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

src/ScadaLink.DataConnectionLayer/Adapters/OpcUaDataConnection.cs

@@ -38,7 +38,12 @@ public class OpcUaDataConnection : IDataConnection
         _logger = logger;
     }
 
-    private volatile bool _disconnectFired;
+    // DataConnectionLayer-013: an int flag toggled with Interlocked.Exchange so the
+    // "only the first caller fires Disconnected" guard in RaiseDisconnected is genuinely
+    // atomic. A plain volatile bool gives visibility but not atomicity — two threads
+    // (e.g. the keep-alive thread and a ReadAsync failure path) could both observe it
+    // false and both raise the event. 0 = not fired, 1 = fired.
+    private int _disconnectFired;
 
     public ConnectionHealth Status => _status;
     public event Action? Disconnected;
@@ -82,7 +87,7 @@ public class OpcUaDataConnection : IDataConnection
         await _client.ConnectAsync(_endpointUrl, options, cancellationToken);
 
         _status = ConnectionHealth.Connected;
-        _disconnectFired = false;
+        Interlocked.Exchange(ref _disconnectFired, 0);
         _logger.LogInformation("OPC UA connected to {Endpoint}", _endpointUrl);
 
         await StartHeartbeatMonitorAsync(config.Heartbeat, cancellationToken);
@@ -285,12 +290,15 @@ public class OpcUaDataConnection : IDataConnection
 
     /// <summary>
     /// Marks the connection as disconnected and fires the Disconnected event once.
-    /// Thread-safe: only the first caller triggers the event.
+    /// Thread-safe: the firing guard is an atomic compare-and-set
+    /// (<see cref="Interlocked.Exchange(ref int, int)"/>), so when several threads race
+    /// here — e.g. the keep-alive thread via <see cref="OnClientConnectionLost"/> and a
+    /// <c>ReadAsync</c> failure path — exactly one of them observes the 0→1 transition
+    /// and invokes <see cref="Disconnected"/>.
     /// </summary>
     private void RaiseDisconnected()
     {
-        if (_disconnectFired) return;
-        _disconnectFired = true;
+        if (Interlocked.Exchange(ref _disconnectFired, 1) != 0) return;
         _status = ConnectionHealth.Disconnected;
         _logger.LogWarning("OPC UA connection to {Endpoint} lost", _endpointUrl);
         Disconnected?.Invoke();