Maybe the problem is that it is "lib"systemd, meaning it doesn't really have control over when the process forks, it's the application code's responsibility. Although I would probably also avoid this design, but that would probably make the library interface a bit unfriendly, with the application needing to pass its own pid to library functions where appropriate.
The audacity of the design to detect a pid change by looking at it in a loop, is interesting. I don't think I could have come up with something like that. Feels very "sync the web DOM by traversing it and comparing with a shadow DOM".
If only we had an API to tell us when a fork happened. We could place it into the usual library for threading stuff: pthread, and could call it, oh I don't know, maybe pthread_atfork()
It is mentioned in the glibc bug tracker entry as a replacement to the polling. systemd didn't use it because they thought it would be impossible for a library using to be dlclose'd as there is no way to unregister the callback. But apparently glibc has (undocumented) magic under the hood to take care of it.
