Bug: ClientMessageQueueInterceptor breaks the broker due to QoS levels

Describe the bug

If a Client A subscribes a topic with QoS 0 and a client B publishes with QoS 1 AND a ClientMessageQueueInterceptor is used, the broker throws an exception.

Which project is your bug related to?

• Server

To Reproduce

Steps to reproduce the behavior:

1. Using this version of MQTTnet ‘3.0.12’.
2. Add a ClientMessageQueueInterceptor.
3. Subscribe with a Client to a topic “A” and QoS 0
4. Publish with a Client to topic “A” and QoS 1
5. Exception is thrown (MqttCommunicationException)

Expected behavior

The usage of a ClientMessageQueueInterceptor should work correctly.

Additional context / logging

The following lines lead to the exception:

• QoS is set to the publishPacket correctly from queuedApplicationMessage.QualityOfServiceLevel (i.e. QoS from the subscription) https://github.com/chkr1011/MQTTnet/blob/master/Source/MQTTnet/Server/MqttClientConnection.cs#L395
• The MqttClientMessageQueueInterceptorContext is created and passed to the handler. The information queuedApplicationMessage.QualityOfServiceLevel isn’t available on the context. https://github.com/chkr1011/MQTTnet/blob/master/Source/MQTTnet/Server/MqttClientConnection.cs#L407
• The publishPacket.QualityOfServiceLevel is set again BUT from ApplicationMessage.QualityOfServiceLevel which overwrites the correct one from queuedApplicationMessage.QualityOfServiceLevel (i.e. with the QoS from the published message) https://github.com/chkr1011/MQTTnet/blob/master/Source/MQTTnet/Server/MqttClientConnection.cs#L424:

The thrown exceptions:

Message was published with QoS 1 but client subscribed with QoS 0:

MQTT Trace: Sending publish packet failed: Communication exception (ClientId: xy).
MQTTnet.Exceptions.MqttCommunicationException: Packet identifier must be empty if QoS == 0 [MQTT-2.3.1-5]. ---> MQTTnet.Exceptions.MqttProtocolViolationException: Packet identifier must be empty if QoS == 0 [MQTT-2.3.1-5].
   at MQTTnet.Formatter.V3.MqttV310PacketFormatter.EncodePublishPacket(MqttPublishPacket packet, IMqttPacketWriter packetWriter)
   at MQTTnet.Formatter.V3.MqttV310PacketFormatter.Encode(MqttBasePacket packet)
   at MQTTnet.Adapter.MqttChannelAdapter.<SendPacketAsync>d__33.MoveNext()
   --- End of inner exception stack trace ---
   at MQTTnet.Adapter.MqttChannelAdapter.WrapException(Exception exception)
   at MQTTnet.Adapter.MqttChannelAdapter.<SendPacketAsync>d__33.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at MQTTnet.Server.MqttClientConnection.<SendAsync>d__46.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at MQTTnet.Server.MqttClientConnection.<SendPendingPacketsAsync>d__45.MoveNext()

Message was published with QoS 0 but client subscribed with QoS 1:

MQTT Trace: Sending publish packet failed: Communication exception (ClientId: xy).
MQTTnet.Exceptions.MqttCommunicationException: Publish packet has no packet identifier. ---> MQTTnet.Exceptions.MqttProtocolViolationException: Publish packet has no packet identifier.
   at MQTTnet.Formatter.V3.MqttV310PacketFormatter.EncodePublishPacket(MqttPublishPacket packet, IMqttPacketWriter packetWriter)
   at MQTTnet.Formatter.V3.MqttV310PacketFormatter.Encode(MqttBasePacket packet)
   at MQTTnet.Adapter.MqttChannelAdapter.<SendPacketAsync>d__33.MoveNext()
   --- End of inner exception stack trace ---
   at MQTTnet.Adapter.MqttChannelAdapter.WrapException(Exception exception)
   at MQTTnet.Adapter.MqttChannelAdapter.<SendPacketAsync>d__33.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at MQTTnet.Server.MqttClientConnection.<SendAsync>d__46.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at MQTTnet.Server.MqttClientConnection.<SendPendingPacketsAsync>d__45.MoveNext()

Discussion

This is a bug which is very urgent for us and we would like to submit a Pull Request. We see the following possibilities:

1. Don’t allow the interceptor to change the QoS level at all (means, just delete the line)
2. Pass and the correct QoS level (means, if the interceptor doesn’t change something, the correct level as before is set)

[chkr1011]

@msallin I have no plans yet for a new release but I will release an RC soon so you can already make use of the latest changes.

[msallin]

https://github.com/chkr1011/MQTTnet/releases/tag/v3.0.13