bug#25178: 26.0.50; Crash when pressing C-g in TTY mode

[Elias Martenson <elias.martenson@murex.com>]

Eli Zaretskii <eliz@gnu.org> writes:

> > From: Andreas Schwab <schwab@linux-m68k.org>
> > Cc: Elias Martenson <elias.martenson@murex.com>,  25178@debbugs.gnu.org
> > Date: Tue, 13 Dec 2016 20:26:10 +0100
> > 
> > On Dez 13 2016, Eli Zaretskii <eliz@gnu.org> wrote:
> > 
> > > Hmm...  Is calling pthread_mutex_unlock twice in a row, without an
> > > intervening call to pthread_mutex_lock, supposed to segfault?
> > 
> > It's undefined, so anything can happen.  Just don't do that.
> 
> At this point, I'd like to establish whether the fact Emacs does do it
> could explain crashes reported by Elias which I cannot reproduce on a
> different GNU/Linux system.
> 
> What I think happens is that C-g on a TTY produces a SIGINT that
> interrupts the call to pselect and runs handle_interrupt, which then
> longjmps back to read_char, which then calls thread_select.  But since
> pselect inside the previous call to thread_select was interrupted, the
> following call to acquire_global_lock was not done, and we are now
> running with the global lock unlocked.  Then thread_select calls
> release_global_lock which attempts to unlock the (unlocked) mutex.
> 
> The question is whether this is just bad, or causes the crash.  It
> doesn't crash for me.

I'm using Arch Linux, which tends to use quite bleeding edge versions
pretty much everything.

Since I don't recall seeing this issue until recently, I'm
suspecting—although I have no proof of this—that they might have
recently introduced a change that causes this to crash instead of
silently accepting it, perhaps as a way to find bugs like this?

Regards,
Elias
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