bug#61369: Problem with keeping tree-sitter parse tree up-to-date

[Yuan Fu]

> On Feb 17, 2023, at 4:11 PM, Dmitry Gutov <dgutov@yandex.ru> wrote:
> 
> On 18/02/2023 00:32, Yuan Fu wrote:
>> Thank you very much! I thought that clipping the change into the fixed 
>> visible range, and rely on treesit_sync_visible_region to add back the 
>> clipped “tail” when we extend the visible range would be equivalent to not 
>> clipping, but I guess clipping and re-adding affects how incremental parsing 
>> works inside tree-sitter.
> 
> It seems like the "repairing" sync used a different range, one that didn't 
> include the character number 68 inserted from the beginning.
> 
> It just synced the 1 or 2 characters at the end of the buffer, the difference 
> between the computed visible_end and the actual BUF_ZV_BYTE.

That should be enough, no? Because other text didn’t change, they just moved. 
And tree-sitter should know that they moved. Or maybe I’m misunderstanding what 
you mean.

> 
>> I don’t think this change would have any adverse effect, because if you 
>> think of it, inserting text in a narrowed region always extends the region, 
>> rather than pushing text at the end out of the narrowed region. So the right 
>> thing to do here is in fact not clipping new_end_offset.
> 
> I figured it could be a problem if both old_end_byte and new_end_byte extend 
> past the current restriction.

That should be fine (ie, technically correct), since when we widen, the clipped 
text are reparsed by tree-sitter as new text.

> 
> But I'm not sure whether that could actually happen in practice. The obvious 
> attempts (undo a change outside of the narrowing, or revert the buffer when 
> narrowing is in effect) didn't play out, but I'm not sure whether there is an 
> actual hard limit on modifying the text outside of the current restriction.

It is my impression that Emacs in general enforces the narrowing restriction 
strictly. And we are still correct when exceptions occur.

Yuan