How to amalgamate changes across multiple buffers into a single undo entry

[Brian Leung <leungbk@mailfence.com>]

Suppose one action causes a change in two buffers. I'd like a way of performing an undo operation so that both buffers will be undone at once. But I can't figure out how to amalgamate the buffer changes properly. This is what I've got:

(defmacro my/with-single-undo (curr-buf &rest body)
  "Within CURR-BUF, execute BODY as a single undo step."
  (declare (indent 1))
  (let ((marker (cl-gensym "marker")))
    `(let ((,marker
            (nconc (prepare-change-group ,curr-buf)
                   (prepare-change-group (get-buffer-create "b.txt")))))
       (unwind-protect
           (progn ,@body)
         (undo-amalgamate-change-group ,marker)))))

(defun my/insert-some-stuff (&rest _args)
  (interactive)
  (my/with-single-undo (current-buffer)
    (insert "apple banana caterpillar\n")
    (with-current-buffer (get-buffer-create "b.txt")
      (insert "xylophone yak zebra"))))

I test by making "a.txt" and "b.txt" files, opening both up, and then executing M-x my/insert-some-stuff from within a.txt. Right now, undo is only undoing changes within the buffer in which I execute it.

I'd appreciate any help on this.

Best,
Brian