Comment changes.

[gnu-emacs] / lisp / vc.el

;;; vc.el --- drive a version-control system from within Emacs

;; Copyright (C) 1992, 1993 Free Software Foundation, Inc.

;; Author: Eric S. Raymond <esr@snark.thyrsus.com>
;; Version: 5.4

;; This file is part of GNU Emacs.

;; GNU Emacs is free software; you can redistribute it and/or modify
;; it under the terms of the GNU General Public License as published by
;; the Free Software Foundation; either version 2, or (at your option)
;; any later version.

;; GNU Emacs is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
;; GNU General Public License for more details.

;; You should have received a copy of the GNU General Public License
;; along with GNU Emacs; see the file COPYING.  If not, write to
;; the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.

;;; Commentary:

;; This mode is fully documented in the Emacs user's manual.
;;
;; This was designed and implemented by Eric Raymond <esr@snark.thyrsus.com>.
;; Paul Eggert <eggert@twinsun.com>, Sebastian Kremer <sk@thp.uni-koeln.de>,
;; and Richard Stallman contributed valuable criticism, support, and testing.
;;
;; Supported version-control systems presently include SCCS and RCS;
;; your RCS version should be 5.6.2 or later for proper operation of
;; the lock-breaking code.
;;
;; The RCS code assumes strict locking.  You can support the RCS -x option
;; by adding pairs to the vc-master-templates list.
;;
;; Proper function of the SCCS diff commands requires the shellscript vcdiff
;; to be installed somewhere on Emacs's path for executables.
;;
;; If your site uses the ChangeLog convention supported by Emacs, the
;; function vc-comment-to-change-log should prove a useful checkin hook.
;;
;; This code depends on call-process passing back the subprocess exit
;; status.  Thus, you need Emacs 18.58 or later to run it.  For the
;; vc-directory command to work properly as documented, you need 19.
;; You also need Emacs 19's ring.el.
;;
;; The vc code maintains some internal state in order to reduce expensive
;; version-control operations to a minimum.  Some names are only computed
;; once.  If you perform version control operations with RCS/SCCS/CVS while
;; vc's back is turned, or move/rename master files while vc is running,
;; vc may get seriously confused.  Don't do these things!
;;
;; Developer's notes on some concurrency issues are included at the end of
;; the file.

;;; Code:

(require 'vc-hooks)
(require 'ring)

(if (not (assoc 'vc-parent-buffer minor-mode-alist))
    (setq minor-mode-alist
          (cons '(vc-parent-buffer vc-parent-buffer-name)
                minor-mode-alist)))

;; General customization

(defvar vc-default-back-end nil
  "*Back-end actually used by this interface; may be SCCS or RCS.
The value is only computed when needed to avoid an expensive search.")
(defvar vc-suppress-confirm nil
  "*If non-nil, treat user as expert; suppress yes-no prompts on some things.")
(defvar vc-keep-workfiles t
  "*If non-nil, don't delete working files after registering changes.")
(defvar vc-initial-comment nil
  "*Prompt for initial comment when a file is registered.")
(defvar vc-command-messages nil
  "*Display run messages from back-end commands.")
(defvar vc-mistrust-permissions 'file-symlink-p
  "*Don't assume that permissions and ownership track version-control status.")
(defvar vc-checkin-switches nil
  "*Extra switches passed to the checkin program by \\[vc-checkin].")
(defvar vc-path
  (if (file-exists-p "/usr/sccs")
      '("/usr/sccs") nil)
  "*List of extra directories to search for version control commands.")

(defconst vc-maximum-comment-ring-size 32
  "Maximum number of saved comments in the comment ring.")

;;; This is duplicated in diff.el.
(defvar diff-switches "-c"
  "*A string or list of strings specifying switches to be be passed to diff.")

;;;###autoload
(defvar vc-checkin-hook nil
  "*List of functions called after a checkin is done.  See `run-hooks'.")

;; Header-insertion hair

(defvar vc-header-alist
  '((SCCS "\%W\%") (RCS "\$Id\$"))
  "*Header keywords to be inserted when `vc-insert-headers' is executed.")
(defvar vc-static-header-alist
  '(("\\.c$" .
     "\n#ifndef lint\nstatic char vcid[] = \"\%s\";\n#endif /* lint */\n"))
  "*Associate static header string templates with file types.  A \%s in the
template is replaced with the first string associated with the file's
version-control type in `vc-header-alist'.")

(defvar vc-comment-alist
  '((nroff-mode ".\\\"" ""))
  "*Special comment delimiters to be used in generating vc headers only.
Add an entry in this list if you need to override the normal comment-start
and comment-end variables.  This will only be necessary if the mode language
is sensitive to blank lines.")

;; Variables the user doesn't need to know about.
(defvar vc-log-entry-mode nil)
(defvar vc-log-operation nil)
(defvar vc-log-after-operation-hook nil)
(defvar vc-checkout-writable-buffer-hook 'vc-checkout-writable-buffer)
(defvar vc-parent-buffer nil)
(defvar vc-parent-buffer-name nil)

(defvar vc-log-file)
(defvar vc-log-version)

(defconst vc-name-assoc-file "VC-names")

(defvar vc-dired-mode nil)
(make-variable-buffer-local 'vc-dired-mode)

(defvar vc-comment-ring nil)
(defvar vc-comment-ring-index nil)
(defvar vc-last-comment-match nil)

;; File property caching

(defun vc-file-clearprops (file)
  ;; clear all properties of a given file
  (setplist (intern file vc-file-prop-obarray) nil))

(defun vc-clear-context ()
  "Clear all cached file properties and the comment ring."
  (interactive)
  (fillarray vc-file-prop-obarray nil)
  ;; Note: there is potential for minor lossage here if there is an open
  ;; log buffer with a nonzero local value of vc-comment-ring-index.
  (setq vc-comment-ring nil))

;; Random helper functions

(defun vc-registration-error (file)
  (if file
      (error "File %s is not under version control" file)
    (error "Buffer %s is not associated with a file" (buffer-name))))

(defvar vc-binary-assoc nil)

(defun vc-find-binary (name)
  "Look for a command anywhere on the subprocess-command search path."
  (or (cdr (assoc name vc-binary-assoc))
      (catch 'found
        (mapcar
         (function 
          (lambda (s)
            (if s
                (let ((full (concat s "/" name)))
                  (if (file-executable-p full)
                      (progn
                        (setq vc-binary-assoc
                              (cons (cons name full) vc-binary-assoc))
                        (throw 'found full)))))))
         exec-path)
        nil)))

(defun vc-do-command (okstatus command file &rest flags)
  "Execute a version-control command, notifying user and checking for errors.
The command is successful if its exit status does not exceed OKSTATUS.
Output from COMMAND goes to buffer *vc*.  The last argument of the command is
the master name of FILE; this is appended to an optional list of FLAGS."
  (setq file (expand-file-name file))
  (if vc-command-messages
      (message "Running %s on %s..." command file))
  (let ((obuf (current-buffer)) (camefrom (current-buffer))
        (squeezed nil)
        (vc-file (and file (vc-name file)))
        status)
    (set-buffer (get-buffer-create "*vc*"))
    (set (make-local-variable 'vc-parent-buffer) camefrom)
    (set (make-local-variable 'vc-parent-buffer-name)
         (concat " from " (buffer-name camefrom)))
    
    (erase-buffer)

    ;; This is so that command arguments typed in the *vc* buffer will
    ;; have reasonable defaults.
    (setq default-directory (file-name-directory file))

    (mapcar
     (function (lambda (s) (and s (setq squeezed (append squeezed (list s))))))
     flags)
    (if vc-file
        (setq squeezed (append squeezed (list vc-file))))
    (let ((default-directory (file-name-directory (or file "./")))
          (exec-path (if vc-path (append exec-path vc-path) exec-path)))
      (setq status (apply 'call-process command nil t nil squeezed)))
    (goto-char (point-max))
    (forward-line -1)
    (if (or (not (integerp status)) (< okstatus status))
        (progn
          (pop-to-buffer "*vc*")
          (goto-char (point-min))
          (shrink-window-if-larger-than-buffer)
          (error "Running %s...FAILED (%s)" command
                 (if (integerp status)
                     (format "status %d" status)
                   status))
          )
      (if vc-command-messages
          (message "Running %s...OK" command))
      )
    (set-buffer obuf)
    status)
  )

;;; Save a bit of the text around POSN in the current buffer, to help
;;; us find the corresponding position again later.  This works even
;;; if all markers are destroyed or corrupted.
(defun vc-position-context (posn)
  (list posn
        (buffer-size)
        (buffer-substring posn
                          (min (point-max) (+ posn 100)))))

