MqttClient Publish a message when you are inside the InterceptingPublishAsync result to an Error when the Client Publish a QoS 1

[miaooss]

Note: I'm migrating from version 3.1.2 to 4.3.3.952

Describe the bug

I have a device that is connecting to Mqtt,
The device is subscribing and publishing a message correctly.

But publishing a message from the server will be ignored if the PubAck is not sent.

Here is my setup,
I'm using the InterceptingPublishAsync event to intercept messages from the client.
In one of the messages I received, I would like to send another publish on a specific topic.
Unfortunately, the pubAck will be sent after my event inside the InterceptingPublishAsync
The Client will ignore the Message (The Device/Client is not using MQTTNET, but using a low-level library)

We can find the issue here:

MQTTnet/Source/MQTTnet/Server/Internal/MqttClient.cs

Line 218 in 7f5c437

async Task HandleIncomingPublishPacket(MqttPublishPacket publishPacket, CancellationToken cancellationToken)

Having another event after the end of a pubAck, will help handling the message. OnCompletedPublishAsync

Description of the QoS 1 delivery protocol

In the QoS 1 delivery protocol, the Sender

· MUST assign an unused Packet Identifier each time it has a new Application Message to publish.

· MUST send a PUBLISH Packet containing this Packet Identifier with QoS=1, DUP=0.

· MUST treat the PUBLISH Packet as “unacknowledged” until it has received the corresponding PUBACK packet from the receiver. See Section 4.4 for a discussion of unacknowledged messages.

1 The receiver is not required to complete delivery of the Application Message before sending the PUBACK. When its original sender receives the PUBACK packet, ownership of the Application Message is transferred to the receiver

Which component is your bug related to?

• QoS 1 delivery protocol
• MqttClient

To Reproduce

Steps to reproduce the behavior:

1. Using this version of MQTTnet '4.3.3.952'.
2. Subscribe to the server with InterceptingPublishAsync
3. Publish a message inside the event method
4. The Error can be seen with WireShark

Expected behavior

The pubAck should be sent before I even handle the business logic of the event itself

Screenshots

MqttNet 3

MqttNet 4

Code example

using MQTTnet;
using MQTTnet.Server;

var mqttFactory = new MqttFactory();

var mqttServerOptions = new MqttServerOptionsBuilder().WithDefaultEndpoint().Build();

using (var mqttServer = mqttFactory.CreateMqttServer(mqttServerOptions))
{
    await mqttServer.StartAsync();

    mqttServer.InterceptingPublishAsync += async (InterceptingPublishEventArgs arg) =>
    {
        // Create a new message using the builder as usual.
        var message = new MqttApplicationMessageBuilder().WithTopic("HelloWorld").WithPayload("Test").Build();

        // Now inject the new message at the broker.
        await mqttServer.InjectApplicationMessage(
            new InjectedMqttApplicationMessage(message)
            {
                SenderClientId = "SenderClientId"
            });
    };

    Console.ReadLine();

    // Stop and dispose the MQTT server if it is no longer needed!
    await mqttServer.StopAsync();
}