Fix undo of changes in cloned indirect buffers

* lisp/simple.el (primitive-undo): If the visited-modtime of the
indirect buffer's file is bogus, use the modtime of the file
visited by its base buffer.

* src/undo.c (record_first_change): Call
'buffer_visited_file_modtime' with the correct buffer, instead of
always calling 'Fvisited_file_modtime', which returns possibly
bogus values for indirect buffers.
* src/fileio.c (Fset_visited_file_modtime): Signal a meaningful
error for indirect buffers.
(buffer_visited_file_modtime): New function, with implementation
taken from 'Fvisited_file_modtime'.
(Fvisited_file_modtime): Call 'buffer_visited_file_modtime'.
* src/lisp.h: Add prototype for 'buffer_visited_file_modtime'.
(Bug#56397)

3 files changed, 14 insertions, 5 deletions

diff --git a/src/fileio.c b/src/fileio.c
index 10d4b8bc15e..d07e62a1212 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -5832,6 +5832,15 @@ See Info node `(elisp)Modification Time' for more details.  */)
   return Qnil;
 }
 
+Lisp_Object
+buffer_visited_file_modtime (struct buffer *buf)
+{
+  int ns = buf->modtime.tv_nsec;
+  if (ns < 0)
+    return make_fixnum (UNKNOWN_MODTIME_NSECS - ns);
+  return make_lisp_time (buf->modtime);
+}
+
 DEFUN ("visited-file-modtime", Fvisited_file_modtime,
        Svisited_file_modtime, 0, 0, 0,
        doc: /* Return the current buffer's recorded visited file modification time.
@@ -5841,10 +5850,7 @@ visited file doesn't exist.
 See Info node `(elisp)Modification Time' for more details.  */)
   (void)
 {
-  int ns = current_buffer->modtime.tv_nsec;
-  if (ns < 0)
-    return make_fixnum (UNKNOWN_MODTIME_NSECS - ns);
-  return make_lisp_time (current_buffer->modtime);
+  return buffer_visited_file_modtime (current_buffer);
 }
 
 DEFUN ("set-visited-file-modtime", Fset_visited_file_modtime,
@@ -5871,6 +5877,8 @@ in `current-time' or an integer flag as returned by `visited-file-modtime'.  */)
       current_buffer->modtime = mtime;
       current_buffer->modtime_size = -1;
     }
+  else if (current_buffer->base_buffer)
+    error ("An indirect buffer does not have a visited file");
   else
     {
       register Lisp_Object filename;
diff --git a/src/lisp.h b/src/lisp.h
index e4a49b8ef94..35cc7f5a098 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -4733,6 +4733,7 @@ extern bool internal_delete_file (Lisp_Object);
 extern Lisp_Object check_emacs_readlinkat (int, Lisp_Object, char const *);
 extern bool file_directory_p (Lisp_Object);
 extern bool file_accessible_directory_p (Lisp_Object);
+extern Lisp_Object buffer_visited_file_modtime (struct buffer *);
 extern void init_fileio (void);
 extern void syms_of_fileio (void);
 
diff --git a/src/undo.c b/src/undo.c
index 36664d16424..f76977dbe50 100644
--- a/src/undo.c
+++ b/src/undo.c
@@ -218,7 +218,7 @@ record_first_change (void)
     base_buffer = base_buffer->base_buffer;
 
   bset_undo_list (current_buffer,
-                  Fcons (Fcons (Qt, Fvisited_file_modtime ()),
+                  Fcons (Fcons (Qt, buffer_visited_file_modtime (base_buffer)),
                          BVAR (current_buffer, undo_list)));
 }