[mosquitto-dev] Callback on_message called multiple times when using MQT

[Aleksandar Nikolic <an010@xxxxxxxx>]

Hello,

why is the on_message function called 3 times when I am using MQTT v5.0, and exactly once when I am not using MQTT v5.0? The mosquitto version is 2.0.14. I have also debugged a little and the message is really published multiple times when I use the MQTT v5.0. Why is that?

Here is the code:

```

#include <mosquitto.h>
#include <mqtt_protocol.h>
#include <thread>
#include <cstring>

#define SERVER_HOST "127.0.0.1"
#define SERVER_PORT 1883

/* Callback called when the client receives a message. */
static void on_message(struct mosquitto* mosq, void* obj, const struct mosquitto_message* msg, const mosquitto_property* props)
{
    printf("%s called with topic %s\n", __FUNCTION__, msg->topic);
    return;
}

int main(void)
{
    struct mosquitto* mosq_;
    char msg[] = "testmsg";
    int qos = 0;

    mosquitto_lib_init();
    mosq_ = mosquitto_new("tx", false, NULL);
    if (mosq_ == NULL)
        return 1;

    // Commenting this function call results in the `on_message` being called exactly once.
    // With this line it is called 3 times (for each mosquitto_subscribe).
    mosquitto_int_option(mosq_, MOSQ_OPT_PROTOCOL_VERSION, MQTT_PROTOCOL_V5);

    mosquitto_message_v5_callback_set(mosq_, on_message);
    mosquitto_connect(mosq_, SERVER_HOST, SERVER_PORT, 60);
    mosquitto_loop_start(mosq_);
    mosquitto_subscribe(mosq_, NULL, "#", qos);
    mosquitto_subscribe(mosq_, NULL, "1/#", qos);
    mosquitto_subscribe(mosq_, NULL, "1/2/#", qos);

    mosquitto_publish_v5(mosq_, NULL, "1/2/3", strlen(msg), static_cast<void*>(msg), qos, false, NULL);
    std::this_thread::sleep_for(std::chrono::seconds(1));

    mosquitto_disconnect(mosq_);
    mosquitto_destroy(mosq_);
    mosquitto_lib_cleanup();

    return 0;
}
```

Regards