QMqttClient: Possible race condition on messageReceived signal

User informed us that they  have a lambda that is called upon a messageReceived signal connected to subscription A. Within that lambda I subscribe to a set of other topics.

Sometimes (not always) this triggers the same message to be delivered twice.

They noticed that the delivery is done within a loop over all active subscriptions within the QMqttClient (finalize_publish method). They think that IF a subscription is added that inserts an entry in that activeSubcriptions table that is BEFORE the current sub, upon the next loop the sub is signaled again.

They sent an example with a workaround:

This is the code that works now we made the connection queued so that the subscribe within does not apply while the messageReceived is being executed:

QMqttSubscription *sub = cl->subscribe("Identity");    
connect(
        sub,
        &QMqttSubscription::messageReceived,
        this,        
        [this](QMqttMessage data) {
            qDebug() << "Voertuiggroepeerder" << id << "received identity message." << this
                     << sender() << data.topic();
                QString DetailTopic = QJsonDocument::fromJson(data.payload())["Unique_identifier"])
                  QMqttSubscription *sub2 = cl->subscribe(DetailTopic);
                 connect(
                       sub2,
                       &QMqttSubscription::messageReceived,
                       this,
                       [this](QMqttMessage data) {
                              qDebug() << data.topic;
                       }
                 );
        },
        Qt::QueuedConnection); // to make it work

Important for the demo is that there should already be messages waiting on the identity channel (message was published in retained mode by multiple
publishers). this because we need messsages to arrivre while we are dispatching. The problem is a race condition: It does not always occur and the more
publishers exist, the higher the chance of breakdown.