User properties not available on MOSQ_EVT_ACL_CHECK events

[da-mkay]

When listening for MOSQ_EVT_ACL_CHECK events in a plugin you get event_data of type mosquitto_evt_acl_check which includes properties.
However, those properties are set explicitly to NULL inside mosquitto_acl_check before the event listeners are called. So at the time of the ACL check the plugins have no access to user properties.

In case of a publish-packet the MOSQ_EVT_MESSAGE event is fired after the ACL check. And the event_data there (type mosquitto_evt_message) will contain the user properties, because they are handled inside handle_publish, stored in msg and passed over to the listeners.

In case of a subscribe-packet there is no additional event that gets fired. So inside your plugin you cannot access user properties for subscriptions. But these are part of the MQTT 5 spec.

More interestingly, I found the following comment inside handle_subscribe:

/* Note - User Property not handled */

Is there a reason why user properties are not handled during subscribe and are being explicitly set to NULL inside mosquitto_acl_check ❓

I did a quick test: I copied and adjusted the property-handling-code from handle_publish to handle_subscribe and passed the properties as a new parameter to mosquitto_acl_check which then adds them to the event_data which gets passed to the event listeners listening for MOSQ_EVT_ACL_CHECK. Seems to work 😉