emacs-30 ef5466c2675: Avoid aborts when buffer is modified during its redisplay

[Eli Zaretskii]

branch: emacs-30
commit ef5466c267542970f28548c820783f3257c1b0cf
Author: Eli Zaretskii <eliz@gnu.org>
Commit: Eli Zaretskii <eliz@gnu.org>

    Avoid aborts when buffer is modified during its redisplay
    
    * src/xdisp.c (redisplay_window): Restore point from saved byte
    position only if the buffer was not changed meanwhile.
    (Bug#72165)
---
 src/xdisp.c | 22 +++++++++++++++++++---
 1 file changed, 19 insertions(+), 3 deletions(-)

diff --git a/src/xdisp.c b/src/xdisp.c
index baf181cba1c..1dadb372911 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -19957,6 +19957,7 @@ redisplay_window (Lisp_Object window, bool 
just_this_one_p)
   int frame_line_height, margin;
   bool use_desired_matrix;
   void *itdata = NULL;
+  modiff_count lchars_modiff = CHARS_MODIFF, ochars_modiff = lchars_modiff;
 
   SET_TEXT_POS (lpoint, PT, PT_BYTE);
   opoint = lpoint;
@@ -20050,6 +20051,7 @@ redisplay_window (Lisp_Object window, bool 
just_this_one_p)
   end_unchanged = END_UNCHANGED;
 
   SET_TEXT_POS (opoint, PT, PT_BYTE);
+  ochars_modiff = CHARS_MODIFF;
 
   specbind (Qinhibit_point_motion_hooks, Qt);
 
@@ -21082,14 +21084,28 @@ redisplay_window (Lisp_Object window, bool 
just_this_one_p)
     TEMP_SET_PT_BOTH (BEGV, BEGV_BYTE);
   else if (CHARPOS (opoint) > ZV)
     TEMP_SET_PT_BOTH (Z, Z_BYTE);
-  else
+  else if (ochars_modiff == CHARS_MODIFF)
     TEMP_SET_PT_BOTH (CHARPOS (opoint), BYTEPOS (opoint));
-
+  else
+    {
+      /* If the buffer was modified while we were redisplaying it, we
+         cannot trust the correspondence between character and byte
+         positions.  This can happen, for example, if we are
+         redisplaying *Messages* and some Lisp, perhaps invoked by
+         display_mode_lines, signals an error which caused something
+         added/deleted to/from the buffer text.  */
+      TEMP_SET_PT_BOTH (CHARPOS (opoint), CHAR_TO_BYTE (CHARPOS (opoint)));
+    }
   set_buffer_internal_1 (old);
   /* Avoid an abort in TEMP_SET_PT_BOTH if the buffer has become
      shorter.  This can be caused by log truncation in *Messages*.  */
   if (CHARPOS (lpoint) <= ZV)
-    TEMP_SET_PT_BOTH (CHARPOS (lpoint), BYTEPOS (lpoint));
+    {
+      if (lchars_modiff == CHARS_MODIFF)
+       TEMP_SET_PT_BOTH (CHARPOS (lpoint), BYTEPOS (lpoint));
+      else
+       TEMP_SET_PT_BOTH (CHARPOS (lpoint), CHAR_TO_BYTE (CHARPOS (lpoint)));
+    }
 
   unbind_to (count, Qnil);
 }