mjhjmtu

I'm trying to shut down the computer when a specific list of systemd units stop.

The background is that I have a compute farm in Google cloud & AWS EC2, and each instance launches a couple services which perform a bunch of work and then exit when complete. Since these are cloud instances and cost money, the instance should shut down once it is no longer doing anything.

Most suggestions I've seen around this involve putting an ExecStopPost=/sbin/init 0 (or similar) in the systemd unit file. However I cannot shut the instance down until all the units have completed.
It's easy enough for me to create a target unit which wraps the individual worker units, and then put a StopWhenUnneeded=true in it so that the target stops once all the workers are stopped. But I don't see a way to use this to trigger the system to shut down (for example by invoking systemd-halt.service).

So the question is: how can I get the system to shut down once a list of units, or my target unit, stops.

After some experimentation, one working, not entirely ugly, solution I've come up with is to use the idea of the wrapper target unit, but use a service instead which shuts the system down on exit.

For example, the worker service:

# worker@.service
[Unit]
BindsTo=master.service
After=master.service

[Service]
ExecStart=/bin/myworker

[Install]
WantedBy=master.service

And then for the wrapper:

# master.service
[Unit]
StopWhenUnneeded=true

[Service]
Type=oneshot
RemainAfterExit=true
ExecStart=/bin/true
ExecStop=/sbin/init 0

I can then do systemctl start master.service to start all the workers (after systemctl enableing them), and when they all exit, master.service will stop and execute init 0.
systemctl stop master.service will also signal the worker units to stop, but will wait for them before executing the init 0.

The ugliness is having to create a fake service which executes /bin/true. It would be nicer if there were an existing systemd unit that when stopped would result in the system shutting down, but this is at least functional.