python program won’t handle systemd KillSignal at reboot

I have a set of LEDs that I start and turn into a specific colour when the machine starts up and during other events. At power-off/reboot I want the LEDs to reset to the default colour.

To achieve this I start a systemd service that starts up the LED code and have a handle for the signal in the python code. Unfortunately I’m not either receiving the signal or the signal is not getting handled.

import signal
from threading import Event

stop_event = Event()

def read_led_status():
    while True:
        #do stuff (read from pipe and update LED status)
        stop_event.wait(0.5)


def system_exit():
    stop_event.set()
    #set default colour


if __name__ == "__main__":
    #handle kill signal
    signal.signal(signal.SIGTERM, system_exit)
    signal.signal(signal.SIGINT, system_exit)
    signal.signal(signal.SIGHUP, system_exit)

    read_led_status()

The system service

[Unit]
Description=LED
ConditionPathExistsGlob=/dev/ttyUSB*
Before=startup.service

[Service]
Type=simple
User=admin
ExecStart=sudo python3 /led/led_start.py
KillSignal=SIGTERM
StandardOutput=syslog
StandardError=syslog
Restart=always
RestartSec=10

[Install]
WantedBy=multi-user.target

What am I doing wrong here? It’s running on Ubuntu. Any help would be appreciated.

Answer

The callback executed by signal expects two arguments. Your implementation does not take any. The right prototype is:

def system_exit(signum, frame):
    stop_event.set()

This might have thrown a TypeError upon signal receive and execution. As you forward your logs output to syslog, then the journalctl should have kept a track of this.

Using the above prototype should make your script properly catch and handle the SIGTERM signal (i.e.: the signal registered in the systemd service definition).