;;; Return the position of CONTEXT in the current buffer, or nil if we
;;; couldn't find it.
(defun vc-find-position-by-context (context)
  (let ((context-string (nth 2 context)))
    (if (equal "" context-string)
        (point-max)
      (save-excursion
        (let ((diff (- (nth 1 context) (buffer-size))))
          (if (< diff 0) (setq diff (- diff)))
          (goto-char (nth 0 context))
          (if (or (search-forward context-string nil t)
                  ;; Can't use search-backward since the match may continue
                  ;; after point.
                  (progn (goto-char (- (point) diff (length context-string)))
                         ;; goto-char doesn't signal an error at
                         ;; beginning of buffer like backward-char would
                         (search-forward context-string nil t)))
              ;; to beginning of OSTRING
              (- (point) (length context-string))))))))

(defun vc-revert-buffer1 (&optional arg no-confirm)
  ;; Most of this was shamelessly lifted from Sebastian Kremer's rcs.el mode.
  ;; Revert buffer, try to keep point and mark where user expects them in spite
  ;; of changes because of expanded version-control key words.
  ;; This is quite important since otherwise typeahead won't work as expected.
  (interactive "P")
  (widen)
  (let ((point-context (vc-position-context (point)))
        ;; Use mark-marker to avoid confusion in transient-mark-mode.
        (mark-context  (if (eq (marker-buffer (mark-marker)) (current-buffer))
                           (vc-position-context (mark-marker))))
        ;; Make the right thing happen in transient-mark-mode.
        (mark-active nil)
        ;; We may want to reparse the compilation buffer after revert
        (reparse (and (boundp 'compilation-error-list) ;compile loaded
                      (let ((curbuf (current-buffer)))
                        ;; Construct a list; each elt is nil or a buffer
                        ;; iff that buffer is a compilation output buffer
                        ;; that contains markers into the current buffer.
                        (save-excursion
                          (mapcar (function
                                   (lambda (buffer)
                                    (set-buffer buffer)
                                    (let ((errors (or
                                                   compilation-old-error-list
                                                   compilation-error-list))
                                          (buffer-error-marked-p nil))
                                      (while (and (consp errors)
                                                  (not buffer-error-marked-p))
                                        (and (markerp (cdr (car errors)))
                                             (eq buffer
                                                 (marker-buffer
                                                  (cdr (car errors))))
                                             (setq buffer-error-marked-p t))
                                        (setq errors (cdr errors)))
                                      (if buffer-error-marked-p buffer))))
                                  (buffer-list)))))))

    ;; the actual revisit
    (revert-buffer arg no-confirm)

    ;; Reparse affected compilation buffers.
    (while reparse
      (if (car reparse)
          (save-excursion
            (set-buffer (car reparse))
            (let ((compilation-last-buffer (current-buffer)) ;select buffer
                  ;; Record the position in the compilation buffer of
                  ;; the last error next-error went to.
                  (error-pos (marker-position
                              (car (car-safe compilation-error-list)))))
              ;; Reparse the error messages as far as they were parsed before.
              (compile-reinitialize-errors '(4) compilation-parsing-end)
              ;; Move the pointer up to find the error we were at before
              ;; reparsing.  Now next-error should properly go to the next one.
              (while (and compilation-error-list
                          (/= error-pos (car (car compilation-error-list))))
                (setq compilation-error-list (cdr compilation-error-list))))))
      (setq reparse (cdr reparse)))

    ;; Restore point and mark
    (let ((new-point (vc-find-position-by-context point-context)))
      (if new-point (goto-char new-point)))
    (if mark-context
        (let ((new-mark (vc-find-position-by-context mark-context)))
          (if new-mark (set-mark new-mark))))))


(defun vc-buffer-sync ()
  ;; Make sure the current buffer and its working file are in sync
  (if (and (buffer-modified-p)
           (or
            vc-suppress-confirm
            (y-or-n-p (format "%s has been modified.  Write it out? "
                              (buffer-name)))))
      (save-buffer)))

(defun vc-workfile-unchanged-p (file)
  ;; Has the given workfile changed since last checkout?
  (let ((checkout-time (vc-file-getprop file 'vc-checkout-time))
        (lastmod (nth 5 (file-attributes file))))
    (if checkout-time
     (equal lastmod checkout-time)
     (if (zerop (vc-backend-diff file nil))
         (progn
           (vc-file-setprop file 'vc-checkout-time lastmod)
           t)
       (progn
           (vc-file-setprop file 'vc-checkout-time '(0 . 0))
           nil
         ))
     )))

(defun vc-next-action-on-file (file verbose &optional comment)
  ;;; If comment is specified, it will be used as an admin or checkin comment.
  (let (owner version (vc-file (vc-name file)))
    (cond

     ;; if there is no master file corresponding, create one
     ((not vc-file)
      (vc-register verbose comment)
      (if vc-initial-comment
          (setq vc-log-after-operation-hook
                'vc-checkout-writable-buffer-hook)
        (vc-checkout-writable-buffer file)))

     ;; if there is no lock on the file, assert one and get it
     ((not (setq owner (vc-locking-user file)))
      (vc-checkout-writable-buffer file))

     ;; a checked-out version exists, but the user may not own the lock
     ((not (string-equal owner (user-login-name)))
      (if comment
          (error "Sorry, you can't steal the lock on %s this way" file))
      (vc-steal-lock
       file
       (and verbose (read-string "Version to steal: "))
       owner))
     
     ;; OK, user owns the lock on the file
     (t
          (find-file file)

          ;; give luser a chance to save before checking in.
          (vc-buffer-sync)

          ;; Revert if file is unchanged and buffer is too.
          ;; If buffer is modified, that means the user just said no
          ;; to saving it; in that case, don't revert,
          ;; because the user might intend to save
          ;; after finishing the log entry.
          (if (and (vc-workfile-unchanged-p file)
                   (not (buffer-modified-p)))
              (progn
                (vc-backend-revert file)
                ;; DO NOT revert the file without asking the user!
                (vc-resynch-window file t nil))

            ;; user may want to set nonstandard parameters
            (if verbose
                (setq version (read-string "New version level: ")))

            ;; OK, let's do the checkin
            (vc-checkin file version comment)
            )))))

(defun vc-next-action-dired (file rev comment)
  ;; We've accepted a log comment, now do a vc-next-action using it on all
  ;; marked files.
  (set-buffer vc-parent-buffer)
  (dired-map-over-marks
   (save-window-excursion
     (let ((file (dired-get-filename)))
       (message "Processing %s..." file)
       (vc-next-action-on-file file nil comment)
       (message "Processing %s...done" file)))
   nil t)
  )

;; Here's the major entry point.

;;;###autoload
(defun vc-next-action (verbose)
  "Do the next logical checkin or checkout operation on the current file.
   If the file is not already registered, this registers it for version
control and then retrieves a writable, locked copy for editing.
   If the file is registered and not locked by anyone, this checks out
a writable and locked file ready for editing.
   If the file is checked out and locked by the calling user, this
first checks to see if the file has changed since checkout.  If not,
it performs a revert.
   If the file has been changed, this pops up a buffer for entry
of a log message; when the message has been entered, it checks in the
resulting changes along with the log message as change commentary.  If
the variable `vc-keep-workfiles' is non-nil (which is its default), a
read-only copy of the changed file is left in place afterwards.
   If the file is registered and locked by someone else, you are given
the option to steal the lock.
   If you call this from within a VC dired buffer with no files marked,
it will operate on the file in the current line.
   If you call this from within a VC dired buffer, and one or more
files are marked, it will accept a log message and then operate on
each one.  The log message will be used as a comment for any register
or checkin operations, but ignored when doing checkouts.  Attempted
lock steals will raise an error."
  (interactive "P")
  (catch 'nogo
    (if vc-dired-mode
        (let ((files (dired-get-marked-files)))
          (if (= (length files) 1)
              (find-file-other-window (dired-get-filename))
            (vc-start-entry nil nil nil
                            "Enter a change comment for the marked files."
                            'vc-next-action-dired)
            (throw 'nogo nil))))
    (while vc-parent-buffer
      (pop-to-buffer vc-parent-buffer))
    (if buffer-file-name
        (vc-next-action-on-file buffer-file-name verbose)
      (vc-registration-error nil))))

;;; These functions help the vc-next-action entry point

(defun vc-checkout-writable-buffer (&optional file)
  "Retrieve a writable copy of the latest version of the current buffer's file."
  (vc-checkout (or file (buffer-file-name)) t)
  )

;;;###autoload
(defun vc-register (&optional override comment)
  "Register the current file into your version-control system."
  (interactive "P")
  (if (vc-name buffer-file-name)
      (error "This file is already registered"))
  ;; Watch out for new buffers of size 0: the corresponding file
  ;; does not exist yet, even though buffer-modified-p is nil.
  (if (and (not (buffer-modified-p))
           (zerop (buffer-size))
           (not (file-exists-p buffer-file-name)))
      (set-buffer-modified-p t))
  (vc-buffer-sync)
  (vc-admin
   buffer-file-name
   (and override
        (read-string
         (format "Initial version level for %s: " buffer-file-name))))
  )

