reverting buffer from non-existent file

[Martin Rudalics]

When reverting a buffer and the file to revert from is not or no more
readable, all undo information gets killed nevertheless.  This is an
unhappy effect because I occasionally forget that I have not yet saved a
buffer I just created, do `revert-buffer', and loose the entire
information needed to undo the latest changes.  The patch below fixes
the problem for me.

============================================================
*** files.el    Wed Feb  1 10:17:44 2006
--- files.el    Fri Feb  3 20:47:02 2006
***************
*** 3840,3845 ****
--- 3840,3848 ----
                    (setq buffer-backed-up nil))
               ;; Get rid of all undo records for this buffer.
               (or (eq buffer-undo-list t)
+                    ;; Do not clear buffer-undo-list if reverting will fail.
+                    (and (not (file-readable-p file-name))
+                         (not revert-buffer-insert-file-contents-function))
                   (setq buffer-undo-list nil))
               ;; Effectively copy the after-revert-hook status,
               ;; since after-find-file will clobber it.
***************
*** 3847,3863 ****
                     (local-hook-p (local-variable-p 'after-revert-hook))
                     (local-hook (and (local-variable-p 'after-revert-hook)
                                      after-revert-hook)))
!                (let (buffer-read-only
!                      ;; Don't make undo records for the reversion.
!                      (buffer-undo-list t))
!                  (if revert-buffer-insert-file-contents-function
!                      (funcall revert-buffer-insert-file-contents-function
!                               file-name auto-save-p)
!                    (if (not (file-exists-p file-name))
!                        (error (if buffer-file-number
!                                   "File %s no longer exists!"
!                                 "Cannot revert nonexistent file %s")
!                               file-name))
                     ;; Bind buffer-file-name to nil
                     ;; so that we don't try to lock the file.
                     (let ((buffer-file-name nil))
--- 3850,3873 ----
                     (local-hook-p (local-variable-p 'after-revert-hook))
                     (local-hook (and (local-variable-p 'after-revert-hook)
                                      after-revert-hook)))
!                  (cond
!                   (revert-buffer-insert-file-contents-function
!                    (let (buffer-read-only
!                          ;; Don't make undo records for the reversion.
!                          (buffer-undo-list t))
!                      (funcall revert-buffer-insert-file-contents-function
!                               file-name auto-save-p)))
!                   ((not (file-exists-p file-name))
!                    (error (if buffer-file-number
!                               "File %s no longer exists!"
!                             "Cannot revert nonexistent file %s")
!                           file-name))
!                   ((not (file-readable-p file-name))
!                    (error "File %s is not readable" file-name))
!                   (t
!                    (let (buffer-read-only
!                          ;; Don't make undo records for the reversion.
!                          (buffer-undo-list t))
                     ;; Bind buffer-file-name to nil
                     ;; so that we don't try to lock the file.
                     (let ((buffer-file-name nil))
***************
*** 3865,3871 ****
                           (unlock-buffer)))
                     (widen)
                     (let ((coding-system-for-read
!                           ;; Auto-saved file shoule be read by Emacs'
                            ;; internal coding.
                            (if auto-save-p 'auto-save-coding
                              (or coding-system-for-read
--- 3875,3881 ----
                           (unlock-buffer)))
                     (widen)
                     (let ((coding-system-for-read
!                           ;; Auto-saved file should be read by Emacs'
                            ;; internal coding.
                            (if auto-save-p 'auto-save-coding
                              (or coding-system-for-read
***************
*** 3881,3887 ****
                             (insert-file-contents file-name (not auto-save-p)
                                                   nil nil t))
                         (insert-file-contents file-name (not auto-save-p)
!                                              nil nil t)))))
                 ;; Recompute the truename in case changes in symlinks
                 ;; have changed the truename.
                 (setq buffer-file-truename
--- 3891,3897 ----
                             (insert-file-contents file-name (not auto-save-p)
                                                   nil nil t))
                         (insert-file-contents file-name (not auto-save-p)
!                                              nil nil t))))))
                 ;; Recompute the truename in case changes in symlinks
                 ;; have changed the truename.
                 (setq buffer-file-truename
============================================================