bug#67977: 30.0.50; tree-sitter: Emacs crashes when accessing treesit-nodes in a narrowed buffer

[Yuan Fu]

> On Dec 28, 2023, at 8:16 AM, Dmitry Gutov <dmitry@gutov.dev> wrote:
> 
> On 28/12/2023 15:53, Eli Zaretskii wrote:
>>> Date: Thu, 28 Dec 2023 13:44:43 +0200
>>> Cc: Denis Zubarev<dvzubarev@yandex.ru>,67977@debbugs.gnu.org
>>> From: Dmitry Gutov<dmitry@gutov.dev>
>>> 
>>>>> Could font-lock-dont-widen help, perhaps?
>>>> Yes. If font-lock doesn’t widen, then there wouldn’t be back-and-forth 
>>>> reparses.
>>> But then treesit major modes will be affected by user narrowing (e.g. if
>>> the user narrowed to inside the string, the buffer won't be highlighted
>>> as a string).
>> Why is that a problem?  When the user narrows the buffer, the part
>> outside the narrowing doesn't exist as far as Emacs is concerned.
> 
> That's not how it works in most major modes, at least since the introduction 
> of font-lock-dont-widen 20 years ago.
> 
> Like its docstring says, the exceptions were supposed to be weird modes like 
> RMAIL and Info which use narrowing for their own purposes (that seems buggy 
> in Info's case, when 'C-x n w' breaks the intended display right away). But 
> even Info-mode doesn't actually change font-lock-dont-widen, actually, 
> because the apparent behavior would be the same. But it could.
> 
> I don't have a personal stake in this (I never use narrowing interactively). 
> But maybe you'll want to make a poll, to ask the users that do.

I guess that depends on how you view narrowing, I assume most of the time the 
user considers narrowing to be something that narrows the view to a region, 
rather than effectively removing the rest of the buffer. IIRC We’ve had 
discussions on adding “types” to narrows but to no conclusion.

Anyway, if the reparse caused by narrowing prove to be problematic, I can 
implement the optimization I mentioned earlier, where we use two parsers, one 
for when buffer is widened and one for when buffer is narrowed. This will be in 
C and is transparent to lisp.

Yuan