(defun vc-resynch-window (file &optional keep noquery)
  ;; If the given file is in the current buffer,
  ;; either revert on it so we see expanded keyworks,
  ;; or unvisit it (depending on vc-keep-workfiles)
  ;; NOQUERY if non-nil inhibits confirmation for reverting.
  ;; NOQUERY should be t *only* if it is known the only difference
  ;; between the buffer and the file is due to RCS rather than user editing!
  (and (string= buffer-file-name file)
       (if keep
           (progn
             (vc-revert-buffer1 t noquery)
             (vc-mode-line buffer-file-name))
         (progn
           (delete-window)
           (kill-buffer (current-buffer))))))

(defun vc-start-entry (file rev comment msg action)
  ;; Accept a comment for an operation on FILE revision REV.  If COMMENT
  ;; is nil, pop up a VC-log buffer, emit MSG, and set the
  ;; action on close to ACTION; otherwise, do action immediately.
  ;; Remember the file's buffer in parent-buffer (current one if no file).
  (let ((parent (if file (find-file-noselect file) (current-buffer))))
    (if comment
        (set-buffer (get-buffer-create "*VC-log*"))
      (pop-to-buffer (get-buffer-create "*VC-log*")))
    (set (make-local-variable 'vc-parent-buffer) parent)
    (set (make-local-variable 'vc-parent-buffer-name)
         (concat " from " (buffer-name vc-parent-buffer)))
    (vc-mode-line (or file " (no file)"))
    (vc-log-mode)
    (setq vc-log-operation action)
    (setq vc-log-file file)
    (setq vc-log-version rev)
    (if comment
        (progn
          (erase-buffer)
          (if (eq comment t)
              (vc-finish-logentry t)
            (insert comment)
            (vc-finish-logentry nil)))
      (message "%s  Type C-c C-c when done." msg))))

(defun vc-admin (file rev &optional comment)
  "Check a file into your version-control system.
FILE is the unmodified name of the file.  REV should be the base version
level to check it in under.  COMMENT, if specified, is the checkin comment."
      (vc-start-entry file rev
                      (or comment (not vc-initial-comment))
                      "Enter initial comment." 'vc-backend-admin))

(defun vc-checkout (file &optional writable)
  "Retrieve a copy of the latest version of the given file."
  ;; If ftp is on this system and the name matches the ange-ftp format
  ;; for a remote file, the user is trying something that won't work.
  (if (and (string-match "^/[^/:]+:" file) (vc-find-binary "ftp"))
      (error "Sorry, you can't check out files over FTP"))
  (vc-backend-checkout file writable)
  (if (string-equal file buffer-file-name)
      (vc-resynch-window file t t))
  )

(defun vc-steal-lock (file rev &optional owner)
  "Steal the lock on the current workfile."
  (interactive)
  (if (not owner)
      (setq owner (vc-locking-user file)))
  (if (not (y-or-n-p (format "Take the lock on %s:%s from %s? " file rev owner)))
      (error "Steal cancelled"))
  (pop-to-buffer (get-buffer-create "*VC-mail*"))
  (setq default-directory (expand-file-name "~/"))
  (auto-save-mode auto-save-default)
  (mail-mode)
  (erase-buffer)
  (mail-setup owner (format "%s:%s" file rev) nil nil nil
              (list (list 'vc-finish-steal file rev)))
  (goto-char (point-max))
  (insert
   (format "I stole the lock on %s:%s, " file rev)
   (current-time-string)
   ".\n")
  (message "Please explain why you stole the lock.  Type C-c C-c when done."))

;; This is called when the notification has been sent.
(defun vc-finish-steal (file version)
  (vc-backend-steal file version)
  (vc-resynch-window file t t))

(defun vc-checkin (file &optional rev comment)
  "Check in the file specified by FILE.
The optional argument REV may be a string specifying the new version level
\(if nil increment the current level).  The file is either retained with write
permissions zeroed, or deleted (according to the value of `vc-keep-workfiles').
COMMENT is a comment string; if omitted, a buffer is
popped up to accept a comment."
  (setq vc-log-after-operation-hook 'vc-checkin-hook)
  (vc-start-entry file rev comment "Enter a change comment." 'vc-backend-checkin))

;;; Here is a checkin hook that may prove useful to sites using the
;;; ChangeLog facility supported by Emacs.
(defun vc-comment-to-change-log (&optional whoami file-name)
  "Enter last VC comment into change log file for current buffer's file.
Optional arg (interactive prefix) non-nil means prompt for user name and site.
Second arg is file name of change log.  \
If nil, uses `change-log-default-name'."
  (interactive (if current-prefix-arg
                   (list current-prefix-arg
                         (prompt-for-change-log-name))))
  (let (;; Extract the comment first so we get any error before doing anything.
        (comment (ring-ref vc-comment-ring 0))
        ;; Don't let add-change-log-entry insert a defun name.
        (add-log-current-defun-function 'ignore)
        end)
    ;; Call add-log to do half the work.
    (add-change-log-entry whoami file-name t t)
    ;; Insert the VC comment, leaving point before it.
    (setq end (save-excursion (insert comment) (point-marker)))
    (if (looking-at "\\s *\\s(")
        ;; It starts with an open-paren, as in "(foo): Frobbed."
        ;; So remove the ": " add-log inserted.
        (delete-char -2))
    ;; Canonicalize the white space between the file name and comment.
    (just-one-space)
    ;; Indent rest of the text the same way add-log indented the first line.
    (let ((indentation (current-indentation)))
      (save-excursion
        (while (< (point) end)
          (forward-line 1)
          (indent-to indentation))
        (setq end (point))))
    ;; Fill the inserted text, preserving open-parens at bol.
    (let ((paragraph-separate (concat paragraph-separate "\\|^\\s *\\s("))
          (paragraph-start (concat paragraph-start "\\|^\\s *\\s(")))
      (beginning-of-line)
      (fill-region (point) end))
    ;; Canonicalize the white space at the end of the entry so it is
    ;; separated from the next entry by a single blank line.
    (skip-syntax-forward " " end)
    (delete-char (- (skip-syntax-backward " ")))
    (or (eobp) (looking-at "\n\n")
        (insert "\n"))))


(defun vc-finish-logentry (&optional nocomment)
  "Complete the operation implied by the current log entry."
  (interactive)
  ;; Check and record the comment, if any.
  (if (not nocomment)
      (progn
        (goto-char (point-max))
        (if (not (bolp))
            (newline))
        ;; Comment too long?
        (vc-backend-logentry-check vc-log-file)
        ;; Record the comment in the comment ring
        (if (null vc-comment-ring)
            (setq vc-comment-ring (make-ring vc-maximum-comment-ring-size)))
        (ring-insert vc-comment-ring (buffer-string))
        ))
  ;; OK, do it to it
  (if vc-log-operation
      (save-excursion
        (funcall vc-log-operation 
                 vc-log-file
                 vc-log-version
                 (buffer-string)))
    (error "No log operation is pending"))
  ;; Return to "parent" buffer of this checkin and remove checkin window
  (pop-to-buffer vc-parent-buffer)
  (let ((logbuf (get-buffer "*VC-log*")))
    (delete-windows-on logbuf)
    (kill-buffer logbuf))
  ;; Now make sure we see the expanded headers
  (if buffer-file-name
        (vc-resynch-window buffer-file-name vc-keep-workfiles t))
  (run-hooks vc-log-after-operation-hook))

;; Code for access to the comment ring

(defun vc-previous-comment (arg)
  "Cycle backwards through comment history."
  (interactive "*p")
  (let ((len (ring-length vc-comment-ring)))
    (cond ((<= len 0)
           (message "Empty comment ring")
           (ding))
          (t
           (erase-buffer)
           ;; Initialize the index on the first use of this command
           ;; so that the first M-p gets index 0, and the first M-n gets
           ;; index -1.
           (if (null vc-comment-ring-index)
               (setq vc-comment-ring-index
                     (if (> arg 0) -1
                         (if (< arg 0) 1 0))))
           (setq vc-comment-ring-index
                 (mod (+ vc-comment-ring-index arg) len))
           (message "%d" (1+ vc-comment-ring-index))
           (insert (ring-ref vc-comment-ring vc-comment-ring-index))))))

(defun vc-next-comment (arg)
  "Cycle forwards through comment history."
  (interactive "*p")
  (vc-previous-comment (- arg)))

(defun vc-comment-search-reverse (str)
  "Searches backwards through comment history for substring match."
  (interactive "sComment substring: ")
  (if (string= str "")
      (setq str vc-last-comment-match)
    (setq vc-last-comment-match str))
  (if (null vc-comment-ring-index)
      (setq vc-comment-ring-index -1))
  (let ((str (regexp-quote str))
        (len (ring-length vc-comment-ring))
        (n (1+ vc-comment-ring-index)))
    (while (and (< n len) (not (string-match str (ring-ref vc-comment-ring n))))
      (setq n (+ n 1)))
    (cond ((< n len)
           (vc-previous-comment (- n vc-comment-ring-index)))
          (t (error "Not found")))))

