Avoid crashes when redisplayng a window changes faces or fonts

* src/xdisp.c (redisplay_internal): If redisplaying the selected
window or one of the frames turns on the frame's 'redisplay' flag,
redisplay again.  (Bug#21428)

* src/frame.c (x_set_font): Set the frame's 'fonts_changed' flag.

2 files changed, 23 insertions, 2 deletions

diff --git a/src/frame.c b/src/frame.c
index 98a7a57a988..6d596a48e23 100644
--- a/src/frame.c
+++ b/src/frame.c
@@ -3651,6 +3651,10 @@ x_set_font (struct frame *f, Lisp_Object arg, Lisp_Object oldval)
   /* Attempt to hunt down bug#16028.  */
   SET_FRAME_GARBAGED (f);
 
+  /* This is important if we are called by some Lisp as part of
+     redisplaying the frame, see redisplay_internal.  */
+  f->fonts_changed = true;
+
   recompute_basic_faces (f);
 
   do_pending_window_change (0);
diff --git a/src/xdisp.c b/src/xdisp.c
index a793157f086..986e13f43b5 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -13810,6 +13810,7 @@ redisplay_internal (void)
               bool gcscrollbars
                 /* Only GC scrollbars when we redisplay the whole frame.  */
                 = f->redisplay || !REDISPLAY_SOME_P ();
+              bool f_redisplay_flag = f->redisplay;
               /* Mark all the scroll bars to be removed; we'll redeem
                  the ones we want when we redisplay their windows.  */
               if (gcscrollbars && FRAME_TERMINAL (f)->condemn_scroll_bars_hook)
@@ -13853,6 +13854,20 @@ redisplay_internal (void)
                         goto retry_frame;
                     }
 
+                  /* If the frame's redisplay flag was not set before
+                     we went about redisplaying its windows, but it is
+                     set now, that means we employed some redisplay
+                     optimizations inside redisplay_windows, and
+                     bypassed producing some screen lines.  But if
+                     f->redisplay is now set, it might mean the old
+                     faces are no longer valid (e.g., if redisplaying
+                     some window called some Lisp which defined a new
+                     face or redefined an existing face), so trying to
+                     use them in update_frame will segfault.
+                     Therefore, we must redisplay this frame.  */
+                  if (!f_redisplay_flag && f->redisplay)
+                    goto retry_frame;
+
                   /* Prevent various kinds of signals during display
                      update.  stdio is not robust about handling
                      signals, which can cause an apparent I/O error.  */
@@ -13906,8 +13921,10 @@ redisplay_internal (void)
       /* Compare desired and current matrices, perform output.  */
 
     update:
-      /* If fonts changed, display again.  */
-      if (sf->fonts_changed)
+      /* If fonts changed, display again.  Likewise if redisplay_window_1
+         above caused some change (e.g., a change in faces) that requires
+         considering the entire frame again.  */
+      if (sf->fonts_changed || sf->redisplay)
         goto retry;
 
       /* Prevent freeing of realized faces, since desired matrices are