MQTT messages received multiple times on QoS 1 subscription

[JWizard]

I am using the SLM app from the 1.9.1 SDK.

I connect to my MQTT broker using TLS offloaded on the modem.

I subscribe to a topic using QoS = 1

Then if a message is published to the same topic using QoS = 1 then the nrf9160 reports the message received over and over again every few seconds.

If I instead subscribe to the topic using QoS = 0 then I will only get the messages (published at QoS = 1) one time.

Is this the intended behavior? I feel unsure that I am actually operating under QoS = 1 conditions when the subscription is set to QoS = 0.

[Øyvind]

Hello, have confirmed that this was a bug. A bug fix has been registered https://github.com/nrfconnect/sdk-nrf/pull/7363 

Kind regards,
Øyvind

[Øyvind]

In order to fix the issue in the nRF Connect SDK tag 1.9.1 - please add the following to the beginning of handle_mqtt_publish_evt() in nrf\applications\serial_lte_modem\src\mqtt_c\slm_at_mqtt.c

const struct mqtt_publish_param *p = &evt->param.publish;
LOG_INF("MQTT PUBLISH result=%d len=%d",
		evt->result, p->message.payload.len);
	
ret = publish_get_payload(c, evt->param.publish.message.payload.len);
if (ret < 0) {
	return ret;
}

if (p->message.topic.qos == MQTT_QOS_1_AT_LEAST_ONCE) {
	const struct mqtt_puback_param ack = {
	.message_id = p->message_id
	};
	/* Send acknowledgment. */
	mqtt_publish_qos1_ack(&client, &ack);
}