bug#56682: locked narrowing

[Gregory Heytings]

> > 1. M-: (let ((large-file-warning-threshold nil)) (find-file "dictionary.json") 
> > (narrow-to-region 4000000 4004000)) RET
> > 2. C-x n w
> > 3. Kaboom!
>
> By "Kaboom!" you mean what? a crash?  Because it doesn't crash here. 
> This is a build from the latest emacs-29 branch.

I mean that Emacs is trapped in the long line problem again, don't you see 
it?

> This is all beyond argument.  We do want the heuristic.  I just want it 
> to be cheaper than it is now, especially for buffers without any long 
> lines, where each time we run this loop we waste CPU cycles.  So I'm 
> looking for ways of wasting less of them.

There are none, or at least none that I can see.  Your proposed heuristic 
to limit the scan to some portion of the buffer around point cannot work, 
precisely because the loop is not called during each redisplay cycle, but 
only from time to time.  See the example above.  Again, we cannot know 
what happened in the buffer between these two moments, which could be far 
away in time. We can either use a heuristic on the whole buffer, or not 
use a heuristic and scan only a portion of the buffer.  We cannot combine 
both, and use a heuristic on a portion of the buffer.  It would simply be 
too unsafe.