This is the first I've heard of using an open pipe to poll for subprocess termination. Don't get me wrong, I don't hate it, but you could just as easily have a SIGCHLD handler write to your pipe (or do nothing, since poll(2) will be fail with EINTR), and you don't have to worry about the subprocess closing the pipe or considering it some weird stddata fd like tree does here.

`SIGCHLD` is extremely unreliable in a lot of ways, `pidfd` is better (but Linux-specific), though it doesn't handle the case of wanting to be notified of all grandchildren's terminations after the direct child dies early.