master 66eeaaa: * lisp/subr.el (activate-change-group): Fix bug#33341

[Stefan Monnier]

branch: master
commit 66eeaaa48858572d47ff3b0b177d5b765df93418
Author: Stefan Monnier <monnier@iro.umontreal.ca>
Commit: Stefan Monnier <monnier@iro.umontreal.ca>

    * lisp/subr.el (activate-change-group): Fix bug#33341
---
 lisp/subr.el            |  5 ++++-
 test/lisp/subr-tests.el | 12 ++++++++++++
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/lisp/subr.el b/lisp/subr.el
index e009dcc..1cf3a49 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -3036,7 +3036,10 @@ to `accept-change-group' or `cancel-change-group'."
   (dolist (elt handle)
     (with-current-buffer (car elt)
       (if (eq buffer-undo-list t)
-         (setq buffer-undo-list nil)))))
+         (setq buffer-undo-list nil)
+       ;; Add a boundary to make sure the upcoming changes won't be
+       ;; merged with any previous changes (bug#33341).
+       (undo-boundary)))))
 
 (defun accept-change-group (handle)
   "Finish a change group made with `prepare-change-group' (which see).
diff --git a/test/lisp/subr-tests.el b/test/lisp/subr-tests.el
index 67f7fc9..e3f798d 100644
--- a/test/lisp/subr-tests.el
+++ b/test/lisp/subr-tests.el
@@ -551,5 +551,17 @@ See https://debbugs.gnu.org/cgi/bugreport.cgi?bug=19350.";
   (should (equal (replace-regexp-in-string "\\`\\|x" "z" "--xx--")
                  "z--zz--")))
 
+(ert-deftest subr-tests--change-group-33341 ()
+  (with-temp-buffer
+    (buffer-enable-undo)
+    (insert "0\n")
+    ;; (undo-boundary)
+    (let ((g (prepare-change-group)))
+      (activate-change-group g)
+      (insert "b\n")
+      (insert "c\n")
+      (cancel-change-group g))
+    (should (equal (buffer-string) "0\n"))))
+
 (provide 'subr-tests)
 ;;; subr-tests.el ends here