> who knew how many scripts just haphazardly hand programs random file descriptors, that's surely not a problem.

Oh for fuck's sake! Why are you using random file descriptors nobody told you about? Those open fds are there for a reason, thank you: I've put an end of an open pipe specifically so I could notice when it will become closed.

If the user set up the environment of your application in a specific way, that means he wants your application to run in such an environment. If you were invoked with 10 non-standard file descriptors open and two injected threads — you'll have to live with it. Because, believe it or not, your application's purpose is to serve the user's goals. So don't break composability that the user relies on, please.

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.