bug#34405: 26.1; atomic change group after undo fails to cancel

["Braun Gábor" <braungb88@gmail.com>]

Hi,

-------------------- File test.el ----------------------------------
;; -*- lexical-binding: t; -*-
(defun test-fun ()
  "Test atomic change group, no visible effect."
  (interactive)
  (catch 'test
    (atomic-change-group
      (save-excursion
        (goto-char (point-min))
        (insert "!!! TEST: you shouldn't see this !!!")
        (throw 'test t)))))

(global-set-key [(control c) ?a] #'test-fun)
--------------------------------------------------------------------

Start emacs by the command

emacs -Q -l test.el

Press the following keys: a C-_ C-c a

A "!!! TEST: you shouldn't see this !!!" gets inserted at the top of
buffer *scratch*, and the message "Undoing to some unrelated state"
appears in the echo area.  I expect that "C-c a" has no visible effect
(as is the case if one omits "a C-_" before "C-c a").


A variant: in function cancel-change-group change line
"(unless (eq last-command 'undo) (undo-start))"
into "(undo-start)".
I.e. to have a self-containd test:

------------------- file test2.el ----------------------------------
;; -*- lexical-binding: t; -*-
(defun test-fun ()
  "Test atomic change group, no visible effect."
  (interactive)
  (catch 'test
    (atomic-change-group
      (save-excursion
        (goto-char (point-min))
        (insert "!!! TEST: you shouldn't see this !!!")
        (throw 'test t)))))

(global-set-key [(control c) ?a] #'test-fun)

(defun cancel-change-group (handle)
  "Finish a change group made with `prepare-change-group' (which see).
This finishes the change group by reverting all of its changes."
  (dolist (elt handle)
    (with-current-buffer (car elt)
      (setq elt (cdr elt))
      (save-restriction
        ;; Widen buffer temporarily so if the buffer was narrowed within
        ;; the body of `atomic-change-group' all changes can be undone.
        (widen)
        (let ((old-car (car-safe elt))
              (old-cdr (cdr-safe elt)))
          (unwind-protect
              (progn
                ;; Temporarily truncate the undo log at ELT.
                (when (consp elt)
                  (setcar elt nil) (setcdr elt nil))
                (undo-start)
                ;; Make sure there's no confusion.
                (when (and (consp elt) (not (eq elt (last pending-undo-
list))))
                  (error "Undoing to some unrelated state"))
                ;; Undo it all.
                (save-excursion
                  (while (listp pending-undo-list) (undo-more 1)))
                ;; Revert the undo info to what it was when we grabbed
                ;; the state.
                (setq buffer-undo-list elt))
            ;; Reset the modified cons cell ELT to its original content.
            (when (consp elt)
              (setcar elt old-car)
              (setcdr elt old-cdr))))))))
--------------------------------------------------------------------

Run emacs as

emacs -Q -l test2.el

Press the following keys (same as above): a C-_ C-c a

Now the displayed buffer *scratch* is in its original form, no message
in the echo area as expected.

Discussion:

I do not claim that the change above in cancel-change-group is a correct
fix, especially no claim that pending-undo-list will not get corrupted.
It is intended only as a proof that in its original definition
cancel-change-group fails while it was still possible to properly cancel
the change group.

Best wishes,

     Gábor

System information (for the first test, not the second one):

In GNU Emacs 26.1 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.2)
 of 2018-12-26, modified by Debian built on x86-ubc-01
Windowing system distributor 'The X.Org Foundation', version 
11.0.12003000
System Description:	Debian GNU/Linux buster/sid

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Undo!
cancel-change-group: Undoing to some unrelated state

Configured using:
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Memory information:
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