availability_topic for Home assistant

[phbasler]

With this PR a Birth message and a Will message is set.
Upon connection the message "online" is published to switchbot_mqtt/availability and upon loosing connection (KeepAlve Timeout or job was killed) the Will publishes "offline" to the same topic.
For the mqtt sensors in home assistant you can set the availability_topic to this topic and then you can check if the service is running or not.

If you want we could add a CLI Option to set this topic to have different topics for multiple Instances.

[fphammerle]

Thank you, @phbasler !

Could you take a look at my suggestions and format the changes with black?

[phbasler]

@fphammerle I applied the changes and formatted with black. I also changed the topic to end with "status" like suggested.

[fphammerle]

Thank you!

What happens if switchbot-mqtt shuts down gracefully? (planned disconnect)
Will "offline" be reported in that case?

[fphammerle]

Shall we set retain=True in .will_set and .publish (to preserve the status information in case some recipients reset/restarted, e.g. home assistant)?

[phbasler]

@fphammerle Good idea with the retain message. I added the flags.
In case of a clean disconnect (the client actually sends the disconnect message), the will is not triggered and "offline" is not send. Is there an actual way for the user to call disconnect? I always ended the code by stopping the systemd service or ctrl+c it

Edit: It doesn't seem that you are calling the disconnect function of the mqtt client anywhere in your code, so it never exits with a clean disconnect

[fphammerle]

Thank you!

I expected mqtt_client.loop_forever() to trigger a clean disconnect when receiving SIGTERM, but apparently it does not:
https://github.com/eclipse/paho.mqtt.python/blob/master/src/paho/mqtt/client.py#L1740

[fphammerle]

released here: https://pypi.org/project/switchbot-mqtt/3.3.0/