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

[Eli Zaretskii <eliz@gnu.org>]

> From: Pip Cet <pipcet@gmail.com>
> Date: Fri, 12 Jul 2019 13:40:15 +0000
> Cc: politza@hochschule-trier.de, 36609@debbugs.gnu.org
> 
> When a thread is signalled (by thread-signal, which sets another
> thread's error_symbol) while the signalled thread is inside a
> select(), thread_select() will return non-locally for that thread, and
> we fail to release an internal GLib lock through
> g_main_context_release(). That's the first bug.

We should either release the global lock before the thread exits, or
defer the acting upon the signal until later.  We cannot disable the
signal handling altogether because it is entirely legitimate to signal
another thread, and when we do, that other thread will _always_ be
inside thread_select.

For the main thread, handling the signal in that situation shouldn't
be a problem, because it is not going to exit.  Right?

> When xg_select() fails to acquire the internal GLib lock, it simply
> does a select() on the remaining file descriptors:

Why does it fail to acquire that lock?

>   context_acquired = g_main_context_acquire (context);
>   /* FIXME: If we couldn't acquire the context, we just silently proceed
>      because this function handles more than just glib file descriptors.
>      Note that, as implemented, this failure is completely silent: there is
>      no feedback to the caller.  */
> 
> This seems like a second, albeit documented, bug to me. I think we're
> risking not waking up from the actual select because another
> (non-Emacs) thread happened to hold the main context at the time.

So what is the proposal for that? spin waiting for the lock?