Prepare recommended fix

Source/MQTTnet.Server/Internal/MqttConnectedClient.cs

@@ -218,6 +218,8 @@ public sealed class MqttConnectedClient : IDisposable
         HandleTopicAlias(publishPacket);
 
         var applicationMessage = MqttApplicationMessageFactory.Create(publishPacket);
+        // Topic aliases are scoped to this network connection and must not be forwarded to other clients.
+        applicationMessage.TopicAlias = 0;
 
         var dispatchApplicationMessageResult =
             await _sessionsManager.DispatchApplicationMessage(Id, UserName, Session.Items, applicationMessage, cancellationToken).ConfigureAwait(false);
@@ -579,4 +581,4 @@ public sealed class MqttConnectedClient : IDisposable
             _logger.Warning(exception, "Client '{0}': Error while sending DISCONNECT packet (ReasonCode = {1})", Id, options.ReasonCode);
         }
     }
-}
+}

Source/MQTTnet.Tests/Clients/MqttClient/MqttClient_Tests.cs

@@ -454,6 +454,67 @@ public sealed class MqttClient_Tests : BaseTestClass
         }
     }
 
+    [TestMethod]
+    public async Task Raise_Disconnected_On_KeepAlive_Timeout()
+    {
+        using var listener = new TcpListener(System.Net.IPAddress.Loopback, 0);
+        listener.Start();
+
+        var port = ((System.Net.IPEndPoint)listener.LocalEndpoint).Port;
+
+        using var testEnvironment = new TestEnvironment(TestContext);
+        using var client = testEnvironment.CreateClient();
+
+        var options = new MqttClientOptionsBuilder()
+            .WithTcpServer("127.0.0.1", port)
+            .WithKeepAlivePeriod(TimeSpan.FromSeconds(5))
+            .WithTimeout(TimeSpan.FromSeconds(10)) // For ping timeout and others
+            .Build();
+
+        var disconnectedTask = new TaskCompletionSource<MqttClientDisconnectedEventArgs>();
+        client.DisconnectedAsync += e =>
+        {
+            disconnectedTask.TrySetResult(e);
+            return CompletedTask.Instance;
+        };
+
+        // Fake server implementation
+        _ = Task.Run(async () =>
+        {
+            using var clientSocket = await listener.AcceptTcpClientAsync();
+            await using var stream = clientSocket.GetStream();
+
+            // Read CONNECT
+            var buffer = new byte[1024];
+            _ = await stream.ReadAsync(buffer.AsMemory(), CancellationToken.None);
+
+            // Send CONNACK
+            var connack = new byte[] { 0x20, 0x02, 0x00, 0x00 };
+            await stream.WriteAsync(connack.AsMemory(), CancellationToken.None);
+
+            // Wait until client drops connection because of ping timeout
+            while (true)
+            {
+                var readCount = await stream.ReadAsync(buffer.AsMemory(), CancellationToken.None);
+                if (readCount == 0)
+                {
+                    break; // connection closed
+                }
+            }
+        });
+
+        await client.ConnectAsync(options, CancellationToken.None);
+
+        var timeoutTask = Task.Delay(TimeSpan.FromSeconds(10));
+        var completedTask = await Task.WhenAny(disconnectedTask.Task, timeoutTask);
+
+        listener.Stop();
+
+        Assert.AreEqual(disconnectedTask.Task, completedTask, "DisconnectedAsync was not raised.");
+        var result = await disconnectedTask.Task;
+        Assert.IsNotNull(result);
+    }
+
     [TestMethod]
     public async Task Publish_QoS_1_In_ApplicationMessageReceiveHandler()
     {

Source/MQTTnet.Tests/MQTTv5/Client_Tests.cs

@@ -114,7 +114,7 @@ public sealed class Client_Tests : BaseTestClass
 
         Assert.IsNotNull(receivedMessage);
         Assert.AreEqual(applicationMessage.Topic, receivedMessage.Topic);
-        Assert.AreEqual(applicationMessage.TopicAlias, receivedMessage.TopicAlias);
+        Assert.AreEqual(0, receivedMessage.TopicAlias);
         Assert.AreEqual(applicationMessage.ContentType, receivedMessage.ContentType);
         Assert.AreEqual(applicationMessage.ResponseTopic, receivedMessage.ResponseTopic);
         Assert.AreEqual(applicationMessage.MessageExpiryInterval, receivedMessage.MessageExpiryInterval);
@@ -344,4 +344,4 @@ public sealed class Client_Tests : BaseTestClass
         memoryStream.Position = 0;
         return memoryStream;
     }
-}
+}

Source/MQTTnet.Tests/Server/Topic_Alias_Tests.cs

@@ -34,6 +34,7 @@ public sealed class Topic_Alias_Tests : BaseTestClass
         await testEnvironment.StartServer();
 
         var receivedTopics = new List<string>();
+        var receivedTopicAliases = new List<ushort>();
 
         var c1 = await testEnvironment.ConnectClient(options => options.WithProtocolVersion(MqttProtocolVersion.V500));
         c1.ApplicationMessageReceivedAsync += e =>
@@ -41,6 +42,7 @@ public sealed class Topic_Alias_Tests : BaseTestClass
             lock (receivedTopics)
             {
                 receivedTopics.Add(e.ApplicationMessage.Topic);
+                receivedTopicAliases.Add(e.ApplicationMessage.TopicAlias);
             }
 
             return CompletedTask.Instance;
@@ -68,5 +70,6 @@ public sealed class Topic_Alias_Tests : BaseTestClass
         Assert.HasCount(3, receivedTopics);
         CollectionAssert.AllItemsAreNotNull(receivedTopics);
         Assert.IsTrue(receivedTopics.All(t => t.Equals("this_is_the_topic", System.StringComparison.Ordinal)));
+        Assert.IsTrue(receivedTopicAliases.All(a => a == 0));
     }
-}
+}

Source/MQTTnet/MqttClient.cs

@@ -1049,6 +1049,12 @@ public sealed class MqttClient : Disposable, IMqttClient
             switch (exception)
             {
                 case OperationCanceledException:
+                    // if (cancellationToken.IsCancellationRequested)
+                    // {
+                    //     return;
+                    // }
+
+                    //break;
                     return;
                 case MqttCommunicationException:
                     _logger.Warning(exception, "Communication error while sending/receiving keep alive packets");

Source/ReleaseNotes.md

@@ -1,11 +1 @@
-* Core: Used new language features across the entire library
+- Server: Fixed exposinbg Topic Alias to clients (#2250, thanks to @suhashollakc)
-* Core: Performance improvements
-* Core: Added validations for variable byte integers which do not match the .NET uint perfectly
-* Client: Added support for pre-encoded UTF-8 binary buffers for user properties (#2228, thanks to @koepalex)
-* Server: Improved performance of retained messages when no event handler is attached (#2093, thanks to @zhaowgit)
-* Server: The event `InterceptingClientEnqueue` is now also called for retained messages (BREAKING CHANGE!)
-* Server: The local end point is now also exposed in the channel adapter (#2179)
-* Marked all projects as AOT compatible
-* Restored the strong name of the nugets
-* Embedded license file in all nugets (#2197, thanks to @JensNordenbro)
-* Added support for dotnet10