MQTT Last Will and Testament

[ckuethe]

Register a lost connection message with the MQTT broker to publish if the client loses connection. Also publish online when the client connects and offline when the client politely disconnects to a status topic. This way you get a clue if the device was shut down or it somehow failed.

From my MQTT viewer:

...
[0 ] rdz_sonde_server/csk/uptime > {"uptime": 106.6, "user": "radiosonde", "time": 2025-01-02 21:23:35, "SW": "rdzTTGOsonde", "VER": "dev20250102"}
(powered off device)
[0 ] rdz_sonde_server/csk/status > lost connection
[0 ] rdz_sonde_server/csk/status > online
[0 ] rdz_sonde_server/csk/uptime > {"uptime": 15.8, "user": "radiosonde", "time": 2025-01-02 21:24:17, "SW": "rdzTTGOsonde", "VER": "dev20250102"}
...

[dl9rdz]

I am hesitating to merge this as it is, as in the past I have tried to minimize the use of global variables (like the char lwt[128];). Its just 128 bytes, but that is already something on the TTGO. Whenever easily feasible I prefer local variables on the stack that only use RAM during function execution, this is why the topic string is in a function local char topic[128];

It course its a trade-off between space use and CPU time use for constructing the topic on each invocation - besides the difference of being able to change the topic in the config without restarting). Unless there are more good reasons for the global lwt variable, I would prefer having the topic for the additional messages in a stack (function local) variable as well...

[ckuethe]

No problem. Happy to follow your preferred style. Will update PR later today.

[ckuethe]

lwt global removed, LWT publication moved into its own function