emacs-29 03e897c08fa: Fix rare crashes in redisplay due to problems with fontsets

[Eli Zaretskii]

branch: emacs-29
commit 03e897c08facf8e920ee04cd505b72f5aa5a30cf
Author: Eli Zaretskii <eliz@gnu.org>
Commit: Eli Zaretskii <eliz@gnu.org>

    Fix rare crashes in redisplay due to problems with fontsets
    
    * src/xdisp.c (get_next_display_element): If we have no usable
    face to display a character/composition, treat that as glyphless.
    (produce_glyphless_glyph): If neither it->face nor its ASCII face
    are usable, fall back to the frame's default font.  (Bug#65198)
---
 src/xdisp.c | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/src/xdisp.c b/src/xdisp.c
index 9cddcfeda27..22508978298 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -8312,9 +8312,17 @@ get_next_display_element (struct it *it)
       && success_p
       && FRAME_WINDOW_P (it->f))
     {
-      struct face *face = FACE_FROM_ID (it->f, it->face_id);
+      struct face *face = FACE_FROM_ID_OR_NULL (it->f, it->face_id);
 
-      if (it->what == IT_COMPOSITION && it->cmp_it.ch >= 0)
+      /* It shouldn't happen, ever, that FACE is NULL here, but
+         evidently some faulty fonts/fontsets can sometimes cause it.
+         In that case, we punt and consider the stuff undisplayable.  */
+      if (!face)
+       {
+         it->what = IT_GLYPHLESS;
+         it->glyphless_method = GLYPHLESS_DISPLAY_EMPTY_BOX;
+       }
+      else if (it->what == IT_COMPOSITION && it->cmp_it.ch >= 0)
        {
          /* Automatic composition with glyph-string.   */
          Lisp_Object gstring = composition_gstring_from_id (it->cmp_it.id);
@@ -31781,9 +31789,12 @@ produce_glyphless_glyph (struct it *it, bool 
for_no_font, Lisp_Object acronym)
   int len;
 
   /* Get the metrics of the base font.  We always refer to the current
-     ASCII face.  */
-  face = FACE_FROM_ID (it->f, it->face_id)->ascii_face;
-  font = face->font ? face->font : FRAME_FONT (it->f);
+     ASCII face, but if some faulty setup of fontsets causes that to
+     be NULL, we fall back to the frame's default font.  */
+  face = FACE_FROM_ID_OR_NULL (it->f, it->face_id);
+  if (face)
+    face = face->ascii_face;
+  font = (face && face->font) ? face->font : FRAME_FONT (it->f);
   normal_char_ascent_descent (font, -1, &it->ascent, &it->descent);
   it->ascent += font->baseline_offset;
   it->descent -= font->baseline_offset;