mqtt_connect does not generate a client.error if wrong username and/or password is provided

[PitHerm]

i tried your simple publisher example
and modified the parameter of mqtt_connect
from:

/* Send connection request to the broker. */
mqtt_connect(&client, client_id, NULL, NULL, 0, NULL, NULL, connect_flags, 400);

/* check that we don't have any errors */
if (client.error != MQTT_OK) {
    fprintf(stderr, "error: %s\n", mqtt_error_str(client.error));
    exit_example(EXIT_FAILURE, sockfd, NULL);
}

with the correct username and password to:
/* Send connection request to the broker. */
mqtt_connect(&client, client_id, NULL, NULL, 0, "paul", "paula", connect_flags, 400);

/* check that we don't have any errors */
if (client.error != MQTT_OK) {
    fprintf(stderr, "error: %s\n", mqtt_error_str(client.error));
    exit_example(EXIT_FAILURE, sockfd, NULL);
}

i received no errors and a connection which worked.
then i tried wrong username and wrong password

/* Send connection request to the broker. */
mqtt_connect(&client, client_id, NULL, NULL, 0, "otto", "otto", connect_flags, 400);

/* check that we don't have any errors */
if (client.error != MQTT_OK) {
    fprintf(stderr, "error: %s\n", mqtt_error_str(client.error));
    exit_example(EXIT_FAILURE, sockfd, NULL);
}

i received no errors and no connection to the broker was established
after pressing the ENTER key to send the time i received an error

./simple_publisher is ready to begin publishing the time.
Press ENTER to publish the current time.
Press CTRL-D (or any other key) to exit.

./simple_publisher published : "The time is 2023-10-25 06:55:24"
datetimeThe time is 2023-10-25 06:55:24
error: MQTT_ERROR_SOCKET_ERROR

if no connection was established to the broker there should be an error message.

[conggalam12]

@PitHerm I have the same error , do you fix it ?

[mwetterw]

If I'm not mistaken, this is because mqtt_connect seems to only queue the CONNECT message without actually sending it.
It's the first mqtt_sync call after the mqtt_connect that will really send the pending CONNECT request.

So, when mqtt_connect returns, the library cannot possibly yet know if the username/password are correct.
You can see that connect failed if mqtt_sync returns either:

• MQTT_ERROR_CONNECT_CLIENT_ID_REFUSED or
• MQTT_ERROR_CONNECTION_REFUSED