bug#59630: 29.0.50; treesitter-buffer-root-node doesn't change when changing buffer restriction

[Yuan Fu]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Yuan Fu <casouri@gmail.com>
>> Date: Sun, 27 Nov 2022 14:40:41 -0800
>> Cc: 59630@debbugs.gnu.org,
>>  Eli Zaretskii <eliz@gnu.org>
>> 
>> >  ELISP> (widen)
>> >  ELISP> (treesit-buffer-root-node 'bash)
>> >  #<treesit-node
>> >  (program)
>> >  in 1-4>  ;; <---- This is not expected, the root node should span 1-9
>> > 
>> >  ELISP> (buffer-string)
>> >  "echo '123'"
>> 
>> Thanks. We didn’t edit the buffer after widening, so tree-sitter
>> didn’t reparse and used the old tree, which sees the narrowed
>> buffer. Eli, what would be a good and reliable way to know that
>> narrowing has changed? I see current_buffer->clip_changed set to 1
>> in narrow-to-region and widen, but when are they set to 0?
>
> Not sure what exactly are you after.  If you want to catch the moment when
> we change the buffer restriction, you will have to add something to
> Fnarrow_to_region and Fwiden.  However, why does tree-sitter need to know
> about changes in the narrowing, unless it is asked to update something or
> produce a tree?  I thought we decided to update this stuff lazily, only when
> actually needed?  Being sensitive to these changes would require you to have
> some logic, because a buffer can be narrowed and widened several times in a
> sequence without any consequences for tree-sitter, and asking the parser to
> update itself will just burn CPU cycles.  So if this is really needed, let's
> discuss for which purposes and under what conditions.
>
> I actually don't think why we should be worried by the above scenario; can
> you explain?
>

We still parse lazily, and narrow/widen wouldn’t affect the parse tree,
until user requests for a node when the restriction is different from
last time we parsed the buffer.  Basically:

request-node <-- last time we parsed
narrow
widen
narrow
widen
request-node <-- no need to reparse (1)

request-node <-- last time we parsed
edits-buffer
request-node <-- need to reparse (2)

request-node <-- last time we parsed
narrow
request-node <-- need to reparse (3)

Right now in case (3) we don’t reparse the buffer. I have a reasonable
fix in f794263da20.

> To answer your question: the clip_changed flag is reset in
> reconsider_clip_changes, which is called at the beginning of redisplay, and
> when a window's redisplay is successfully completed (see
> mark_window_display_accurate_1).  However, this flag is not meaningful
> outside of redisplay, in particular when redisplay cycle is NOT running.  So
> I'm not sure it is what you want.

Thanks. Yeah that’s not something we want to use.

Yuan