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

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

Braun Gábor
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:
 'configure --build x86_64-linux-gnu --prefix=/usr
 --sharedstatedir=/var/lib --libexecdir=/usr/lib
 --localstatedir=/var/lib --infodir=/usr/share/info
 --mandir=/usr/share/man --enable-libsystemd --with-pop=yes
 --enable-locallisppath=/etc/emacs:/usr/local/share/emacs/26.1/site-
lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/26.1/site-lisp:/
usr/share/emacs/site-lisp
 --with-sound=alsa --without-gconf --with-mailutils --build
 x86_64-linux-gnu --prefix=/usr --sharedstatedir=/var/lib
 --libexecdir=/usr/lib --localstatedir=/var/lib
 --infodir=/usr/share/info --mandir=/usr/share/man --enable-libsystemd
 --with-pop=yes
 --enable-locallisppath=/etc/emacs:/usr/local/share/emacs/26.1/site-
lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/26.1/site-lisp:/
usr/share/emacs/site-lisp
 --with-sound=alsa --without-gconf --with-mailutils --with-x=yes
 --with-x-toolkit=gtk3 --with-toolkit-scroll-bars 'CFLAGS=-g -O2
 -fdebug-prefix-map=/build/emacs-3ThesY/emacs-26.1+1=. -fstack-
protector-strong
 -Wformat -Werror=format-security -Wall' 'CPPFLAGS=-Wdate-time
 -D_FORTIFY_SOURCE=2' LDFLAGS=-Wl,-z,relro'

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GSETTINGS NOTIFY
ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11 THREADS LIBSYSTEMD LCMS2

Important settings:
  value of $LANG: hu_HU.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny seq byte-opt gv
bytecomp byte-compile cconv cl-loaddefs cl-lib dired dired-loaddefs
format-spec rfc822 mml easymenu mml-sec password-cache epa derived epg
epg-config gnus-util rmail rmail-loaddefs mm-decode mm-bodies mm-encode
mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail rfc2047
rfc2045 ietf-drums mm-util mail-prsvr mail-utils elec-pair time-date
mule-util tooltip eldoc electric uniquify ediff-hook vc-hooks
lisp-float-type mwheel term/x-win x-win term/common-win x-dnd tool-bar
dnd fontset image regexp-opt fringe tabulated-list replace newcomment
text-mode elisp-mode lisp-mode prog-mode register page menu-bar
rfn-eshadow isearch timer select scroll-bar mouse jit-lock font-lock
syntax facemenu font-core term/tty-colors frame cl-generic cham georgian
utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao korean
japanese eucjp-ms cp51932 hebrew greek romanian slovak czech european
ethiopic indian cyrillic chinese composite charscript charprop
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text-properties overlay sha1 md5 base64 format env code-pages mule
custom widget hashtable-print-readable backquote dbusbind inotify lcms2
dynamic-setting system-font-setting font-render-setting move-toolbar gtk
x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 95434 9148)
 (symbols 48 20367 1)
 (miscs 40 44 118)
 (strings 32 28319 1131)
 (string-bytes 1 742545)
 (vectors 16 14644)
 (vector-slots 8 496930 10762)
 (floats 8 49 118)
 (intervals 56 263 0)
 (buffers 992 11))