[throwawaysysd]

A pid file is never actually reliable though. Since the supervised process has control over it, it can write any number it wants in there and trick the service manager into signaling the wrong process. As long as the process is not root and can't escape its own cgroup, systemd's pid detection is going to be less error-prone in basically every case.

I can't stress this enough. Pid files are really bad. The fact that we used to have to use them is a flaw in the OS design. Using them for startup notification is also a hack in and of itself. The kernel has enough information to fully track lifecycle of these processes without writing information into files that are inherently prone to race conditions, and we now have better APIs to expose this information, so we shouldn't need to use these hacks anymore. I don't think there is any Unix-like system left that considers the old forking daemon style to be a good idea, and systemd's handling of pidfiles is really just a legacy compatibility thing.