bug#70367: 30.0.50; Inconsistent Syntax Highlighting

[Amol Surati]

Hello, Alan.

On Sun, 14 Apr 2024 at 08:16, Alan Mackenzie <acm@muc.de> wrote:
>
> Hello, Amol.
>
> Thanks for taking the trouble to report this bug, and thanks even more
> for the convenient test file generator, which was extremely helpful.

Thank you for the kind words.

>
> On Sun, Apr 14, 2024 at 03:44:01 +0530, Amol Surati wrote:
> > On Sun, 14 Apr 2024 at 00:35, Eli Zaretskii <eliz@gnu.org> wrote:
>
> > > > From: Stephen Berman <stephen.berman@gmx.net>
> > > > Cc: suratiamol@gmail.com,  70367@debbugs.gnu.org
> > > > Date: Sat, 13 Apr 2024 21:00:16 +0200
>
> > > > On Sat, 13 Apr 2024 20:48:28 +0300 Eli Zaretskii <eliz@gnu.org> wrote:
>
> > > > >> Cc: 70367@debbugs.gnu.org
> > > > >> Date: Sat, 13 Apr 2024 20:44:37 +0300
> > > > >> From: Eli Zaretskii <eliz@gnu.org>
>
> > > > >> > From: Amol Surati <suratiamol@gmail.com>
> > > > >> > Date: Sat, 13 Apr 2024 18:12:54 +0530
>
> > > > >> > The problem is not found in terminal emacs built from the released 
> > > > >> > 29.3.tar.gz,
> > > > >> > or with emacs running under GUI (i.e. under PGTK).
>
> > > > >> > The problem is seen with terminal emacs built from the master 
> > > > >> > branch, at various
> > > > >> > commit levels.
>
> > > > >> > Problem: When a large file (for e.g. vulkan_core.h) is opened, 
> > > > >> > certain
> > > > >> > constructs have their syntax highlighting broken. The video found 
> > > > >> > at [1] shows
> > > > >> > the behaviour. At the end of the video, one can see one instance 
> > > > >> > of the problem;
> > > > >> > the syntax highlighting for the enum constant
> > > > >> > 'VK_STRUCTURE_TYPE_EVENT_CREATE_INFO = 10,' abruptly breaks. The 
> > > > >> > entire
> > > > >> > identifier VK_STRUCTURE_TYPE_EVENT_CREATE_INFO must be one colour. 
> > > > >> > Instead,
> > > > >> > 'VK_STRUCTURE_TYPE_EVENT_CREA' is of the expected colour, while
> > > > >> > 'TE_INFO' is of the colour that is expected with '= 10,'. You may 
> > > > >> > want to
> > > > >> > download the video and then play it, if Google Drive plays it at a 
> > > > >> > resolution
> > > > >> > that is lower than the video's native resolution.
>
> > > > >> > Within this same session, there were other such enum constants 
> > > > >> > with broken
> > > > >> > highlighting, though they have not been captured in the video.
> > > > >> > The termscript is attached at [2].
>
> > > > >> > The graphics session is Wayland with swaywm as its compositor; 
> > > > >> > XWayland is
> > > > >> > not enabled. The terminal emulator is 'foot'. Another terminal 
> > > > >> > emulator,
> > > > >> > 'alacritty' was also tested; the problem occurred there too.
>
> > > > >> > The problem doesn't seem to occur with small-sized files; After 
> > > > >> > reducing the
> > > > >> > vulkan_core.h to contain only around 235 lines, emacs was able to 
> > > > >> > show the
> > > > >> > (reduced) file with consistent highlighting.
>
> > > > I see exactly the same misfontification as the OP in the same file
> > > > (which I happen to have on my system), as well as several more similar
> > > > misfontifications further down in that file -- but only with c-mode from
> > > > cc-mode.el.  With c-ts-mode I see no misfontifications in that file.
> > > > This is with GNU Emacs 30.0.50 (build 1, x86_64-pc-linux-gnu, GTK+
> > > > Version 3.24.41, cairo version 1.18.0) of 2024-04-11.
>
> > > Strange.  I see no misfontifications with either mode.
>
> > Apologies. I missed Eli's email about the C modes.
>
> > My emacs build is devoid of most of the settings and
> > features, including GUI and tree-sitter (the config command is in
> > the original report). So it is likely that only cc-mode is affected,
> > and not c-ts-mode.
>
> This is indeed the case.

Understood.

>
> > Note also that vulkan_core.h isn't special. A C source/header file
> > with a long enough enum definition also works. Attached is a C
> > program that generates to stdout the contents of such a header
> > file. Opening the contents (after they are saved to a file by stdout
> > redirection, etc.) in emacs demonstrates the problem.
>
> The problem is long stretches of code (>= 500 characters) where there're
> no statement boundaries or braces.  These frequently occur in enums.  An
> ad hoc limit to 500 characters backward search is there for speed.

Consistent with the observed behaviour, that it is mostly enums that are
affected.

>
> However, this bit of code was not checking whether it found a
> brace/statement or hit the 500 char limit, hence the mis-fontification.
>
> The patch below tries to fix this.  Would you please apply it to
> cc-mode.el (in .../lisp/progmodes), byte compile the result, and load it
> into your Emacs (or restart Emacs).  Then please try it out on the real
> files that showed the bug.  Please let me know if the bug really is
> fixed.  (If you want any help with patching or byte compiling, feel free
> to send me private email.)

Thanks for the patch. It indeed fixes the highlighting problem on the
real file vulkan_core.h (I know about only this one real file that's affected),
as well as it does on the test file.

-Amol

>
>
>
> diff -r 709b797bdef8 cc-mode.el
> --- a/cc-mode.el        Tue Mar 26 20:26:16 2024 +0000
> +++ b/cc-mode.el        Sun Apr 14 02:39:32 2024 +0000
> @@ -2437,7 +2437,7 @@
>                            (backward-char)
>                            (setq pseudo (c-cheap-inside-bracelist-p 
> (c-parse-state)))))))
>                (goto-char pseudo))
> -            t)
> +            pseudo)
>            ;; Move forward to the start of the next declaration.
>            (progn (c-forward-syntactic-ws)
>                   ;; Have we got stuck in a comment at EOB?
>
>
> > -Amol
>
>
> > > Alan, would you please have a look?
>
> --
> Alan Mackenzie (Nuremberg, Germany).