GNU bug report logs - #79201
30.1.90; set-process-thread can permanently break fd_callback_info slots

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Message #14 received at 79201 <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: Spencer Baugh <sbaugh <at> janestreet.com>
Cc: 79201 <at> debbugs.gnu.org, dmitry <at> gutov.dev, johnw <at> gnu.org,
 app-emacs-dev <at> janestreet.com
Subject: Re: bug#79201: 30.1.90;
 set-process-thread can permanently break fd_callback_info slots
Date: Sat, 09 Aug 2025 13:33:40 +0300

> Cc: 79201 <at> debbugs.gnu.org, dmitry <at> gutov.dev, app-emacs-dev <at> janestreet.com
> Date: Fri, 08 Aug 2025 16:34:10 -0400
> From:  Spencer Baugh via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs <at> gnu.org>
> Here's a patch to fix it by initializing these fields.

Thanks, but I don't think this is right.  First,
update_processes_for_thread_death clears the .thread member of
fd_callback_info of those processes that were locked to the dying
thread.  Second, wait_reading_process_output_unwind clears the
.waiting_thread member of fd_callback_info for all descriptors, when
we exit wait_reading_process_output (which is called by
accept-process-output).  And finally, IMO it is wrong for the add_*_fd
functions to overwrite these two members, because they are set and
reset by other functions, mainly those which compute the wait mask for
the pselect call.  add_*_fd should not touch these members.

I believe in this case update_processes_for_thread_death doesn't do
its job because your scenario kills the process before the thread
dies, and update_processes_for_thread_death only looks at the
processes that are in Vprocess_alist.  So it is possible that all
that's missing is that delete_read_fd and delete_write_fd (which are
called from deactivate_process) should clear the .thread member of
fd_callback_info corresponding to the process's input and output
descriptors.

If that doesn't fix the issue with your reproducer, please investigate
why, and let's take it from there.

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