bug#18162: 24.3.92; segfault on null face pointer in face_for_char

[Eli Zaretskii <eliz@gnu.org>]

> From: Ken Raeburn <raeburn@permabit.com>
> Date: Sun, 3 Aug 2014 02:51:38 -0400
> Cc: 18162@debbugs.gnu.org
> 
> On Aug 2, 2014, at 08:50, Eli Zaretskii <eliz@gnu.org> wrote:
> > Moreover, even if I force the call to clear_font_cache by invoking
> > clear-font-cache in the progn, I don't see a crash, and the use count
> > of the frame's face cache is not zero.
> > 
> > So one way of tracking this down would be to put a breakpoint in
> > Fclear_face_cache, and when it breaks, step through the function until
> > it assigns the frame pointer to 'f', and put a watchpoint on
> > f->face_cache->used, to see which code zeroes it.  My guess would be
> > that some code calls free_realized_faces (I misremembered earlier:
> > clear_face_cache doesn't do that).
> 
> In the code I'm looking at, clear_face_cache can call free_all_realized_faces, but only if FRAME_DISPLAY_INFO(f)->n_fonts is larger than 10 for some frame.

Yes, I meant clear_face_cache doesn't do that directly.  And since
free_realized_faces is called from several other places, it's best not
to assume this particular call is necessarily the culprit.

> (And either clear_fonts_p is set, or you've made over 500 calls since the last font cache cleaning.) In my first evaluation of the lisp code I gave, the n_fonts field had the value 7; on the second evaluation it had the value 12, the cache got cleared, the null pointer was stored, and Emacs segfaulted.
> 
> I've done next to nothing with fonts and faces in Emacs source code, so I'm not sure what a good way is to drive up the number. If I run my test again without list-faces-display, the font counts I'm seeing are first 4 and then 7 for repeated evaluations, as displayed via gdb breakpoint commands. If I split the window with C-x 2 then the count goes up to 8 (new inactive mode line?). Reducing the font size (C-x -) made it jump to 14, and 15 on repeated evaluations, without triggering the problem, but when I set the font size back (C-x +) and evaluated the expression one more time, I got the crash.

Does the patch below help?

=== modified file 'src/fontset.c'
--- src/fontset.c	2014-01-01 07:43:34 +0000
+++ src/fontset.c	2014-08-03 15:00:11 +0000
@@ -1860,6 +1860,8 @@ DEFUN ("internal-char-font", Finternal_c
     return Qnil;
   if (!FRAME_WINDOW_P (f))
     return Qnil;
+  if (FRAME_FACE_CACHE (f)->used == 0)
+    recompute_basic_faces (f);
   face_id = FACE_FOR_CHAR (f, FACE_FROM_ID (f, face_id), c, pos, Qnil);
   face = FACE_FROM_ID (f, face_id);
   if (face->font)