bug#38316: 26.1; Undo with undo-inhibit-record-point set doesn't restore unmodified state

[Braun Gábor]

1. Start Emacs:

emacs -Q --eval '(setq undo-inhibit-record-point t)'

I see the scratch buffer with -- in the modeline indicating it is not 
modified.

2. Enter 'abc' in the scratch buffer: press keys a b c.

3. Press C-_.

The inserted 'abc' is deleted, but in the modeline I see ** indicating
that the buffer is modified.  The expectation is that the buffer is no
longer modified.


Do the same as above but start Emacs with 'emacs -Q', no additional
options.  Now after C-_ I see -- in the modeline indicating that the
buffer is not modified, as expected.


Discussion:

1. The value of buffer-undo-list is (as shown by C-h v buffer-undo-list)
in the first case

(nil
 (#("abc" 0 3
    (fontified t))
   . 146)
 (#<marker at 146 in *scratch*> . -3)
 nil
 (146 . 149)
 nil
 (1 . 146))

while in the second case

(nil
 (#("abc" 0 3
    (fontified t))
   . 146)
 (#<marker at 146 in *scratch*> . -3)
 nil
 (146 . 149)
 (t . 0)
 nil
 (1 . 146)
 (t . 0))


The only change I see is the additional (t . 0) entries in buffer-undo-
list.


2. The difference in buffer-undo-list is explained by the function
record_point in src/undo.c, which I include here in an abbreviated form
with ... denoting omissions:


static void
record_point (ptrdiff_t beg)
{
  /* Don't record position of pt when undo_inhibit_record_point holds.  
*/
  if (undo_inhibit_record_point)
    return;
    
  ...
  
  /* If this is the first change since save, then record this.*/
  if (MODIFF <= SAVE_MODIFF)
    record_first_change ();

  /* We may need to record point ...*/
  if (at_boundary
      && point_before_last_command_or_undo != beg
      && buffer_before_last_command_or_undo == current_buffer )
    bset_undo_list (current_buffer,
                    Fcons (make_fixnum (point_before_last_command_or_undo),
                           BVAR (current_buffer, undo_list)));
}


here the call to record_first_change() is responsible for adding the
(t . ...)  entries.  Note that record_first_change() is called only if
undo-inhibit-record-point is nil regardles of whether point actually
gets recorded in buffer-undo-list (i.e. the condition of the last `if'
is true).

By the way, functions record_insert and record_delete seem to rely on
record_point adding necessary (t . ...) entries.  So these functions
never add (t . ...) entries when undo-inhibit-record-point is t,
and that's why they are missing in the first example above.

Best wishes,

     Gábor


System information:

In GNU Emacs 26.1 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.5)
 of 2019-09-23, modified by Debian built on x86-grnet-01
Windowing system distributor 'The X.Org Foundation', version 
11.0.12004000
System Description:     Debian GNU/Linux 10 (buster)

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