From 682fdae7efe5a36636539b953a657445f2f3382b Mon Sep 17 00:00:00 2001
From: Stefan Monnier <monnier@iro.umontreal.ca>
Date: Tue, 26 Jul 2016 13:14:50 -0400
Subject: [PATCH] * lisp/subr.el (cancel-change-group): Reset cell in case of
 error

Since the setcdr/setcdr is supposed to be temporary, use unwind-protect
to make sure we properly undo the temporary change even in case of error.
---
 lisp/subr.el | 41 +++++++++++++++++++++--------------------
 1 file changed, 21 insertions(+), 20 deletions(-)

diff --git a/lisp/subr.el b/lisp/subr.el
index 937a0506826..2c0be204e0a 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -2535,26 +2535,27 @@ This finishes the change group by reverting all of its changes."
 	;; 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
-	       (if (consp elt) (car elt)))
-	      (old-cdr
-	       (if (consp elt) (cdr elt))))
-	  ;; 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"))
-	  ;; Undo it all.
-	  (save-excursion
-	    (while (listp pending-undo-list) (undo-more 1)))
-	  ;; Reset the modified cons cell ELT to its original content.
-	  (when (consp elt)
-	    (setcar elt old-car)
-	    (setcdr elt old-cdr))
-	  ;; Revert the undo info to what it was when we grabbed the state.
-	  (setq buffer-undo-list elt))))))
+	(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))
+                (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"))
+                ;; 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))))))))
 
 ;;;; Display-related functions.
 
-- 
2.39.2