Re: scratch/sigchld-fd 8f0ce42 1/2: Fix deadlock when receiving SIGCHLD during 'pselect'.

[Philipp Stephani <p.stephani2@gmail.com>]

Am Di., 19. Jan. 2021 um 16:09 Uhr schrieb Eli Zaretskii <eliz@gnu.org>:
>
> > Date: Sat, 16 Jan 2021 13:49:49 -0500 (EST)
> > From: p.stephani2@gmail.com (Philipp Stephani)
> >
> > branch: scratch/sigchld-fd
> > commit 8f0ce42d3eb9b212424a4a25a376287ffc94a73e
> > Author: Philipp Stephani <phst@google.com>
> > Commit: Philipp Stephani <phst@google.com>
> >
> >     Fix deadlock when receiving SIGCHLD during 'pselect'.
> >
> >     If we receive and handle a SIGCHLD signal for a process while waiting
> >     for that process, 'pselect' might never return.  Instead, we have to
> >     explicitly 'pselect' that the process status has changed.  We do this
> >     by writing to a pipe in the SIGCHLD handler and having
> >     'wait_reading_process_output' select on it.
> >
> >     * src/process.c (child_signal_init): New helper function to create a
> >     pipe for SIGCHLD notifications.
> >     (child_signal_read, child_signal_notify): New helper functions to
> >     read from/write to the child signal pipe.
> >     (create_process): Initialize the child signal pipe on first use.
> >     (handle_child_signal): Notify waiters that a process status has
> >     changed.
> >     (wait_reading_process_output): Make sure that we also catch
> >     SIGCHLD/process status changes.
> >
> >     * test/src/process-tests.el
> >     (process-tests/fd-setsize-no-crash/make-process): Remove workaround,
> >     which is no longer needed.
>
> Philipp, can you please elaborate about this changeset (which was in
> the meantime merged with master)?  The comments you added mention some
> kind of deadlock, but don't describe what kind and in which situations
> it could happen, and without that it's hard to understand what
> problem(s) it tries to solve.

I had hoped that the fixed unit test and the commit description would
be clear enough. The issue appears to be that accept-process-output
frequently hangs (doesn't return) even though the process has already
finished. That's what process-tests/fd-setsize-no-crash/make-process
relies on. After some printf debugging, I concluded that
wait_reading_process_output wouldn't be notified about the process
status change in any way, and thus hang forever. A form like (sit-for
0.1) would trigger the process status change to be processed.

>
> In general, pselect is supposed to return with EINTR when SIGCHLD
> happoens while we are inside the call to pselect, and EINTR seems to
> be already handled by wait_reading_process_output.  So I wonder why we
> need that additional "self-pipe" to be watched by pselect.

Yes, I'm wondering about that as well, but it's definitely the
behavior I see. Before commiting to master, I ran the test
process-tests/fd-setsize-no-crash/make-process multiple times with and
without the commit, and the outcome was clear: without the commit
accept-process-output would frequently hang, with the commit it never
hangs.
This is pure speculation, but I could imagine multiple things going on:
- Maybe there's no guarantee that pselect actually returns EINTR on SIGCHLD.
- Maybe EINTR is returned too early, before the signal handler got the
chance to update the process status.

>
> In addition, AFAIU this pipe should not be needed on MS-Windows, where
> the pselect emulation waits on the sub-process handles together with
> the other file descriptors, and so gets awakened when a process exits
> or dies.  But again, without knowing the exact situations against
> which this changeset tries to protect, it is hard to make a decision.
>

It's definitely not needed on Windows, which has a superior mechanism
anyway (process handles are waitable objects in Windows). I opted to
create the additional pipe on Windows as well - the costs should be
small, and it keeps the code more consistent between the operating
systems.