Add support for getting unsent messages out of mqtt outbox (IDFGH-8355)

[bbinet]

I'm using esp-mqtt in a battery powered project, so to save energy I'm deep sleeping most of the time.

I send sensor data from time to time through mqtt, but if it takes too much time, I would like to return to deep sleep, and save the data which has not been actually sent (ack has not been received) to flash memory (NVS), so that I can try again the next time I wake up.

I couldn't find any API to get unsent data out of mqtt outbox. Can you think of any way to do it?

[bbinet]

Also when MQTT_EVENT_DELETED event is triggered, it would be helpful to be able to retrieve the data which has been deleted in case we want to do anything with it.

[euripedesrocha]

Hi @bbinet, in your case, I would suggest taking the route of writing your own outbox.

[Alvin1Zhang]

Thanks for reporting, would close due to short of feedback, feel free to reopen.