(defun vc-comment-search-forward (str)
  "Searches forwards through comment history for substring match."
  (interactive "sComment substring: ")
  (if (string= str "")
      (setq str vc-last-comment-match)
    (setq vc-last-comment-match str))
  (if (null vc-comment-ring-index)
      (setq vc-comment-ring-index 0))
  (let ((str (regexp-quote str))
        (len (ring-length vc-comment-ring))
        (n vc-comment-ring-index))
    (while (and (>= n 0) (not (string-match str (ring-ref vc-comment-ring n))))
      (setq n (- n 1)))
    (cond ((>= n 0)
           (vc-next-comment (- n vc-comment-ring-index)))
          (t (error "Not found")))))

;; Additional entry points for examining version histories

;;;###autoload
(defun vc-diff (historic)
  "Display diffs between file versions.
Normally this compares the current file and buffer with the most recent 
checked in version of that file.  This uses no arguments.
With a prefix argument, it reads the file name to use
and two version designators specifying which versions to compare."
  (interactive "P")
  (if vc-dired-mode
      (set-buffer (find-file-noselect (dired-get-filename))))
  (while vc-parent-buffer
      (pop-to-buffer vc-parent-buffer))
  (if historic
      (call-interactively 'vc-version-diff)
    (if (or (null buffer-file-name) (null (vc-name buffer-file-name)))
        (error
         "There is no version-control master associated with this buffer"))
    (let ((file buffer-file-name)
          unchanged)
      (or (and file (vc-name file))
          (vc-registration-error file))
      (vc-buffer-sync)
      (setq unchanged (vc-workfile-unchanged-p buffer-file-name))
      (if unchanged
          (message "No changes to %s since latest version." file)
        (vc-backend-diff file nil)
        ;; Ideally, we'd like at this point to parse the diff so that
        ;; the buffer effectively goes into compilation mode and we
        ;; can visit the old and new change locations via next-error.
        ;; Unfortunately, this is just too painful to do.  The basic
        ;; problem is that the `old' file doesn't exist to be
        ;; visited.  This plays hell with numerous assumptions in
        ;; the diff.el and compile.el machinery.
        (pop-to-buffer "*vc*")
        (pop-to-buffer "*vc*")
        (if (= 0 (buffer-size))
            (progn
              (setq unchanged t)
              (message "No changes to %s since latest version." file))
          (goto-char (point-min))
          (shrink-window-if-larger-than-buffer)))
      (not unchanged))))

(defun vc-version-diff (file rel1 rel2)
  "For FILE, report diffs between two stored versions REL1 and REL2 of it.
If FILE is a directory, generate diffs between versions for all registered
files in or below it."
  (interactive "FFile or directory to diff: \nsOlder version: \nsNewer version: ")
  (if (string-equal rel1 "") (setq rel1 nil))
  (if (string-equal rel2 "") (setq rel2 nil))
  (if (file-directory-p file)
      (let ((camefrom (current-buffer)))
        (set-buffer (get-buffer-create "*vc-status*"))
        (set (make-local-variable 'vc-parent-buffer) camefrom)
        (set (make-local-variable 'vc-parent-buffer-name)
             (concat " from " (buffer-name camefrom)))
        (erase-buffer)
        (insert "Diffs between "
                (or rel1 "last version checked in")
                " and "
                (or rel2 "current workfile(s)")
                ":\n\n")
        (set-buffer (get-buffer-create "*vc*"))
        (cd file)
        (vc-file-tree-walk
         (function (lambda (f)
                     (message "Looking at %s" f)
                     (and
                      (not (file-directory-p f))
                      (vc-registered f)
                      (vc-backend-diff f rel1 rel2)
                      (append-to-buffer "*vc-status*" (point-min) (point-max)))
                     )))
        (pop-to-buffer "*vc-status*")
        (insert "\nEnd of diffs.\n")
        (goto-char (point-min))
        (set-buffer-modified-p nil)
        )
    (if (zerop (vc-backend-diff file rel1 rel2))
        (message "No changes to %s between %s and %s." file rel1 rel2)
      (pop-to-buffer "*vc*"))))

;;;###autoload
(defun vc-version-other-window (rev)
  "Visit version REV of the current buffer in another window.
If the current buffer is named `F', the version is named `F.~REV~'.
If `F.~REV~' already exists, it is used instead of being re-created."
  (interactive "sVersion to visit (default is latest version): ")
  (if vc-dired-mode
      (set-buffer (find-file-noselect (dired-get-filename))))
  (while vc-parent-buffer
      (pop-to-buffer vc-parent-buffer))
  (if (and buffer-file-name (vc-name buffer-file-name))
      (let* ((version (if (string-equal rev "")
                          (vc-latest-version buffer-file-name)
                        rev))
             (filename (concat buffer-file-name ".~" version "~")))
         (or (file-exists-p filename)
             (vc-backend-checkout buffer-file-name nil version filename))
         (find-file-other-window filename))
    (vc-registration-error buffer-file-name)))

;; Header-insertion code

;;;###autoload
(defun vc-insert-headers ()
  "Insert headers in a file for use with your version-control system.
Headers desired are inserted at the start of the buffer, and are pulled from
the variable `vc-header-alist'."
  (interactive)
  (if vc-dired-mode
      (find-file-other-window (dired-get-filename)))
  (while vc-parent-buffer
      (pop-to-buffer vc-parent-buffer))
  (save-excursion
    (save-restriction
      (widen)
      (if (or (not (vc-check-headers))
              (y-or-n-p "Version headers already exist.  Insert another set? "))
          (progn
            (let* ((delims (cdr (assq major-mode vc-comment-alist)))
                   (comment-start-vc (or (car delims) comment-start "#"))
                   (comment-end-vc (or (car (cdr delims)) comment-end ""))
                   (hdstrings (cdr (assoc (vc-backend-deduce (buffer-file-name)) vc-header-alist))))
              (mapcar (function (lambda (s)
                                  (insert comment-start-vc "\t" s "\t"
                                          comment-end-vc "\n")))
                      hdstrings)
              (if vc-static-header-alist
                  (mapcar (function (lambda (f)
                                      (if (string-match (car f) buffer-file-name)
                                          (insert (format (cdr f) (car hdstrings))))))
                          vc-static-header-alist))
              )
            )))))

;; The VC directory submode.  Coopt Dired for this.
;; All VC commands get mapped into logical equivalents.

(defvar vc-dired-prefix-map (make-sparse-keymap))
(define-key vc-dired-prefix-map "\C-xv" vc-prefix-map)

(or (not (boundp 'minor-mode-map-alist))
    (assq 'vc-dired-mode minor-mode-map-alist)
    (setq minor-mode-map-alist
           (cons (cons 'vc-dired-mode vc-dired-prefix-map)
                 minor-mode-map-alist)))

(defun vc-dired-mode ()
  "The augmented Dired minor mode used in VC directory buffers.
All Dired commands operate normally.  Users currently locking listed files
are listed in place of the file's owner and group.
Keystrokes bound to VC commands will execute as though they had been called
on a buffer attached to the file named in the current Dired buffer line."
  (setq vc-dired-mode t)
  (setq vc-mode " under VC"))

(defun vc-dired-reformat-line (x)
  ;; Hack a directory-listing line, plugging in locking-user info in
  ;; place of the user and group info.  Should have the beneficial
  ;; side-effect of shortening the listing line.  Each call starts with
  ;; point immediately following the dired mark area on the line to be
  ;; hacked.
  ;;
  ;; Simplest possible one:
  ;; (insert (concat x "\t")))
  ;;
  ;; This code, like dired, assumes UNIX -l format.
  (forward-word 1)      ;; skip over any extra field due to -ibs options
  (if x (setq x (concat "(" x ")")))
  (if (re-search-forward "\\([0-9]+ \\).................\\( .*\\)" nil 0)
      (let ((rep (substring (concat x "                 ") 0 9)))
        (replace-match (concat "\\1" rep "\\2") t)))
  )

;;; Note in Emacs 18 the following defun gets overridden
;;; with the symbol 'vc-directory-18.  See below.
;;;###autoload
(defun vc-directory (verbose)
  "Show version-control status of all files under the current directory."
  (interactive "P")
  (let (nonempty
        (dl (length default-directory))
        (filelist nil) (userlist nil)
        dired-buf
        dired-buf-mod-count)
    (vc-file-tree-walk
     (function (lambda (f)
                 (if (vc-registered f)
                     (let ((user (vc-locking-user f)))
                       (and (or verbose user)
                            (setq filelist (cons (substring f dl) filelist))
                            (setq userlist (cons user userlist))))))))
    (save-excursion
      ;; This uses a semi-documented feature of dired; giving a switch
      ;; argument forces the buffer to refresh each time.
      (dired
       (cons default-directory (nreverse filelist))
       dired-listing-switches)
      (setq dired-buf (current-buffer))
      (setq nonempty (not (zerop (buffer-size)))))
    (if nonempty
        (progn
          (pop-to-buffer dired-buf)
          (vc-dired-mode)
          (goto-char (point-min))
          (setq buffer-read-only nil)
          (forward-line 1)      ;; Skip header line
          (mapcar
           (function
            (lambda (x)
             (forward-char 2)   ;; skip dired's mark area
             (vc-dired-reformat-line x)
             (forward-line 1))) ;; go to next line
           (nreverse userlist))
          (setq buffer-read-only t)
          (goto-char (point-min))
          )
      (message "No files are currently %s under %s"
               (if verbose "registered" "locked") default-directory))
    ))

