The current design of the MQTT-SN library just covers the case, that the library has to wait for input. The case, where the library has to wait for bandwidth on the outgoing link is (nearly) not covered.

If the library finds the output link being saturated, it will wait and retry. This leads then to either very poor performance (when that poll interval is large) or very much power consumption (if that poll interval is small).

My current workaround for that is a function (similar to mqtt_sn_input()) that can be called, when there is confidence, that a next attempt to transmit a message might not fail. Would such a workaround be accepted as a fix? Better ideas, as how to fix the issue with the current design?

/**
 * @brief Check the transport for capacity on the outgoing side.
 *
 * Call this function if you have a good reason to believe that there is enough capacity
 * on the outgoing side of the link, so that pending outgoing messages could be send.
 *
 * @param client            The MQTT-SN client to check for incoming data.
 *
 * @return -EINVAL if the client is invalid or the transport contains no means to send data.
 *         Otherwise, 0 is returned.
 */
int mqtt_sn_output(struct mqtt_sn_client *client)
{
    if (!client || !client->transport || !client->transport->msg_send) {
        return -EINVAL;
    }

    k_work_reschedule(&client->process_work, K_NO_WAIT);

    return 0;
}