bug#56487: xgselect race condition leading to abort when USE_GTK not defined

[Po Lu via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Tom Gillespie <tgbugs@gmail.com> writes:

>> Thanks.  Why did the code previously under !USE_GTK have to be removed?
>
> When the !USE_GTK code is used an abort in glib will happen
> stochastically due to an out-of-sync call to release_select_lock
> in thread.c. This happens on my system somewhere between
> approximately 1 in 10 and 1 in 10000 times that the test file
> is run.
>
> As far as I can tell from testing there is no difference in behavior
> between the USE_GTK and !USE_GTK code. Also, as far as
> I can tell from reading, the behavior should be almost identical.
> The only addition is to check for already_has_events before
> calling thread_select, which may be enough to shift the timing
> to prevent a race.
>
> I have not been able to figure out what the actual underlying
> cause is (I tried). All I can say for sure is that there is
> something that calls into g_main_context_release and
> context->owner_count has a negative overflow to 4294967295.
>
> I do not think that it is because something somehow sneaks
> in between the calls to the atomics in acquire_select_lock
> and relese_select_lock. If you would like I can send along
> a couple of patches that include changes I made to try to
> see what is going on.
>
> The real underlying issue would seem to be that there is a
> missing lock somewhere and that the use of atomics is not
> sufficient, but I could be wrong about that.

So I suggest that someone find that problem instead.  Any attempt to
"fix" a race condition by moving code around so that the timings are
slightly different is simply deluding oneself.

Thanks.