;; Emacs 18 version
(defun vc-directory-18 (verbose)
  "Show version-control status of all files under the current directory."
  (interactive "P")
  (let (nonempty (dir default-directory))
    (save-excursion
      (set-buffer (get-buffer-create "*vc-status*"))
      (erase-buffer)
      (cd dir)
      (vc-file-tree-walk
       (function (lambda (f)
                   (if (vc-registered f)
                       (let ((user (vc-locking-user f)))
                         (if (or user verbose)
                             (insert (format
                                      "%s       %s\n"
                                      (concat user) f))))))))
      (setq nonempty (not (zerop (buffer-size)))))
    (if nonempty
        (progn
          (pop-to-buffer "*vc-status*" t)
          (goto-char (point-min))
          (shrink-window-if-larger-than-buffer)))
      (message "No files are currently %s under %s"
               (if verbose "registered" "locked") default-directory))
    )

(or (boundp 'minor-mode-map-alist)
    (fset 'vc-directory 'vc-directory-18))

; Emacs 18 also lacks these.
(or (boundp 'compilation-old-error-list)
    (setq compilation-old-error-list nil))

;; Named-configuration support for SCCS

(defun vc-add-triple (name file rev)
  (save-excursion
    (find-file (concat (vc-backend-subdirectory-name file) "/" vc-name-assoc-file))
    (goto-char (point-max))
    (insert name "\t:\t" file "\t" rev "\n")
    (basic-save-buffer)
    (kill-buffer (current-buffer))
    ))

(defun vc-record-rename (file newname)
  (save-excursion
    (find-file (concat (vc-backend-subdirectory-name file) "/" vc-name-assoc-file))
    (goto-char (point-min))
    (replace-regexp (concat ":" (regexp-quote file) "$") (concat ":" newname))
    (basic-save-buffer)
    (kill-buffer (current-buffer))
    ))

(defun vc-lookup-triple (file name)
  ;; Return the numeric version corresponding to a named snapshot of file
  ;; If name is nil or a version number string it's just passed through
  (cond ((null name) "")
        ((let ((firstchar (aref name 0)))
           (and (>= firstchar ?0) (<= firstchar ?9)))
         name)
        (t
         (car (vc-master-info
               (concat (vc-backend-subdirectory-name file) "/" vc-name-assoc-file)
               (list (concat name "\t:\t" file "\t\\(.+\\)"))))
         )))

;; Named-configuration entry points

(defun vc-locked-example ()
  ;; Return an example of why the current directory is not ready to be snapshot
  ;; or nil if no such example exists.
  (catch 'vc-locked-example
    (vc-file-tree-walk
     (function (lambda (f)
                 (if (and (vc-registered f) (vc-locking-user f))
                     (throw 'vc-locked-example f)))))
    nil))

;;;###autoload
(defun vc-create-snapshot (name)
  "Make a snapshot called NAME.
The snapshot is made from all registered files at or below the current
directory.  For each file, the version level of its latest
version becomes part of the named configuration."
  (interactive "sNew snapshot name: ")
  (let ((locked (vc-locked-example)))
    (if locked
        (error "File %s is locked" locked)
      (vc-file-tree-walk
       (function (lambda (f) (and
                              (vc-name f)
                              (vc-backend-assign-name f name)))))
      )))

;;;###autoload
(defun vc-retrieve-snapshot (name)
  "Retrieve the snapshot called NAME.
This function fails if any files are locked at or below the current directory
Otherwise, all registered files are checked out (unlocked) at their version
levels in the snapshot."
  (interactive "sSnapshot name to retrieve: ")
  (let ((locked (vc-locked-example)))
    (if locked
        (error "File %s is locked" locked)
      (vc-file-tree-walk
       (function (lambda (f) (and
                              (vc-name f)
                              (vc-error-occurred
                               (vc-backend-checkout f nil name))))))
      )))

;; Miscellaneous other entry points

;;;###autoload
(defun vc-print-log ()
  "List the change log of the current buffer in a window."
  (interactive)
  (if vc-dired-mode
      (set-buffer (find-file-noselect (dired-get-filename))))
  (while vc-parent-buffer
      (pop-to-buffer vc-parent-buffer))
  (if (and buffer-file-name (vc-name buffer-file-name))
      (progn
        (vc-backend-print-log buffer-file-name)
        (pop-to-buffer (get-buffer-create "*vc*"))
        (while (looking-at "=*\n")
          (delete-char (- (match-end 0) (match-beginning 0)))
          (forward-line -1))
        (goto-char (point-min))
        (if (looking-at "[\b\t\n\v\f\r ]+")
            (delete-char (- (match-end 0) (match-beginning 0))))
        (shrink-window-if-larger-than-buffer)
        )
    (vc-registration-error buffer-file-name)
    )
  )

;;;###autoload
(defun vc-revert-buffer ()
  "Revert the current buffer's file back to the latest checked-in version.
This asks for confirmation if the buffer contents are not identical
to that version."
  (interactive)
  (if vc-dired-mode
      (find-file-other-window (dired-get-filename)))
  (while vc-parent-buffer
      (pop-to-buffer vc-parent-buffer))
  (let ((file buffer-file-name)
        (obuf (current-buffer)) (changed (vc-diff nil)))
    (if (and changed (or vc-suppress-confirm
                         (not (yes-or-no-p "Discard changes? "))))
        (progn
          (delete-window)
          (error "Revert cancelled"))
      (set-buffer obuf))
    (if changed
        (delete-window))
    (vc-backend-revert file)
    (vc-resynch-window file t t)
    )
  )

;;;###autoload
(defun vc-cancel-version (norevert)
  "Get rid of most recently checked in version of this file.
A prefix argument means do not revert the buffer afterwards."
  (interactive "P")
  (if vc-dired-mode
      (find-file-other-window (dired-get-filename)))
  (while vc-parent-buffer
    (pop-to-buffer vc-parent-buffer))
  (let* ((target (concat (vc-latest-version (buffer-file-name))))
        (yours (concat (vc-your-latest-version (buffer-file-name))))
        (prompt (if (string-equal yours target)
                    "Remove your version %s from master? "
                  "Version %s was not your change.  Remove it anyway? ")))
    (if (null (yes-or-no-p (format prompt target)))
        nil
      (vc-backend-uncheck (buffer-file-name) target)
      (if (or norevert
              (not (yes-or-no-p "Revert buffer to most recent remaining version? ")))
          (vc-mode-line (buffer-file-name))
        (vc-checkout (buffer-file-name) nil)))
    ))

(defun vc-rename-file (old new)
  "Rename file OLD to NEW, and rename its master file likewise."
  (interactive "fVC rename file: \nFRename to: ")
  (let ((oldbuf (get-file-buffer old)))
    (if (and oldbuf (buffer-modified-p oldbuf))
        (error "Please save files before moving them"))
    (if (get-file-buffer new)
        (error "Already editing new file name"))
    (if (file-exists-p new)
        (error "New file already exists"))
    (let ((oldmaster (vc-name old)))
      (if oldmaster
          (progn
            (if (vc-locking-user old)
                (error "Please check in files before moving them"))
            (if (or (file-symlink-p oldmaster)
                    ;; This had FILE, I changed it to OLD. -- rms.
                    (file-symlink-p (vc-backend-subdirectory-name old)))
                (error "This is not a safe thing to do in the presence of symbolic links"))
            (rename-file
             oldmaster
             (let ((backend (vc-backend-deduce old))
                   (newdir (or (file-name-directory new) ""))
                   (newbase (file-name-nondirectory new)))
               (catch 'found
                 (mapcar
                  (function
                   (lambda (s)
                     (if (eq backend (cdr s))
                         (let* ((newmaster (format (car s) newdir newbase))
                                (newmasterdir (file-name-directory newmaster)))
                           (if (or (not newmasterdir)
                                   (file-directory-p newmasterdir))
                               (throw 'found newmaster))))))
                  vc-master-templates)
                 (error "New file lacks a version control directory"))))))
      (if (or (not oldmaster) (file-exists-p old))
          (rename-file old new)))
; ?? Renaming a file might change its contents due to keyword expansion.
; We should really check out a new copy if the old copy was precisely equal
; to some checked in version.  However, testing for this is tricky....
    (if oldbuf
        (save-excursion
          (set-buffer oldbuf)
          (set-visited-file-name new)
          (set-buffer-modified-p nil))))
  ;; This had FILE, I changed it to OLD. -- rms.
  (vc-backend-dispatch old
                       (vc-record-rename old new)
                       nil)
  )

