Is there any way to have EMACS save your undo history between sessions?

Question

Is there any way to have EMACS save your undo history between sessions?

I'm aware of the savehist lib, the saveplace lib, the desktop lib, and the windows lib, these all provide some session control but none seem to save the undo history.

Answer 1

My hunch is "no", simply because by default, emacs throws away your undo history when you re-visit a file.

Answer 2

Here's some code I wrote which seems to do the trick. It isn't bullet-proof, as in, it doesn't handle all the file handling intricacies that Emacs does (e.g. overriding where auto-save files are put, symlink handling, etc.). But, it seemed to do the trick for some simple text files I manipulated.

(defun save-undo-filename (orig-name)
  "given a filename return the file name in which to save the undo list"
  (concat (file-name-directory orig-name)
          "."
          (file-name-nondirectory orig-name)
          ".undo"))

(defun save-undo-list ()
  "Save the undo list to a file"
  (save-excursion
    (ignore-errors
      (let ((undo-to-save `(setq buffer-undo-list ',buffer-undo-list))
            (undo-file-name (save-undo-filename (buffer-file-name))))
        (find-file undo-file-name)
        (erase-buffer)
        (let (print-level
              print-length)
          (print undo-to-save (current-buffer)))
        (let ((write-file-hooks (remove 'save-undo-list write-file-hooks)))
          (save-buffer))
        (kill-buffer))))
  nil)

(defvar handling-undo-saving nil)

(defun load-undo-list ()
  "load the undo list if appropriate"
  (ignore-errors
    (when (and
           (not handling-undo-saving)
           (null buffer-undo-list)
           (file-exists-p (save-undo-filename (buffer-file-name))))
      (let* ((handling-undo-saving t)
             (undo-buffer-to-eval (find-file-noselect (save-undo-filename (buffer-file-name)))))
        (eval (read undo-buffer-to-eval))))))

(add-hook 'write-file-hooks 'save-undo-list)
(add-hook 'find-file-hook 'load-undo-list)