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

tests/ScadaLink.DataConnectionLayer.Tests/DataConnectionActorTests.cs

@@ -836,4 +836,63 @@ public class DataConnectionActorTests : TestKit
         Assert.Equal(5, report.TotalSubscribedTags); // all 5 tags tracked
         Assert.Equal(3, report.ResolvedTags);        // only the 3 good ones resolved
     }
+
+    // ── DataConnectionLayer-008: HandleUnsubscribe shared-tag reference counting ──
+
+    [Fact]
+    public async Task DCL008_Unsubscribe_OnlyReleasesTagWhenLastSubscriberLeaves()
+    {
+        // Regression test for DataConnectionLayer-008. HandleUnsubscribe must release a
+        // tag at the adapter only when no other instance still subscribes to it. The
+        // O(n) per-tag scan over every instance was replaced with an O(1) reference
+        // count; this guards that the reference count tracks shared subscriptions
+        // correctly — a shared tag is kept while any subscriber remains and the
+        // resolved-tag counter and adapter UnsubscribeAsync stay consistent.
+        var unsubscribed = new System.Collections.Concurrent.ConcurrentBag<string>();
+        _mockAdapter.ConnectAsync(Arg.Any<IDictionary<string, string>>(), Arg.Any<CancellationToken>())
+            .Returns(Task.CompletedTask);
+        _mockAdapter.Status.Returns(ConnectionHealth.Connected);
+        _mockAdapter.SubscribeAsync(Arg.Any<string>(), Arg.Any<SubscriptionCallback>(), Arg.Any<CancellationToken>())
+            .Returns(ci => Task.FromResult("sub-" + (string)ci[0]));
+        _mockAdapter.ReadAsync(Arg.Any<string>(), Arg.Any<CancellationToken>())
+            .Returns(new ReadResult(false, null, null));
+        _mockAdapter.UnsubscribeAsync(Arg.Any<string>(), Arg.Any<CancellationToken>())
+            .Returns(ci => { unsubscribed.Add((string)ci[0]); return Task.CompletedTask; });
+
+        var actor = CreateConnectionActor("dcl008-shared");
+        await Task.Delay(300);
+
+        // Two instances both subscribe to the shared tag; instA also has an exclusive tag.
+        actor.Tell(new SubscribeTagsRequest("c1", "instA", "dcl008-shared",
+            ["shared/tag", "exclusive/a"], DateTimeOffset.UtcNow));
+        ExpectMsg<SubscribeTagsResponse>(TimeSpan.FromSeconds(5));
+        actor.Tell(new SubscribeTagsRequest("c2", "instB", "dcl008-shared",
+            ["shared/tag"], DateTimeOffset.UtcNow));
+        ExpectMsg<SubscribeTagsResponse>(TimeSpan.FromSeconds(5));
+
+        // Unsubscribe instA — shared/tag must stay (instB still subscribes); only
+        // exclusive/a is released at the adapter.
+        actor.Tell(new UnsubscribeTagsRequest("c3", "instA", "dcl008-shared", DateTimeOffset.UtcNow));
+        await Task.Delay(300);
+
+        Assert.Contains("sub-exclusive/a", unsubscribed);
+        Assert.DoesNotContain("sub-shared/tag", unsubscribed);
+
+        // Health: 1 tag still subscribed and resolved (shared/tag held by instB).
+        actor.Tell(new DataConnectionActor.GetHealthReport());
+        var report1 = ExpectMsg<DataConnectionHealthReport>(TimeSpan.FromSeconds(3));
+        Assert.Equal(1, report1.TotalSubscribedTags);
+        Assert.Equal(1, report1.ResolvedTags);
+
+        // Unsubscribe instB — now shared/tag has no subscribers and is released.
+        actor.Tell(new UnsubscribeTagsRequest("c4", "instB", "dcl008-shared", DateTimeOffset.UtcNow));
+        await Task.Delay(300);
+
+        Assert.Contains("sub-shared/tag", unsubscribed);
+
+        actor.Tell(new DataConnectionActor.GetHealthReport());
+        var report2 = ExpectMsg<DataConnectionHealthReport>(TimeSpan.FromSeconds(3));
+        Assert.Equal(0, report2.TotalSubscribedTags);
+        Assert.Equal(0, report2.ResolvedTags);
+    }
 }