;;;###autoload
(defun vc-update-change-log (&rest args)
  "Find change log file and add entries from recent RCS logs.
The mark is left at the end of the text prepended to the change log.
With prefix arg of C-u, only find log entries for the current buffer's file.
With any numeric prefix arg, find log entries for all files currently visited.
Otherwise, find log entries for all registered files in the default directory.
From a program, any arguments are passed to the `rcs2log' script."
  (interactive
   (cond ((consp current-prefix-arg)    ;C-u
          (list buffer-file-name))
         (current-prefix-arg            ;Numeric argument.
          (let ((files nil)
                (buffers (buffer-list))
                file)
            (while buffers
              (setq file (buffer-file-name (car buffers)))
              (and file (vc-backend-deduce file)
                   (setq files (cons file files)))
              (setq buffers (cdr buffers)))
            files))
         (t
          (let ((RCS (concat default-directory "RCS")))
            (and (file-directory-p RCS)
                 (mapcar (function
                          (lambda (f)
                            (if (string-match "\\(.*\\),v$" f)
                                (substring f 0 (match-end 1))
                              f)))
                         (directory-files RCS nil "...\\|^[^.]\\|^.[^.]")))))))
  (let ((odefault default-directory))
    (find-file-other-window (find-change-log))
    (barf-if-buffer-read-only)
    (vc-buffer-sync)
    (undo-boundary)
    (goto-char (point-min))
    (push-mark)
    (message "Computing change log entries...")
    (message "Computing change log entries... %s"
             (if (or (null args)
                     (eq 0 (apply 'call-process "rcs2log" nil t nil
                                  (mapcar (function
                                           (lambda (f)
                                             (file-relative-name
                                              (if (file-name-absolute-p f)
                                                  f
                                                (concat odefault f)))))
                                          args))))
                 "done" "failed"))))

;; Functions for querying the master and lock files.

(defun vc-match-substring (bn)
  (buffer-substring (match-beginning bn) (match-end bn)))

(defun vc-parse-buffer (patterns &optional file properties)
  ;; Use PATTERNS to parse information out of the current buffer
  ;; by matching each regular expression in the list and returning \\1.
  ;; If a regexp has two tag brackets, assume the second is a date
  ;; field and we want the most recent entry matching the template.
  ;; If FILE and PROPERTIES are given, the latter must be a list of
  ;; properties of the same length as PATTERNS; each property is assigned 
  ;; the corresponding value.
  (mapcar (function (lambda (p)
             (goto-char (point-min))
             (if (string-match "\\\\(.*\\\\(" p)
                 (let ((latest-date "") (latest-val))
                   (while (re-search-forward p nil t)
                     (let ((date (vc-match-substring 2)))
                       (if (string< latest-date date)
                           (progn
                             (setq latest-date date)
                             (setq latest-val
                                   (vc-match-substring 1))))))
                   latest-val))
             (prog1
                 (and (re-search-forward p nil t)
                      (let ((value (vc-match-substring 1)))
                        (if file
                            (vc-file-setprop file (car properties) value))
                        value))
               (setq properties (cdr properties)))))
          patterns)
  )

(defun vc-master-info (file fields &optional rfile properties)
  ;; Search for information in a master file.
  (if (and file (file-exists-p file))
      (save-excursion
        (let ((buf))
          (setq buf (create-file-buffer file))
          (set-buffer buf))
        (erase-buffer)
        (insert-file-contents file nil)
        (set-buffer-modified-p nil)
        (auto-save-mode nil)
        (prog1
            (vc-parse-buffer fields rfile properties)
          (kill-buffer (current-buffer)))
        )
    (if rfile
        (mapcar
         (function (lambda (p) (vc-file-setprop rfile p nil)))
         properties))
    )
  )

(defun vc-log-info (command file patterns &optional properties)
  ;; Search for information in log program output
  (if (and file (file-exists-p file))
      (save-excursion
        (let ((buf))
          (setq buf (get-buffer-create "*vc*"))
          (set-buffer buf))
        (apply 'vc-do-command 0 command file nil)
        (set-buffer-modified-p nil)
        (prog1
            (vc-parse-buffer patterns file properties)
          (kill-buffer (current-buffer))
          )
        )
    (if file
        (mapcar
         (function (lambda (p) (vc-file-setprop file p nil)))
         properties))
    )
  )

(defun vc-locking-user (file)
  "Return the name of the person currently holding a lock on FILE.
Return nil if there is no such person."
  (setq file (expand-file-name file))   ;; ??? Work around bug in 19.0.4
  (if (or (not vc-keep-workfiles)
          (eq vc-mistrust-permissions 't)
          (and vc-mistrust-permissions
               (funcall vc-mistrust-permissions (vc-backend-subdirectory-name file))))
      (vc-true-locking-user file)
    ;; This implementation assumes that any file which is under version
    ;; control and has -rw-r--r-- is locked by its owner.  This is true
    ;; for both RCS and SCCS, which keep unlocked files at -r--r--r--.
    ;; We have to be careful not to exclude files with execute bits on;
    ;; scripts can be under version control too.  The advantage of this
    ;; hack is that calls to the very expensive vc-fetch-properties
    ;; function only have to be made if (a) the file is locked by someone
    ;; other than the current user, or (b) some untoward manipulation
    ;; behind vc's back has changed the owner or the `group' or `other'
    ;; write bits.
    (let ((attributes (file-attributes file)))
      (cond ((string-match ".r-.r-.r-." (nth 8 attributes))
             nil)
            ((and (= (nth 2 attributes) (user-uid))
                  (string-match ".rw.r-.r-." (nth 8 attributes)))
             (user-login-name))
            (t
             (vc-true-locking-user file))))))

(defun vc-true-locking-user (file)
  ;; The slow but reliable version
  (vc-fetch-properties file)
  (vc-file-getprop file 'vc-locking-user))

(defun vc-latest-version (file)
  ;; Return version level of the latest version of FILE
  (vc-fetch-properties file)
  (vc-file-getprop file 'vc-latest-version))

(defun vc-your-latest-version (file)
  ;; Return version level of the latest version of FILE checked in by you
  (vc-fetch-properties file)
  (vc-file-getprop file 'vc-your-latest-version))

;; Collect back-end-dependent stuff here
;;
;; Everything eventually funnels through these functions.  To implement
;; support for a new version-control system, add another branch to the
;; vc-backend-dispatch macro and fill it in in each call.  The variable
;; vc-master-templates in vc-hooks.el will also have to change.

(defmacro vc-backend-dispatch (f s r)
  "Execute FORM1 or FORM2 depending on whether we're using SCCS or RCS."
  (list 'let (list (list 'type (list 'vc-backend-deduce f)))
        (list 'cond
              (list (list 'eq 'type (quote 'SCCS)) s)   ;; SCCS
              (list (list 'eq 'type (quote 'RCS)) r)    ;; RCS
              )))

(defun vc-lock-file (file)
  ;; Generate lock file name corresponding to FILE
  (let ((master (vc-name file)))
    (and
     master
     (string-match "\\(.*/\\)s\\.\\(.*\\)" master)
     (concat
      (substring master (match-beginning 1) (match-end 1))
      "p."
      (substring master (match-beginning 2) (match-end 2))))))


(defun vc-fetch-properties (file)
  ;; Re-fetch all properties associated with the given file.
  ;; Currently these properties are:
  ;;    vc-locking-user
  ;;    vc-locked-version
  ;;    vc-latest-version
  ;;    vc-your-latest-version
  (vc-backend-dispatch
   file
   ;; SCCS
   (progn
     (vc-master-info (vc-lock-file file)
                     (list
                      "^[^ ]+ [^ ]+ \\([^ ]+\\)"
                      "^\\([^ ]+\\)")
                     file
                     '(vc-locking-user vc-locked-version))
     (vc-master-info (vc-name file)
                  (list
                   "^\001d D \\([^ ]+\\)"
                   (concat "^\001d D \\([^ ]+\\) .* " 
                           (regexp-quote (user-login-name)) " ")
                   )
                  file
                  '(vc-latest-version vc-your-latest-version))
     )
   ;; RCS
   (vc-log-info "rlog" file
                (list
                 "^locks: strict\n\t\\([^:]+\\)"
                 "^locks: strict\n\t[^:]+: \\(.+\\)"
                 "^revision[\t ]+\\([0-9.]+\\).*\ndate: \\([ /0-9:]+\\);"
                 (concat
                  "^revision[\t ]+\\([0-9.]+\\)\n.*author: "
                  (regexp-quote (user-login-name))
                  ";"))
                '(vc-locking-user vc-locked-version
                                  vc-latest-version vc-your-latest-version))
   ))

