Send DISCONNECT packets to MQTT5 clients only (#1506)

Source/MQTTnet.Tests/MQTTv5/Server_Tests.cs

@@ -175,6 +175,49 @@ namespace MQTTnet.Tests.MQTTv5
             }
         }
 
+        [TestMethod]
+        public async Task Disconnect_with_Reason()
+        {
+            using (var testEnvironment = CreateTestEnvironment())
+            {
+                var disconnectReason = MQTTnet.Client.MqttClientDisconnectReason.UnspecifiedError;
+
+                string testClientId = null;
+
+                await testEnvironment.StartServer();
+
+                testEnvironment.Server.ClientConnectedAsync += e =>
+                {
+                    testClientId = e.ClientId;
+                    return PlatformAbstractionLayer.CompletedTask;
+                };
+
+                var client = await testEnvironment.ConnectClient(new MQTTnet.Client.MqttClientOptionsBuilder().WithProtocolVersion(MQTTnet.Formatter.MqttProtocolVersion.V500));
+
+                client.DisconnectedAsync += e =>
+                {
+                    disconnectReason = e.Reason;
+                    return PlatformAbstractionLayer.CompletedTask;
+                };
+
+                await LongTestDelay();
+
+                // Test client should be connected now
+
+                Assert.IsTrue(testClientId != null);
+
+                // Have the server disconnect the client with AdministrativeAction reason
+
+                await testEnvironment.Server.DisconnectClientAsync(testClientId, Protocol.MqttDisconnectReasonCode.AdministrativeAction);
+
+                await LongTestDelay();
+
+                // The reason should be returned to the client in the DISCONNECT packet
+
+                Assert.AreEqual(MQTTnet.Client.MqttClientDisconnectReason.AdministrativeAction, disconnectReason);
+            }
+        }
+
         MqttClientOptions CreateClientOptions(TestEnvironment testEnvironment, string clientId, bool cleanSession, uint sessionExpiryInterval)
         {
             return testEnvironment.Factory.CreateClientOptionsBuilder()

Source/MQTTnet.Tests/Server/General.cs

@@ -935,7 +935,62 @@ namespace MQTTnet.Tests.Server
                 Assert.IsTrue(client.IsConnected);
             }
         }
-        
+
+        [TestMethod]
+        public async Task Disconnect_Client_with_Reason()
+        {
+            using (var testEnvironment = CreateTestEnvironment())
+            {
+                var disconnectPacketReceived = false;
+
+                string testClientId = null;
+
+                await testEnvironment.StartServer();
+
+                testEnvironment.Server.ClientConnectedAsync += e =>
+                {
+                    testClientId = e.ClientId;
+                    return PlatformAbstractionLayer.CompletedTask;
+                };
+
+                var client = testEnvironment.CreateClient();
+
+                client.InspectPackage += e =>
+                {
+                    if (e.Buffer.Length > 0)
+                    {
+                        if (e.Buffer[0] == (byte)Protocol.MqttControlPacketType.Disconnect << 4)
+                        {
+                            disconnectPacketReceived = true;
+                        }
+                    }
+                    return PlatformAbstractionLayer.CompletedTask;
+                };
+
+                await client.ConnectAsync(
+                    new MqttClientOptionsBuilder().WithTcpServer("localhost", testEnvironment.ServerPort)
+                    .WithProtocolVersion(MQTTnet.Formatter.MqttProtocolVersion.V311)
+                    .Build()
+                    );
+
+                await LongTestDelay();
+
+                // Test client should be connected now
+
+                Assert.IsTrue(testClientId != null);
+
+                // Have the server disconnect the client with SessionTakenOver reason; no DISCONNECT should be sent to MQTT 3.1.1 clients
+
+                await testEnvironment.Server.DisconnectClientAsync(testClientId, Protocol.MqttDisconnectReasonCode.SessionTakenOver);
+
+                await LongTestDelay();
+
+                // MQTT 3.1.1 client should not receive a disconnect packet
+
+                Assert.IsFalse(disconnectPacketReceived, "Disconnect packet received when none is expected");
+            }
+        }
+
         void ConnectionValidationHandler(ValidatingConnectionEventArgs eventArgs)
         {
             if (_connected.ContainsKey(eventArgs.ClientId))

Source/MQTTnet/Server/Internal/MqttClient.cs

@@ -147,7 +147,9 @@ namespace MQTTnet.Server
         {
             IsRunning = false;
 
-            if (reason == MqttDisconnectReasonCode.SessionTakenOver || reason == MqttDisconnectReasonCode.KeepAliveTimeout)
+            // Potentially send disconnect reason to MQTT 5 clients
+
+            if (ChannelAdapter.PacketFormatterAdapter.ProtocolVersion == MqttProtocolVersion.V500 && reason != MqttDisconnectReasonCode.NormalDisconnection)            
             {
                 // Is is very important to send the DISCONNECT packet here BEFORE cancelling the
                 // token because the entire connection is closed (disposed) as soon as the cancellation