* lisp/subr.el (cancel-change-group): Fix bug#39680

Don't re-use an existing `pending-undo-list` even if (eq last-command 'undo)
since there might have been changes to the buffer since that `undo` command
and the `pending-undo-list` can hence be invalid for the current
buffer contents.

diff --git a/lisp/subr.el b/lisp/subr.el
index 5b94343e499840fac199305a7f63124f02e09631..a744cfddfd4b41fd931036ac40138e03a98f2d7f 100644 (file)
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -2964,13 +2964,18 @@ This finishes the change group by reverting all of its changes."
        ;; the body of `atomic-change-group' all changes can be undone.
        (widen)
        (let ((old-car (car-safe elt))
-             (old-cdr (cdr-safe elt)))
+             (old-cdr (cdr-safe elt))
+             ;; Use `pending-undo-list' temporarily since `undo-more' needs
+             ;; it, but restore it afterwards so as not to mess with an
+             ;; ongoing sequence of `undo's.
+             (pending-undo-list
+              ;; Use `buffer-undo-list' unconditionally (bug#39680).
+              buffer-undo-list))
           (unwind-protect
               (progn
                 ;; Temporarily truncate the undo log at ELT.
                 (when (consp elt)
                   (setcar elt nil) (setcdr elt nil))
-                (unless (eq last-command 'undo) (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"))