bug#45898: 27.1; wedged in redisplay again

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: larsi@gnus.org,  psainty@orcon.net.nz,  Emacs-hacker2018@jovi.net,
>   45898@debbugs.gnu.org
> Date: Thu, 23 Jun 2022 17:14:50 -0400
> 
> >> For such huge files, it's arguably more important to be more-or-less
> >> usable than it is to get the highlighting right, so there's a good case
> >> for turning off font-lock or breaking it somewhat by using arbitrary
> >> limits in the handling of `font-lock-extend-region-functions` in
> >> `font-lock-default-fontify-region`.
> > I'm talking about cases where the user visits such files without
> > knowing in advance that he/she has better turned off font-lock there.
> > I'm asking whether one of the things we could do when we discover that
> > parse-partial-sexp is called for such a large chunk of text could be
> > automatic introduction of such a limit only for that buffer, so that
> > font-lock won't be such a significant part of the reason for redisplay
> > slowness in these cases.
> 
> Yes, that's exactly what I'm proposing in the paragraph you quoted.
> I think it makes sense to bound the growth of the region due to
> `font-lock-extend-region-functions`.  We could bound it where we handle
> `font-lock-extend-region-functions`, or we could bound it just inside
> `font-lock-extend-region-wholelines`.

I hope this will be done soon.

I've landed the feature on master, let me know (everyone, not just
Stefan) how it fares.

Gerd, your comments will be most appreciated, TIA.

> > I guess this is related to font-lock
> > settings of js.el, but my question is: would any mode that uses
> > syntactic fontification be similarly affected, or is there anything
> > special with js.el specifically?
> 
> I think pretty much any major mode would be affected.  It can be more or
> less severe depending on how much work is done inside
> `font-lock-keywords` (and `syntax-propertize`), but by and large I'd
> expect it to suck for all non-trivial modes.

For now, you will see that the code I installed "punishes" syntax.c
for exploring too many character positions, hope this is "good enough"
for the relevant pathological situations.