GNU bug report logs - #64321
29.0.92; Tree-Sitter/which-function Narrow/Widen causes modified buffer

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Yuan Fu <casouri <at> gmail.com>
Cc: tracker <at> debbugs.gnu.org
Subject: bug#64321: closed (29.0.92; Tree-Sitter/which-function
 Narrow/Widen causes modified buffer)
Date: Wed, 28 Jun 2023 20:11:01 +0000

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Your message dated Wed, 28 Jun 2023 13:09:41 -0700
with message-id <3B76F5DF-F50C-4AFB-ADB6-5F1B76AFD58C <at> gmail.com>
and subject line Re: bug#64321: 29.0.92; Tree-Sitter/which-function Narrow/Widen causes modified buffer
has caused the debbugs.gnu.org bug report #64321,
regarding 29.0.92; Tree-Sitter/which-function Narrow/Widen causes modified buffer
to be marked as done.

(If you believe you have received this mail in error, please contact
help-debbugs <at> gnu.org.)


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From: Troy Brown <brownts <at> troybrown.dev>
To: bug-gnu-emacs <at> gnu.org
Subject: 29.0.92;
 Tree-Sitter/which-function Narrow/Widen causes modified buffer
Date: Tue, 27 Jun 2023 22:23:52 -0400

This problem seems to manifest with multiple tree-sitter modes.  It
appears there is an interaction with which-function-mode and tree-sitter
which causes the buffer to be considered modified either after a narrow
or widen of the buffer.

To reproduce this, use a "hello_world.c" as follows:

--8<---------------cut here---------------start------------->8---
#include <stdio.h>

int main (void)
{
  printf("Hello, world!\n");
  return 0;
}
--8<---------------cut here---------------end--------------->8---

Running "emacs -Q" with the corresponding tree-sitter grammar library
installed.  Move into the main function and narrow-to-defun.  In some
modes, I've seen this cause the buffer to show as modified, however with
this example, it seems to occur after widening.

The following sequence of commands can be used to reproduce the issue:

C-x C-f ~/hello_world.c
M-x c-ts-mode
M-x which-function-mode
C-x n d
C-x n w

At this point, the modeline shows that the buffer is modified, and
running "M-: (buffer-modified-p)" indicates "t".  As previously
mentioned, this appears to only happen with tree-sitter modes when
which-function-mode is enabled in the buffer.

I then repeated the same but also triggered a backtrace on the first
change as follows:

M-: (add-to-list 'first-change-hook #'backtrace)

With that in place, the following is the generated traceback:

  backtrace()
  treesit--font-lock-notifier(((1 . 21)) #<treesit-parser for c>)
  treesit-buffer-root-node(c)
  treesit-node-at(68)
  treesit--things-around(68
"\\(?:class_specifier\\|enum_specifier\\|function_defi..."
c-ts-mode--defun-valid-p)
  treesit-thing-at-point(("\\(?:class_specifier\\|enum_specifier\\|function_defi..."
. c-ts-mode--defun-valid-p) top-level)
  treesit-defun-at-point()
  treesit-add-log-current-defun()
  c-ts-mode--emacs-current-defun-name()
  add-log-current-defun()
  which-function()
  which-func-update-1(#<window 3 on hello_world.c>)
  which-func-update()
  apply(which-func-update nil)
  timer-event-handler([t 0 0 500000 t which-func-update nil idle 0 nil])

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From: Yuan Fu <casouri <at> gmail.com>
To: Troy Brown <brownts <at> troybrown.dev>
Cc: Eli Zaretskii <eliz <at> gnu.org>, 64321-done <at> debbugs.gnu.org
Subject: Re: bug#64321: 29.0.92; Tree-Sitter/which-function Narrow/Widen
 causes modified buffer
Date: Wed, 28 Jun 2023 13:09:41 -0700

> On Jun 28, 2023, at 9:25 AM, Troy Brown <brownts <at> troybrown.dev> wrote:
> 
> On Wed, Jun 28, 2023 at 12:04 AM Yuan Fu <casouri <at> gmail.com> wrote:
>> 
>> Narrowing and widening are transparent to tree-sitter, so when the buffer widens, from tree-sitter’s POV, text are added to the buffer. Which-func calling the tree-sitter current-defund function ultimately leads to tree-sitter calling treesit—font-lock-notifier which puts text properties on the “added” portion of the buffer, which changed the modified status.
>> 
>> treesit—font-lock-notifier should put text properties without changing modified status, like what jit-lock does. This patch should fix it.
>> 
> 
> Thanks for the explanation and the supplied patch.  I can confirm that
> this patch fixes the issue.

Cool, thanks, pushed.

Yuan

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