bug#44818: 27.0.91; wedged

[Eli Zaretskii]

> Cc: 44818@debbugs.gnu.org
> From: Devon Sean McCullough <Emacs-hacker2018@jovi.net>
> Date: Tue, 24 Nov 2020 20:35:00 -0500
> 
> On 24/11/2020 13:48, Eli Zaretskii wrote:
> >> Cc: 44818@debbugs.gnu.org
> >> From: Devon Sean McCullough <Emacs-hacker2018@jovi.net>
> >> Date: Tue, 24 Nov 2020 13:42:03 -0500
> >>
> >> P.S.  Could a watchdog timer safely abort such wedgitude?
> > 
> > That would have to run in a separate thread, right?
> > 
> > But what timeout to set it to? some jobs might legitimately take some
> > time.  And then what to do when the timer expires?
> 
> The job of redisplay cannot legitimately take > 50 milliseconds.

It can, and it does.  Although this is quite rare, it does happen.
Here's a simple example: visit src/xdisp.c, then type M-> and measure
the time.

> When the user repeatedly tries ^G quit but Emacs is unresponsive
> because of redisplay, such redisplay must be prevented while the
> user is offered options to regain control.

I asked "what to do when the timer expires" because the practical
meaning of "redisplay must be prevented" is not at all clear.

> P.S. Could an invisibility overlay offer temporary relief?
> Perhaps a red screen of redisplay death with a short menu?

Anything that has to be done by the display code will suffer from the
same problem.