getProcessExitCode/waitForProcess should throw ECHILD rather than return ExitSuccess for non-child process

[alt-romes]

https://www.man7.org/linux/man-pages/man2/wait.2.html

       ECHILD (for waitpid() or waitid()) The process specified by pid
              (waitpid()) or idtype and id (waitid()) does not exist or
              is not a child of the calling process.  (This can happen
              for one's own child if the action for SIGCHLD is set to
              SIG_IGN.  See also the Linux Notes section about threads.)

In https://github.com/well-typed/haskell-debugger, it took me a long while to uncover a bug that turned out to be caused by getProcessExitCode incorrectly returning Just ExitSuccess for a handle of a non-child process.

I'm "unsafely" (i.e with .Internals) constructing the handle from the PID of a process that was not spawned/is not a child of the process calling getProcessExitCode. It should throw something like ECHILD, following the manpage for wait.

I suspect waitForProcess should also fail with ECHILD here.

I've attached the AI generated counter example program I was using to observe the incorrect behavior outside of the much more complicated haskell-debugger

NonChildProcessHandleDemo.txt

[tomjaguarpaw]

Hi @alt-romes, what do you mean it should "throw" ECHILD? Do you mean it should throw some sort of exception with the same meaning as the ECHILD error code? Do you have candidate for the type of this exception in mind, or would it have to be a new one?

NonChildProcessHandleDemo.txt

Would it be possible to embed that code in a comment here (perhaps using the <details> and <summary> tags to collapse it)? It currently requires a download to view, which is inconvenient.