLink-DCL-Session", (uint)opts.SessionTimeoutMs, userIdentity, null, cancellationToken);
 
         // Detect server going offline via keep-alive failures
-        _connectionLostFired = false;
+        Interlocked.Exchange(ref _connectionLostFired, 0);
         _session.KeepAlive += OnSessionKeepAlive;
 
         // Store options for monitored item creation
@@ -243,14 +246,15 @@ public class RealOpcUaClient : IOpcUaClient
 
     /// <summary>
     /// Called by the OPC UA SDK when a keep-alive response arrives (or fails).
-    /// When CurrentState is bad, the server is unreachable.
+    /// When CurrentState is bad, the server is unreachable. The once-only guard is an
+    /// atomic compare-and-set, so a burst of failed keep-alives raises
+    /// <see cref="ConnectionLost"/> exactly once.
     /// </summary>
     private void OnSessionKeepAlive(ISession session, KeepAliveEventArgs e)
     {
         if (ServiceResult.IsBad(e.Status))
         {
-            if (_connectionLostFired) return;
-            _connectionLostFired = true;
+            if (Interlocked.Exchange(ref _connectionLostFired, 1) != 0) return;
             ConnectionLost?.Invoke();
         }
     }