Client PUB completion (PUBACK, PUBREC/PUBCOMP) should be performed after the application callback.

[CIPop]

The current client implementation will send the PUBACK (likely the same for PUBREC/COMP) before calling the application callback. This can result in messages being lost if the application crashes during the callback.

According to the V3/V5 specs Section 4.3, I believe the intent of the protocol is for the ACK/COMP messages to indicate that the application takes ownership of the message, and not the protocol library.

I propose moving the message acknowledgment after the callback for QoS 1 and 2.

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Details

Currently, send__puback is called before the on_message callback:

mosquitto/lib/handle_publish.c

Line 142 in 1ed275b

rc = send__puback(mosq, mid, 0, NULL);

mosquitto/lib/handle_publish.c

Line 146 in 1ed275b

mosq->on_message(mosq, mosq->userdata, &message->msg);

If the ordering is reversed (on_message before send__puback), even if the application crashes in the callback the message isn't lost and will be sent again by the broker after the application recovers (reconnects).

Version: Mosquitto Client (master branch pulled around tag 2.0.16, built from sources).

Related: #118