(defun vc-backend-subdirectory-name (&optional file)
  ;; Where the master and lock files for the current directory are kept
  (symbol-name
   (or
    (and file (vc-backend-deduce file))
    vc-default-back-end
    (setq vc-default-back-end (if (vc-find-binary "rcs") 'RCS 'SCCS)))))

(defun vc-backend-admin (file &optional rev comment)
  ;; Register a file into the version-control system
  ;; Automatically retrieves a read-only version of the file with
  ;; keywords expanded if vc-keep-workfiles is non-nil, otherwise
  ;; it deletes the workfile.
  (vc-file-clearprops file)
  (or vc-default-back-end
      (setq vc-default-back-end (if (vc-find-binary "rcs") 'RCS 'SCCS)))
  (message "Registering %s..." file)
  (let ((backend
         (cond
          ((file-exists-p (vc-backend-subdirectory-name)) vc-default-back-end)
          ((file-exists-p "RCS") 'RCS)
          ((file-exists-p "SCCS") 'SCCS)
          (t vc-default-back-end))))
    (cond ((eq backend 'SCCS)
           (vc-do-command 0 "admin" file        ;; SCCS
                          (and rev (concat "-r" rev))
                          "-fb"
                          (concat "-i" file)
                          (and comment (concat "-y" comment))
                          (format
                           (car (rassq 'SCCS vc-master-templates))
                           (or (file-name-directory file) "")
                           (file-name-nondirectory file)))
           (delete-file file)
           (if vc-keep-workfiles
               (vc-do-command 0 "get" file)))
          ((eq backend 'RCS)
           (vc-do-command 0 "ci" file   ;; RCS
                          (concat (if vc-keep-workfiles "-u" "-r") rev)
                          (and comment (concat "-t-" comment))
                          file)
           )))
  (message "Registering %s...done" file)
  )

(defun vc-backend-checkout (file &optional writable rev workfile)
  ;; Retrieve a copy of a saved version into a workfile
  (let ((filename (or workfile file)))
    (message "Checking out %s..." filename)
    (vc-backend-dispatch file
     (vc-do-command 0 "get" file        ;; SCCS
                    (if writable "-e")
                    (if workfile  (concat "-G" workfile))
                    (and rev (concat "-r" (vc-lookup-triple file rev))))
     (if workfile ;; RCS
         ;; RCS doesn't let us check out into arbitrary file names directly.
         ;; Use `co -p' and make stdout point to the correct file.
         (let ((vc-modes (logior (file-modes (vc-name file))
                                 (if writable 128 0)))
               (failed t))
           (unwind-protect
               (progn
                   (vc-do-command
                      0 "/bin/sh" file "-c"
                      (format "umask %o; exec >\"$1\" || exit; shift; umask %o; exec co \"$@\""
                              (logand 511 (lognot vc-modes))
                              (logand 511 (lognot (default-file-modes))))
                      "" ; dummy argument for shell's $0
                      filename
                      (if writable "-l")
                      (concat "-p" rev))
                   (setq failed nil))
             (and failed (file-exists-p filename) (delete-file filename))))
       (vc-do-command 0 "co" file
                      (if writable "-l")
                      (and rev (concat "-r" rev))))
     )
    (or workfile
        (vc-file-setprop file 'vc-checkout-time (nth 5 (file-attributes file))))
    (message "Checking out %s...done" filename))
  )

(defun vc-backend-logentry-check (file)
  (vc-backend-dispatch file
   (if (>= (buffer-size) 512)   ;; SCCS
       (progn
         (goto-char 512)
         (error
          "Log must be less than 512 characters; point is now at pos 512")))
   nil)
  )

(defun vc-backend-checkin (file &optional rev comment)
  ;; Register changes to FILE as level REV with explanatory COMMENT.
  ;; Automatically retrieves a read-only version of the file with
  ;; keywords expanded if vc-keep-workfiles is non-nil, otherwise
  ;; it deletes the workfile.
  (message "Checking in %s..." file)
  (save-excursion
    ;; Change buffers to get local value of vc-checkin-switches.
    (set-buffer (or (get-file-buffer file) (current-buffer)))
    (vc-backend-dispatch file
      (progn
        (apply 'vc-do-command 0 "delta" file
               (if rev (concat "-r" rev))
               (concat "-y" comment)
               vc-checkin-switches)
        (if vc-keep-workfiles
            (vc-do-command 0 "get" file))
        )
      (apply 'vc-do-command 0 "ci" file
             (concat (if vc-keep-workfiles "-u" "-r") rev)
             (concat "-m" comment)
             vc-checkin-switches)
      ))
  (vc-file-setprop file 'vc-locking-user nil)
  (message "Checking in %s...done" file)
  )

(defun vc-backend-revert (file)
  ;; Revert file to latest checked-in version.
  (message "Reverting %s..." file)
  (vc-backend-dispatch
   file
   (progn                       ;; SCCS
     (vc-do-command 0 "unget" file nil)
     (vc-do-command 0 "get" file nil))
   (vc-do-command 0 "co" file "-f" "-u")) ;; RCS.  This deletes the work file.
  (vc-file-setprop file 'vc-locking-user nil)
  (message "Reverting %s...done" file)
  )

(defun vc-backend-steal (file &optional rev)
  ;; Steal the lock on the current workfile.  Needs RCS 5.6.2 or later for -M.
  (message "Stealing lock on %s..." file)
  (vc-backend-dispatch file
   (progn
     (vc-do-command 0 "unget" file "-n" (if rev (concat "-r" rev)))
     (vc-do-command 0 "get" file "-g" (if rev (concat "-r" rev)))
     )
   (vc-do-command 0 "rcs" file "-M" (concat "-u" rev) (concat "-l" rev)))
  (vc-file-setprop file 'vc-locking-user (user-login-name))
  (message "Stealing lock on %s...done" file)
  )  

(defun vc-backend-uncheck (file target)
  ;; Undo the latest checkin.  Note: this code will have to get a lot
  ;; smarter when we support multiple branches.
  (message "Removing last change from %s..." file)
  (vc-backend-dispatch file
   (vc-do-command 0 "rmdel" file (concat "-r" target))
   (vc-do-command 0 "rcs" file (concat "-o" target))
   )
  (message "Removing last change from %s...done" file)
  )

(defun vc-backend-print-log (file)
  ;; Print change log associated with FILE to buffer *vc*.
  (vc-do-command 0
                 (vc-backend-dispatch file "prs" "rlog")
                 file)
  )

(defun vc-backend-assign-name (file name)
  ;; Assign to a FILE's latest version a given NAME.
  (vc-backend-dispatch file
   (vc-add-triple name file (vc-latest-version file))   ;; SCCS
   (vc-do-command 0 "rcs" file (concat "-n" name ":"))  ;; RCS
   )
  )

(defun vc-backend-diff (file oldvers &optional newvers)
  ;; Get a difference report between two versions
  (if (eq (vc-backend-deduce file) 'SCCS)
      (setq oldvers (vc-lookup-triple file oldvers))
      (setq newvers (vc-lookup-triple file newvers)))
  (apply 'vc-do-command 1
         (or (vc-backend-dispatch file "vcdiff" "rcsdiff")
             (vc-registration-error file))
         file
         "-q"
         (and oldvers (concat "-r" oldvers))
         (and newvers (concat "-r" newvers))
         (if (listp diff-switches)
             diff-switches
           (list diff-switches))
  ))

(defun vc-check-headers ()
  "Check if the current file has any headers in it."
  (interactive)
  (save-excursion
    (goto-char (point-min))
    (vc-backend-dispatch buffer-file-name
     (re-search-forward  "%[MIRLBSDHTEGUYFPQCZWA]%" nil t)      ;; SCCS
     (re-search-forward "\\$[A-Za-z\300-\326\330-\366\370-\377]+\\(: [\t -#%-\176\240-\377]*\\)?\\$" nil t)             ;; RCS
     )
    ))

;; Back-end-dependent stuff ends here.

;; Set up key bindings for use while editing log messages

(defun vc-log-mode ()
  "Minor mode for driving version-control tools.
These bindings are added to the global keymap when you enter this mode:
\\[vc-next-action]              perform next logical version-control operation on current file
\\[vc-register]                 register current file
\\[vc-toggle-read-only]         like next-action, but won't register files
\\[vc-insert-headers]           insert version-control headers in current file
\\[vc-print-log]                display change history of current file
\\[vc-revert-buffer]            revert buffer to latest version
\\[vc-cancel-version]           undo latest checkin
\\[vc-diff]             show diffs between file versions
\\[vc-version-other-window]             visit old version in another window
\\[vc-directory]                show all files locked by any user in or below .
\\[vc-update-change-log]                add change log entry from recent checkins

While you are entering a change log message for a version, the following
additional bindings will be in effect.

