python API - detect shutdown

I've seen a couple of "solutions" to this, but my question here is: what's the canonical way to detect that linuxcnc is shutting down and/or exiting, or isn't running, using the python linuxcnc.stat.poll() api ?  I can start my python script in hal using loadusr, but there's no unloadusr, and I can't just run it at computer boot because linuxcnc.stat() just fails if linuxcnc isn't running yet.  I tried using the parent PID but that isn't reliable (and prevents you from manually running the script in the background)
I've attached my script.  I'd like to cleanly detect that linuxcnc has exited or the computer is shutting down, and send a final MQTT message to home assistant to update the status to "off".
 

I trying to hook up with a guy in an unfriendly timezone (for an Aussie) who has been working on a implementation of the MTconnect protocol for Linuxcnc that does exactly what you want. The cool thing is that it supports MQTT and there are already integrations to Home Assistant if you have a Fanuc machine.  Right now, I don't know any more detail but keep an eye on my @MrRodW YouTube channel as I will cover it there. Once we get going, I'm sure another like minded helper won't go astray. 

I look forward to testing that, but my tiny script does what I need right now - except exit.  I had hoped to find something in the docs about detecting shutdown, but did not.  It seems a strange oversight, unless you assume linuxcnc never exits until the machine shuts off...
 

your script might be able to look for linuxcnc's rtapi thread (ie the servo thread) and exit if its not found.
AI seems to think it's possible.

I have a component that uses atexit. On my system this works fine to do something when i close linuxcnc. All other attemps failed.



import atexit


def exit():
    some code...
atexit.register(exit)