Linux – How to tell systemd to stop service when it no longer required/wanted

What appears to be a deadlock is actually the Type=oneshot directive working as advertised. The "hang" can be triggered by running your another.service directly:

systemctl start another

Once cancelled, one can review journalctl to find it wasn't "hung" at all, but running an infinite loop as expected. In man systemd.service, we find the documentation for Type=oneshot:

Behavior of oneshot is similar to simple; however, it is expected that the process has to exit before systemd starts follow-up units.

In other words, you have written an infinite loop and then instructed systemd to wait till it finishes before moving on.

You either need to use a different Type=, like Type=simple, or quit using an infinite loop for your main process.

I found the answer: I needed to change the last line of the service file to:

WantedBy=postgresql.service

This way, whenever Postgres is started, my service is started too - but if my service fails, that doesn't stop Postgres.

Directives in the [Install] section only affect enabling and disabling of units. But it wasn't this simple when my service was already enabled:

# systemctl enable tabill.service
Failed to execute operation: Too many levels of symbolic links

The error message was misleading. Fixing it was simple:

# systemctl disable tabill.service
Removed symlink /etc/systemd/system/tabill.service.
Removed symlink /etc/systemd/system/multi-user.target.wants/tabill.service.

# systemctl enable tabill.service
Failed to execute operation: No such file or directory

# systemctl enable /srv/tabill/tabill.service
Created symlink from /etc/systemd/system/postgresql.service.wants/tabill.service to /srv/tabill/tabill.service.
Created symlink from /etc/systemd/system/tabill.service to /srv/tabill/tabill.service.

Now my service stops and starts whenever Postgres does. And naturally Postgres starts when the system boots.