Subject: emacs-23.2 crashes in get_glyph_face_and_encoding with a null face returned by FACE_FROM_ID(0)

From: John Lumby <[email-masked]>
Date: Fri, 9 Jul 2010 09:54:55 -0400

paste this text into any lisp-interaction buffer e.g. *scratch*,  position cursor at end, and press Ctl-X Ctl-E

(progn (modify-frame-parameters (selected-frame) '((menu-bar-lines . 0)))
(set-face-font 'modeline "-misc-fixed-medium-r-normal--13-120-75-75-c-70-iso8859-1")
 (face-set-after-frame-default (selected-frame)) (set-face-background 'default (cdr (assoc 'background-color (frame-parameters (selected-frame))))) (set-face-background 'fringe (cdr (assoc 'background-color (frame-parameters (selected-frame))))) (set-face-foreground 'default (cdr (assoc 'foreground-color (frame-parameters (selected-frame))))) (server-start))

I get a crash in get_glyph_face_and_encoding. 

It did not happen in 23.1

I started emacs with something like
emacs --name=sawlist --title=sawlist -ms "#14892b" -cr red -fn "-Misc-Fixed-bold-R-*--15-*-*-*-C-90-ISO8859-1" -bg "#E0EDED" -fg "#751503" -q -load sawlist.el

I can send my sawlist.el  if needed

I obtained a core file after rebuilding to get the debugging information and here is the relevant piece
(gdb) bt
#0  0x00d57424 in __kernel_vsyscall ()
#1  0x00876006 in kill () at ../sysdeps/unix/syscall-template.S:82
#2  0x0810fe76 in fatal_error_signal (sig=11) at emacs.c:402
#3  <signal handler called>
#4  get_glyph_face_and_encoding (f=0x920c4f0, glyph=0x946b9a0, char2b=0xbfbeb890, two_byte_p=0xbfbeb85c) at xdisp.c:19511
#5  0x0806bc58 in fill_glyph_string (s=0xbfbeb8b0, face_id=0, start=<value optimized out>, end=1, overlaps=0) at xdisp.c:19681
#6  0x0806c68b in draw_glyphs (w=0x920c670, x=27, row=0x93fa530, area=TEXT_AREA, start=0, end=1, hl=DRAW_NORMAL_TEXT, overlaps=0)
    at xdisp.c:20297

This patch (below) fixes one obvious mistake (but not the cause) and also works around the problem.  This mistake is that the assert is located after the dereference of the face pointer  - should be before.   But that just changes the crash to an assert failure.   The workaround to is to try all other faces cached on the frame.    I don't know what the correct fix is for the default face being null.

John Lumby

--- src/dispextern.h.orig	2010-04-03 18:26:13.000000000 -0400
+++ src/dispextern.h	2010-07-08 11:17:41.000000000 -0400
@@ -1664,12 +1664,19 @@ struct face_cache
      else					\
        (void) 0
 
+/* search for the first usable cached face on frame F,
+   or null if such a face doesn't exist.  */
+
+struct face *try_all_cached_faces(struct frame *f);
+
 /* Return a pointer to the face with ID on frame F, or null if such a
-   face doesn't exist.  */
+   face doesn't exist.   But try *all* faces cached for this frame before giving up  */
 
 #define FACE_FROM_ID(F, ID)				\
      (((unsigned) (ID) < FRAME_FACE_CACHE (F)->used)	\
-      ? FRAME_FACE_CACHE (F)->faces_by_id[ID]		\
+         ? (FRAME_FACE_CACHE (F)->faces_by_id[ID] ? FRAME_FACE_CACHE (F)->faces_by_id[ID] : \
+            try_all_cached_faces(F) \
+           ) \
       : NULL)
 
 #ifdef HAVE_WINDOW_SYSTEM

--- src/xdisp.c.orig    2010-04-05 10:05:48.000000000 -0400
+++ src/xdisp.c    2010-07-08 11:23:35.000000000 -0400
@@ -19489,6 +19489,22 @@ get_char_face_and_encoding (f, c, face_i
 }

+/* search for the first usable cached face on frame F,
+   or null if such a face doesn't exist.  */
+
+struct face *try_all_cached_faces(struct frame *f)
+{
+  struct face *local_face = 0;
+  int ix, used;
+
+  used = FRAME_FACE_CACHE(f)->used;
+  for (ix=0; ((ix<used) && (local_face == (struct face *)0)); ix++) {
+     local_face =  FRAME_FACE_CACHE(f)->faces_by_id[ix];
+  }
+
+  return local_face;
+}
+
 /* Get face and two-byte form of character glyph GLYPH on frame F.
    The encoding of GLYPH->u.ch is returned in *CHAR2B.  Value is
    a pointer to a realized face that is ready for display.  */
@@ -19505,6 +19521,9 @@ get_glyph_face_and_encoding (f, glyph, c
   xassert (glyph->type == CHAR_GLYPH);
   face = FACE_FROM_ID (f, glyph->face_id);
 
+  /* Make sure X resources of the face are allocated.  */
+  xassert (face != NULL);
+
   if (two_byte_p)
     *two_byte_p = 0;
 
@@ -19518,8 +19537,6 @@ get_glyph_face_and_encoding (f, glyph, c
     STORE_XCHAR2B (char2b, 0, 0);
     }
 
-  /* Make sure X resources of the face are allocated.  */
-  xassert (face != NULL);
   PREPARE_FACE_FOR_DISPLAY (f, face);
   return face;
 }

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