[Bug 251227] setpgid sometimes returns ESRCH instead of EACCES

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=251227

--- Comment #5 from Mahmoud Al-Qudsi <mqudsi at neosmart.net> ---
Thanks for bringing my attention to the approach taken by sh - it made me
realize what I'd missed. So you're right, there's no broken job control here.

This is a real issue:

> [..] prevents a subsequent process in the same job from getting access to a
> shell that was assigned over to the newly minted pgrp that now contains only
> zombies

but it doesn't apply to FreeBSD, which is why sh can get away with ignoring any
setpgid failures. (Interestingly, tcsh handles this situation by swapping pgrps
in the middle of a single job invocation.)

It turns out that while FreeBSD does not consider zombie processes as valid
targets of a setpgid(2) call, it *does* keep their pgrp around and permits
other processes to join the pgrp even if it only contains zombies.

While the original issue persists and setpgid(zombie_pid, zombie_pid) calls
fail
with ESRCH instead of the expected EACCES, as you say, this is trivially
handled
if you have any degree of confidence that you are specifying a target process
that is or was a valid child pid from the same session... and (more tenuously)
assuming you know this is the behavior on at least some of the platforms you
are
targeting.

It's not the end of the world by any means, but I would argue that the
consideration of zombie child processes should be an everywhere or nowhere at
all type of thing (and this is where my assumption of broken job control came
from). e.g. Linux considers zombie processes in setpgid calls (only to
determine
the error code) and lets new processes join pgrps that contain only zombies,
whereas some kernels (e.g. WSLv1 up until recently) don't consider zombies at
all and reclaim pgrps as soon as the last process in it exits (before it's
reaped).

That aside, I'm assuming we can all agree it would be great for the man pages
to
clarify the behavior when it comes to zombie procs. Are you open to a patch?

Thanks!

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