add mosquitto_delay_puback option

[linuxbasic]

Signed-off-by: Linus Basig [email protected]
Signed-off-by: Fabrizio Lazzaretti [email protected]

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This PR adds a method mosquitto_delay_puback. After calling mosquitto_delay_puback the library will delay the PUBACK message required for QoS 1 message transfers until after the on_message callback returns.

This change ensures the message does not get lost if the client crashes during the processing of the message.

If the mosquitto_delay_puback is not called, the previous behavior (PUBACK before calling on_message) applies.

related issues:

• Support delaying PUBACKs in libmosquitto API eclipse/mosquitto#118
• Mosquitto client implements QoS 1 a zero or more times and QoS 2 as at most once eclipse/mosquitto#188

My use-case is a messaging middleware that forwards the messages to other systems and we need to maintain the "At Least Once" delivery guarantee of QoS 1.