Add asynchronous connection method for MQTT v5

[JacobsJohan]

Description

It is currently only possible to make a blocking MQTT v5 connection to a broker by using mosquitto_connect_bind_v5. The non-blocking alternative mosquitto_connect_bind_async can only be used for MQTT v3, because it does not allow passing MQTT v5 properties.

This pull request adds 2 new API functions to the mosquitto library:

• mosquitto_connect_bind_async_v5: This function will create a non-blocking socket connection to the MQTT broker. It will return with MOSQ_ERR_CONN_INPROGRESS when the socket connection was created, but the connection is still in progress. The socket can be added to an event loop to check when it becomes ready for writing.
• mosquitto_send_connect: This function must be called once the connection is ready for writing to send the actual MQTT CONNECT message.

Started from this issue: #1835

Checklist

Signed-off-by: Johan Jacobs [email protected]

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[JacobsJohan]

The only difference between the existing and new code is a change of return value from MOSQ_ERR_CONN_PENDING to MOSQ_ERR_CONN_INPROGRESS when the connect function returns with errno set to EINPROGRESS. This is needed because the return value needs to be larger than 0 and MOSQ_ERR_CONN_PENDING = -1.

For some reason, the connection setup continues when rc < 0 and only stops when rc > 0. So an MQTT connect message is always sent to the broker, even if the socket is not yet writable. This does not make much sense to me, because the connect message will never arrive in this situation.

Besides this, only new code has been added to extend the asynchronous connect to MQTT v5, so it should not break any existing code.

[ahoarau]

This is a good feature, I would also vote for merging this !

[JacobsJohan]

What would be the next steps to get this integrated @ralight ? Is there anything else I can do?

[dvsmd]

This is a good addition. Can this please be merged?

[Project579]

FYI, the C++ methods for this functionality are missing, they should be added before merging.