bug#39680: 27.0.60; electric-pair-mode broken by undo

[Stefan Monnier]

>> I think we need something like the patch below (not really tested yet).
>> WDYT?
>
> FWIW, I applied it and AFAICT I can't reproduce the issue.

Thanks.  I think the patch wasn't quite right and was overly
complicated: `undo` is already responsible for dealing with the
freshness of `pending-undo-list`.  So I think the better patch is the
one below.

Eli, is the patch below OK for `emacs-27`?


        Stefan


[ BTW, I accidentally installed the previous patch on `master`, and I'm
  waiting for "the right solution" to clean up.  ]

diff --git a/lisp/subr.el b/lisp/subr.el
index 5b94343e499..4c69c9dd044 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -2964,13 +2964,18 @@ cancel-change-group
        ;; 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"))