* src/fileio.c (Finsert_file_contents): Preserve undo info when reverting

a buffer.

Fixes: debbugs:8447

1 files changed, 9 insertions, 2 deletions

diff --git a/src/fileio.c b/src/fileio.c
index f20721251e6..e2b1007fb0b 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -3506,6 +3506,11 @@ by calling `format-decode', which see.  */)
   bool set_coding_system = 0;
   Lisp_Object coding_system;
   bool read_quit = 0;
+  /* If the undo log only contains the insertion, there's no point
+     keeping it.  It's typically when we first fill a file-buffer.  */
+  bool empty_undo_list_p
+    = (!NILP (visit) && NILP (BVAR (current_buffer, undo_list))
+       && BEG == Z);
   Lisp_Object old_Vdeactivate_mark = Vdeactivate_mark;
   bool we_locked_file = 0;
   bool deferred_remove_unwind_protect = 0;
@@ -4108,6 +4113,7 @@ by calling `format-decode', which see.  */)
         {
           del_range_byte (same_at_start, same_at_end, 0);
           temp = GPT;
+          eassert (same_at_start == GPT_BYTE);
           same_at_start = GPT_BYTE;
         }
       else
@@ -4120,6 +4126,7 @@ by calling `format-decode', which see.  */)
         = buf_bytepos_to_charpos (XBUFFER (conversion_buffer),
                                   same_at_start - BEGV_BYTE
                                   + BUF_BEG_BYTE (XBUFFER (conversion_buffer)));
+      eassert (same_at_start_charpos == temp - (BEGV - BEG));
       inserted_chars
         = (buf_bytepos_to_charpos (XBUFFER (conversion_buffer),
                                    same_at_start + inserted - BEGV_BYTE
@@ -4404,7 +4411,7 @@ by calling `format-decode', which see.  */)
 
   if (!NILP (visit))
     {
-      if (!EQ (BVAR (current_buffer, undo_list), Qt) && !nochange)
+      if (empty_undo_list_p)
         bset_undo_list (current_buffer, Qnil);
 
       if (NILP (handler))
@@ -4546,7 +4553,7 @@ by calling `format-decode', which see.  */)
           p = XCDR (p);
         }
 
-      if (NILP (visit))
+      if (!empty_undo_list_p)
         {
           bset_undo_list (current_buffer, old_undo);
           if (CONSP (old_undo) && inserted != old_inserted)