fix(data-connection-layer): resolve DataConnectionLayer-002/003/004/005 — Resume supervision, concurrent dicts, subscribe-failure classification, write timeout

src/ScadaLink.DataConnectionLayer/Adapters/RealOpcUaClient.cs

@@ -1,3 +1,4 @@
+using System.Collections.Concurrent;
 using System.Security.Cryptography.X509Certificates;
 using Opc.Ua;
 using Opc.Ua.Client;
@@ -13,8 +14,14 @@ public class RealOpcUaClient : IOpcUaClient
 {
     private ISession? _session;
     private Subscription? _subscription;
-    private readonly Dictionary<string, MonitoredItem> _monitoredItems = new();
-    private readonly Dictionary<string, Action<string, object?, DateTime, uint>> _callbacks = new();
+
+    // DataConnectionLayer-003: these maps are read from the OPC Foundation SDK's
+    // internal publish threads (the MonitoredItem.Notification handler reads
+    // _callbacks) concurrently with subscribe/disconnect mutations that run on
+    // thread-pool threads. Plain Dictionary access during a concurrent resize or
+    // 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;
     private OpcUaConnectionOptions _options = new();
     private readonly OpcUaGlobalOptions _globalOptions;
@@ -180,8 +187,8 @@ public class RealOpcUaClient : IOpcUaClient
         {
             _subscription.RemoveItem(item);
             await _subscription.ApplyChangesAsync(cancellationToken);
-            _monitoredItems.Remove(subscriptionHandle);
-            _callbacks.Remove(subscriptionHandle);
+            _monitoredItems.TryRemove(subscriptionHandle, out _);
+            _callbacks.TryRemove(subscriptionHandle, out _);
         }
     }