bug#52298: 29.0.50; Frequent redisplay cycles induced by c-type-finder-timer-func timer in CC Mode

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sat, 11 Dec 2021 14:52:22 +0000
> Cc: 52298@debbugs.gnu.org, acm@muc.de
> From: Alan Mackenzie <acm@muc.de>
> 
> c-is-sws (along with c-in-sws) marks syntactic whitespace in a buffer so
> that especially for long comments, passing over that WS is rapid (after
> the first pass has marked the properties).
> 
> c-type marks certain types of identifiers and positions related to a CC
> Mode declaration, e.g. the start of a declarator, or the end of the
> previous statement.
> 
> > Could we perhaps refrain from putting them on buffer text when those
> > functions are called from the timer?
> 
> That would not be sensible.  Both of them are for optimisation, and
> preventing them being used from the timer would involve an involved
> (slow) mechanism.

But we are talking about the timer whose job is to find type
declarations.  Does that job require these properties?

> OK, I think I see what the problem is, now.  It's the middle line in ....
> 
>   redisplay_internal 0
>   071a03c8 (xdisp.c): try_window_id 2
>   redisplay_preserve_echo_area (8)
> 
> ..... , which indicates deep processing in redisplay.  (Yes, I know you've
> been telling me this for a while...)  The question is why does the code
> get that deep in rather than being aborted earlier?

I already established that, it's the fact that the buffer's modified
tick is increasing.  This then causes this test:

  current_matrix_up_to_date_p
    = (w->window_end_valid
       && !current_buffer->clip_changed
       && !current_buffer->prevent_redisplay_optimizations_p
       && !window_outdated (w)
       && !hscrolling_current_line_p (w));

to fail because window_outdated returns non-zero.  That's how I knew
that the buffer's modified tick is the culprit.

> Another thing.  After waiting the ~2 minutes for the background scanning
> to complete, I had a look at which character positions had the
> `fontified' text property, using a simple utility I wrote some years ago:
> [...]
> Using the [f10] key (or just typing M-x get-fontified, if F10 is
> otherwise occupied) the following positions ended up fontified in
> X-Windows after that 2 minute pause:
> 
>     "Fontified regions: ((1 . 1740))"
> 
> ,  That is, at the end, only the visible portion and a bit more were
> fontified.  This suggests (though not conclusively) that no fontification
> happened anywhere else in the buffer.

So why is the timer function keep running for so long, and why does it
put those two other properties on the rest of the buffer?  It sounds
to me like you could stop the timer once the visible portion of the
buffer has been reached, because no type after that can affect
fontification.  You could then restart the timer when the buffer is
modified, or if the window is scrolled to reveal a portion of the
buffer below the current end-of-window.