execute processMsgQueue on connected callback in separate thread because producer.send() will execute in Producer worker thread and can block messages producing

common/transport/mqtt/src/main/java/org/thingsboard/server/transport/mqtt/MqttTransportHandler.java

@@ -825,7 +825,7 @@ public class MqttTransportHandler extends ChannelInboundHandlerAdapter implement
                     ctx.writeAndFlush(createMqttConnAckMsg(CONNECTION_ACCEPTED, connectMessage));
                     deviceSessionCtx.setConnected(true);
                     log.info("[{}] Client connected!", sessionId);
-                    processMsgQueue(ctx);
+                    transportService.getCallbackExecutor().execute(() -> processMsgQueue(ctx)); //this callback will execute in Producer worker thread and hard or blocking work have to be submitted to the separate thread.
                 }
 
                 @Override

common/transport/transport-api/src/main/java/org/thingsboard/server/common/transport/TransportService.java

@@ -56,6 +56,8 @@ import org.thingsboard.server.gen.transport.TransportProtos.ValidateDeviceLwM2MC
 import org.thingsboard.server.gen.transport.TransportProtos.ValidateDeviceTokenRequestMsg;
 import org.thingsboard.server.gen.transport.TransportProtos.ValidateDeviceX509CertRequestMsg;
 
+import java.util.concurrent.ExecutorService;
+
 /**
  * Created by ashvayka on 04.10.18.
  */
@@ -131,4 +133,6 @@ public interface TransportService {
     void log(SessionInfoProto sessionInfo, String msg);
 
     void notifyAboutUplink(SessionInfoProto sessionInfo, TransportProtos.UplinkNotificationMsg build, TransportServiceCallback<Void> empty);
+
+    ExecutorService getCallbackExecutor();
 }

common/transport/transport-api/src/main/java/org/thingsboard/server/common/transport/service/DefaultTransportService.java

@@ -1141,4 +1141,9 @@ public class DefaultTransportService implements TransportService {
             callback.onError(e);
         }
     }
+
+    @Override
+    public ExecutorService getCallbackExecutor() {
+        return transportCallbackExecutor;
+    }
 }