Subject: bug#64321: 29.0.92; Tree-Sitter/which-function Narrow/Widen causes modified buffer

From: Yuan Fu <[email-masked]>
Date: Tue, 27 Jun 2023 21:04:22 -0700

> On Jun 27, 2023, at 7:23 PM, Troy Brown <[email-masked]> wrote:
> 
> 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])

Thanks for the backtrace, it’s very helpful.

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.

Yuan