bug#25279: 26.0.50; Slowdown/crash on certain characters

[Eli Zaretskii <eliz@gnu.org>]

> Date: Tue, 21 Feb 2017 22:28:05 +0200
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 25279@debbugs.gnu.org
> 
> > From: Richard Copley <rcopley@gmail.com>
> > Date: Tue, 21 Feb 2017 20:05:28 +0000
> > Cc: 25279@debbugs.gnu.org
> > 
> > Here's how to make Emacs hang without the need to uninstall any fonts,
> > on Windows (not reproducible on GNU/Linux).
> > 1. Type "M-x view-hello-file" (don't press return).
> > 2. In quick succession, type "C-m" and then "C-g". *
> 
> Thanks.
> 
> > Eli, should I retitle this bug to "26.0.50; calling SendMessage inside
> > critical section causes deadlock", or close this bug as a duplicate of
> > the other "slow fonts on Windows" bugs and file a new report about
> > the hanging?
> 
> The former, I guess.  I will take a look when I have time.

I think I found the reason: due to C-g, we were jumping to top-level
while in a critical section, which is a very bad idea, because any
non-main thread that tries to acquire the critical section after that
will necessarily hang.

Please try the latest master and see if any such hangs are still
there.

Thanks for the recipe, it made it quite easy to catch the villain
red-handed.