BugFix(MQTT): send an event in ReceiveMessageHandler before completin…

…g received message (Azure#3116)

e2e/test/iothub/messaging/MessageReceiveE2ETests.cs

@@ -938,6 +938,9 @@ await deviceClient.SetReceiveMessageHandlerAsync(
                 async (message, context) =>
                 {
                     VerboseTestLogger.WriteLine($"Received message over the first message handler: MessageId={message.MessageId}");
+                    var messageD2C = new Client.Message(Encoding.UTF8.GetBytes("DeviceToCloud"));
+                    // sending an event within message handler to make sure its flow is not affected by the event.
+                    await deviceClient.SendEventAsync(messageD2C);
                     await deviceClient.CompleteAsync(message).ConfigureAwait(false);
                     firstHandlerSemaphore.Release();
                 },

iothub/device/src/Transport/Mqtt/MqttTransportHandler.cs

@@ -578,9 +578,7 @@ private async Task HandleIncomingMessagesAsync()
             // We are intentionally not awaiting _deviceMessageReceivedListener callback.
             // This is a user-supplied callback that isn't required to be awaited by us. We can simply invoke it and continue.
             _ = _deviceMessageReceivedListener?.Invoke(message);
-            // Messages with QoS = 1 need to be Acknowledged otherwise it results in mismatched Ack to IoT Hub
-            // causing next message being replayed and all subsequent messages being queued.
-            await CompleteIncomingMessageAsync(message).ConfigureAwait(false);
+            await TaskHelpers.CompletedTask.ConfigureAwait(false);
 
             if (Logging.IsEnabled)
                 Logging.Exit(this, "Process C2D message via callback", nameof(HandleIncomingMessagesAsync));