Re: Crash with --enable-checking and some glyphs

[Eli Zaretskii <eliz@gnu.org>]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: rpluim@gmail.com,  emacs-devel@gnu.org
> Date: Fri, 26 Nov 2021 15:30:45 +0100
> 
> (gdb) p it->what
> $1 = IT_CHARACTER
> (gdb) p/x it->c
> $2 = 0xace0
> (gdb) pgrowx it->glyph_row
> TEXT: 2 glyphs
>   0    0: CHAR[^] pos=1 blev=0,btyp=L w=22 a+d=35+9 MB
>   1   22: CHAR[0xace0] pos=2 blev=0,btyp=L w=24 a+d=25+-1 face=21 MB

OK, so the glyph for this character, U+ACE0, comes out with a negative
descent value of -1.

Please run Emacs again, under GDB, with a breakpoint like this:

  (gdb) break xdisp.c:30866 if it->c == 0xace0

The breakpoint should be on the line shown with "<<<<" below:

  void
  gui_produce_glyphs (struct it *it)
  {
    int extra_line_spacing = it->extra_line_spacing;

    it->glyph_not_available_p = false;

    if (it->what == IT_CHARACTER)
      {
	unsigned char2b;
	struct face *face = FACE_FROM_ID (it->f, it->face_id);
	struct font *font = face->font;
	struct font_metrics *pcm = NULL;
	int boff;			/* Baseline offset.  */

	if (font == NULL)  <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
	  {
	    /* When no suitable font is found, display this character by
	       the method specified in the first extra slot of
	       Vglyphless_char_display.  */
		Lisp_Object acronym = lookup_glyphless_char_display (-1, it);

Then step with the "next" command through the code there, and see
where it->descent gets its negative value.  In particular, when you
get to this part:

	  if (get_char_glyph_code (it->char_to_display, font, &char2b))
	    {
	      pcm = get_per_char_metric (font, &char2b);
	      if (pcm->width == 0
		  && pcm->rbearing == 0 && pcm->lbearing == 0)
		pcm = NULL;
	    }

	  if (pcm)
	    {
	      it->phys_ascent = pcm->ascent + boff;
	      it->phys_descent = pcm->descent - boff;
	      it->pixel_width = pcm->width;

please tell what are the values of pcm->ascent and pcm->descent.

I want to know whether the font is the culprit or some code of ours
overrides the value from the font.  Then we can devise the solution.

Thanks.