bug#36609: 27.0.50; Possible race-condition in threading implementation

[Eli Zaretskii]

> From: Pip Cet <pipcet@gmail.com>
> Date: Fri, 12 Jul 2019 09:02:22 +0000
> Cc: 36609@debbugs.gnu.org
> 
> On Thu, Jul 11, 2019 at 8:52 PM Andreas Politz
> <politza@hochschule-trier.de> wrote:
> > I think there is a race-condition in the implementation of threads.  I
> > tried to find a minimal test-case, without success.  Thus, I've attached
> > a lengthy source-file.  Loading that file should trigger this bug and
> > may freeze your session.
> 
> It does here, so I can provide further debugging information if
> needed.

Thanks, can you provide the info I asked for?

> On first glance, it appears that xgselect returns abnormally with
> g_main_context acquired in one thread, and then other threads fail
> to acquire it and loop endlessly.

If you can describe what causes this to happen, I think we might be
half-way to a solution.

> +  ptrdiff_t count = SPECPDL_INDEX ();

I don't think we should do that at this low level.