Re: timers and undo

[Richard Stallman]

    Wait until once more a bunch of lines are inserted.  I now expected
    that C-/ would get rid of these inserted lines once more.  But no.
    It deletes the introductory comment lines.  The "aaaaaaa" lines
    stay and they are impossible to undo after repaeting C-/

This is because it sees that the previous command was also `undo'.
So it continues the undo sequence previously started.

One fix is to make the command `undo' defensively notice that the undo
list has changed since its previous execution.  Does the fix below
give good results?

One drawback of this patch is that the space in the undo list
will still be wasted if buffer-undo-list is cleared out

*** simple.el   20 Dec 2004 15:37:08 -0500      1.673
--- simple.el   26 Dec 2004 15:12:32 -0500      
*** 1215,1220 ****
--- 1234,1243 ----
  (defvar undo-no-redo nil
    "If t, `undo' doesn't go through redo entries.")
  
+ (defvar undo-list-saved nil
+   "The value of `buffer-undo-list' saved by the last undo command.")
+ (make-variable-buffer-local 'undo-list-saved)
+ 
  (defun undo (&optional arg)
    "Undo some previous changes.
  Repeat this command to undo more changes.
***************
*** 1237,1243 ****
      ;; So set `this-command' to something other than `undo'.
      (setq this-command 'undo-start)
  
!     (unless (eq last-command 'undo)
        (setq undo-in-region
            (if transient-mark-mode mark-active (and arg (not (numberp arg)))))
        (if undo-in-region
--- 1260,1267 ----
      ;; So set `this-command' to something other than `undo'.
      (setq this-command 'undo-start)
  
!     (unless (and (eq last-command 'undo)
!                (eq undo-list-saved buffer-undo-list))
        (setq undo-in-region
            (if transient-mark-mode mark-active (and arg (not (numberp arg)))))
        (if undo-in-region
***************
*** 1289,1295 ****
              (setq tail (cdr tail)))
            (setq tail nil)))
        (setq prev tail tail (cdr tail))))
! 
      (and modified (not (buffer-modified-p))
         (delete-auto-save-file-if-necessary recent-save))))
  
--- 1313,1319 ----
              (setq tail (cdr tail)))
            (setq tail nil)))
        (setq prev tail tail (cdr tail))))
!     (setq saved-undo-list buffer-undo-list)
      (and modified (not (buffer-modified-p))
         (delete-auto-save-file-if-necessary recent-save))))