Is there a way to retain the undo list in Emacs after reverting a buffer from file?

Question

How can I make Emacs retain its undo history for my buffer after doing revert-buffer or using auto-revert-mode?

In Vim, if a file that is open in a buffer is changed on disc, Vim prompts me to reload the file. I can then simply click 'u' to undo the reload if I so wish and even go back further from then. Emacs seems to trash all the undo information the moment I revert the buffer.

Answer 1

I guess the obvious approach would be a function which kills the current buffer content, and then calls insert-file to read in the current content from the file.

If the changes to the file included changes to the character encoding, there might be problems? I haven't tested that.

Here's my current attempt. It's a little hairy IMO, but it works okay.

;; Allow buffer reverts to be undone
(defun my-revert-buffer (&optional ignore-auto noconfirm preserve-modes)
  "Revert buffer from file in an undo-able manner."
  (interactive)
  (when (buffer-file-name)
    ;; Based upon `delphi-save-state':
    ;; Ensure that any buffer modifications do not have any side
    ;; effects beyond the actual content changes.
    (let ((buffer-read-only nil)
          (inhibit-read-only t)
          (before-change-functions nil)
          (after-change-functions nil))
      (unwind-protect
          (progn
            ;; Prevent triggering `ask-user-about-supersession-threat'
            (set-visited-file-modtime)
            ;; Kill buffer contents and insert from associated file.
            (widen)
            (kill-region (point-min) (point-max))
            (insert-file-contents (buffer-file-name))
            ;; Mark buffer as unmodified.
            (set-buffer-modified-p nil))))))

(defadvice ask-user-about-supersession-threat
  (around my-supersession-revert-buffer)
  "Use my-revert-buffer in place of revert-buffer."
  (let ((real-revert-buffer (symbol-function 'revert-buffer)))
    (fset 'revert-buffer 'my-revert-buffer)
    ;; Note that `ask-user-about-supersession-threat' calls
    ;; (signal 'file-supersession ...), so we need to handle
    ;; the error in order to restore revert-buffer.
    (unwind-protect
        ad-do-it
      (fset 'revert-buffer real-revert-buffer))))

(ad-activate 'ask-user-about-supersession-threat)

Annoyingly, I've only just noticed all the relevant-looking information in the revert-buffer docs, so there's probably a much simpler way to do this.

If the value of revert-buffer-function is non-nil, it is called to do all the work for this command. Otherwise, the hooks before-revert-hook and after-revert-hook are run at the beginning and the end, and if revert-buffer-insert-file-contents-function is non-nil, it is called instead of rereading visited file contents.