Re: master 64f2c96 2/2: Make a process test faster.

[Philipp Stephani]

Am Sa., 9. Jan. 2021 um 19:12 Uhr schrieb Philipp Stephani
<p.stephani2@gmail.com>:
>
> Am Fr., 8. Jan. 2021 um 02:38 Uhr schrieb Glenn Morris <rgm@gnu.org>:
> >
> >
> > It is now unstable, ie mostly passes but sometimes fails.
> >
> > Ref eg
> > https://hydra.nixos.org/build/134561612
> > https://hydra.nixos.org/build/134556213
>
> Interesting. I can't see why it would deadlock.

OK, I think I've found 2 bugs.
1. I've introduced one myself when writing emacs_spawn - we need to
mark processes as live before unblocking SIGCHLD, otherwise the
SIGCHLD handler would never call waitpid for them.
2. The other one happens if SIGCHLD is signaled during
wait_reading_process_output. Then wait_reading_process_output will
wait forever, since the stdout FD never gets closed and it doesn't see
the process status update in time. Not sure how to fix this - the most
elegant way would be pidfd/pdfork, but that's not available on all
systems.