\\[vc-finish-logentry]  proceed with check in, ending log message entry

Whenever you do a checkin, your log comment is added to a ring of
saved comments.  These can be recalled as follows:

\\[vc-next-comment]     replace region with next message in comment ring
\\[vc-previous-comment] replace region with previous message in comment ring
\\[vc-comment-search-reverse]   search backward for regexp in the comment ring
\\[vc-comment-search-forward]   search backward for regexp in the comment ring

Entry to the change-log submode calls the value of text-mode-hook, then
the value of vc-log-mode-hook.

Global user options:
        vc-initial-comment      If non-nil, require user to enter a change
                                comment upon first checkin of the file.

        vc-keep-workfiles       Non-nil value prevents workfiles from being
                                deleted when changes are checked in

        vc-suppress-confirm     Suppresses some confirmation prompts,
                                notably for reversions.

        vc-header-alist         Which keywords to insert when adding headers
                                with \\[vc-insert-headers].  Defaults to
                                '(\"\%\W\%\") under SCCS, '(\"\$Id\$\") under RCS.

        vc-static-header-alist  By default, version headers inserted in C files
                                get stuffed in a static string area so that
                                ident(RCS) or what(SCCS) can see them in the
                                compiled object code.  You can override this
                                by setting this variable to nil, or change
                                the header template by changing it.

        vc-command-messages     if non-nil, display run messages from the
                                actual version-control utilities (this is
                                intended primarily for people hacking vc
                                itself).
"
  (interactive)
  (set-syntax-table text-mode-syntax-table)
  (use-local-map vc-log-entry-mode)
  (setq local-abbrev-table text-mode-abbrev-table)
  (setq major-mode 'vc-log-mode)
  (setq mode-name "VC-Log")
  (make-local-variable 'vc-log-file)
  (make-local-variable 'vc-log-version)
  (make-local-variable 'vc-comment-ring-index)
  (set-buffer-modified-p nil)
  (setq buffer-file-name nil)
  (run-hooks 'text-mode-hook 'vc-log-mode-hook)
)

;; Initialization code, to be done just once at load-time
(if vc-log-entry-mode
    nil
  (setq vc-log-entry-mode (make-sparse-keymap))
  (define-key vc-log-entry-mode "\M-n" 'vc-next-comment)
  (define-key vc-log-entry-mode "\M-p" 'vc-previous-comment)
  (define-key vc-log-entry-mode "\M-r" 'vc-comment-search-reverse)
  (define-key vc-log-entry-mode "\M-s" 'vc-comment-search-forward)
  (define-key vc-log-entry-mode "\C-c\C-c" 'vc-finish-logentry)
  )

;;; These things should probably be generally available

(defun vc-file-tree-walk (func &rest args)
  "Walk recursively through default directory.
Invoke FUNC f ARGS on each non-directory file f underneath it."
  (vc-file-tree-walk-internal default-directory func args)
  (message "Traversing directory %s...done" default-directory))

(defun vc-file-tree-walk-internal (file func args)
  (if (not (file-directory-p file))
      (apply func file args)
    (message "Traversing directory %s..." file)
    (let ((dir (file-name-as-directory file)))
      (mapcar
       (function
        (lambda (f) (or
                     (string-equal f ".")
                     (string-equal f "..")
                     (let ((dirf (concat dir f)))
                        (or
                         (file-symlink-p dirf) ;; Avoid possible loops
                         (vc-file-tree-walk-internal dirf func args))))))
       (directory-files dir)))))

(provide 'vc)

;;; DEVELOPER'S NOTES ON CONCURRENCY PROBLEMS IN THIS CODE
;;;
;;; These may be useful to anyone who has to debug or extend the package.
;;; 
;;; A fundamental problem in VC is that there are time windows between
;;; vc-next-action's computations of the file's version-control state and
;;; the actions that change it.  This is a window open to lossage in a
;;; multi-user environment; someone else could nip in and change the state
;;; of the master during it.
;;; 
;;; The performance problem is that rlog/prs calls are very expensive; we want
;;; to avoid them as much as possible.
;;; 
;;; ANALYSIS:
;;; 
;;; The performance problem, it turns out, simplifies in practice to the
;;; problem of making vc-locking-user fast.  The two other functions that call
;;; prs/rlog will not be so commonly used that the slowdown is a problem; one
;;; makes snapshots, the other deletes the calling user's last change in the
;;; master.
;;; 
;;; The race condition implies that we have to either (a) lock the master
;;; during the entire execution of vc-next-action, or (b) detect and
;;; recover from errors resulting from dispatch on an out-of-date state.
;;; 
;;; Alternative (a) appears to be unfeasible.  The problem is that we can't
;;; guarantee that the lock will ever be removed.  Suppose a user starts a
;;; checkin, the change message buffer pops up, and the user, having wandered
;;; off to do something else, simply forgets about it?
;;; 
;;; Alternative (b), on the other hand, works well with a cheap way to speed up
;;; vc-locking-user.  Usually, if a file is registered, we can read its locked/
;;; unlocked state and its current owner from its permissions.
;;; 
;;; This shortcut will fail if someone has manually changed the workfile's
;;; permissions; also if developers are munging the workfile in several
;;; directories, with symlinks to a master (in this latter case, the
;;; permissions shortcut will fail to detect a lock asserted from another
;;; directory).
;;; 
;;; Note that these cases correspond exactly to the errors which could happen
;;; because of a competing checkin/checkout race in between two instances of
;;; vc-next-action.
;;; 
;;; For VC's purposes, a workfile/master pair may have the following states:
;;; 
;;; A. Unregistered.  There is a workfile, there is no master.
;;; 
;;; B. Registered and not locked by anyone.
;;; 
;;; C. Locked by calling user and unchanged.
;;; 
;;; D. Locked by the calling user and changed.
;;; 
;;; E. Locked by someone other than the calling user.
;;; 
;;; This makes for 25 states and 20 error conditions.  Here's the matrix:
;;; 
;;; VC's idea of state
;;;  |
;;;  V  Actual state   RCS action              SCCS action          Effect
;;;    A  B  C  D  E
;;;  A .  1  2  3  4   ci -u -t-          admin -fb -i<file>      initial admin
;;;  B 5  .  6  7  8   co -l              get -e                  checkout
;;;  C 9  10 .  11 12  co -u              unget; get              revert
;;;  D 13 14 15 .  16  ci -u -m<comment>  delta -y<comment>; get  checkin
;;;  E 17 18 19 20 .   rcs -u -M ; rcs -l unget -n ; get -g       steal lock
;;; 
;;; All commands take the master file name as a last argument (not shown).
;;; 
;;; In the discussion below, a "self-race" is a pathological situation in
;;; which VC operations are being attempted simultaneously by two or more
;;; Emacsen running under the same username.
;;; 
;;; The vc-next-action code has the following windows:
;;; 
;;; Window P:
;;;    Between the check for existence of a master file and the call to
;;; admin/checkin in vc-buffer-admin (apparent state A).  This window may
;;; never close if the initial-comment feature is on.
;;; 
;;; Window Q:
;;;    Between the call to vc-workfile-unchanged-p in and the immediately
;;; following revert (apparent state C).
;;; 
;;; Window R:
;;;    Between the call to vc-workfile-unchanged-p in and the following
;;; checkin (apparent state D).  This window may never close.
;;; 
;;; Window S:
;;;    Between the unlock and the immediately following checkout during a
;;; revert operation (apparent state C).  Included in window Q.
;;; 
;;; Window T:
;;;    Between vc-locking-user and the following checkout (apparent state B).
;;; 
;;; Window U:
;;;    Between vc-locking-user and the following revert (apparent state C).
;;; Includes windows Q and S.
;;; 
;;; Window V:
;;;    Between vc-locking-user and the following checkin (apparent state
;;; D).  This window may never be closed if the user fails to complete the
;;; checkin message.  Includes window R.
;;; 
;;; Window W:
;;;    Between vc-locking-user and the following steal-lock (apparent
;;; state E).  This window may never close if the user fails to complete
;;; the steal-lock message.  Includes window X.
;;; 
;;; Window X:
;;;    Between the unlock and the immediately following re-lock during a
;;; steal-lock operation (apparent state E).  This window may never cloce
;;; if the user fails to complete the steal-lock message.
;;; 
;;; Errors:
;;; 
;;; Apparent state A ---
;;;
;;; 1. File looked unregistered but is actually registered and not locked.
;;; 
;;;    Potential cause: someone else's admin during window P, with
;;; caller's admin happening before their checkout.
;;; 
;;;    RCS: ci will fail with a "no lock set by <user>" message.
;;;    SCCS: admin will fail with error (ad19).
;;; 
;;;    We can let these errors be passed up to the user.
;;; 
;;; 2. File looked unregistered but is actually locked by caller, unchanged.
;;; 
;;;    Potential cause: self-race during window P.
;;; 
;;;    RCS: will revert the file to the last saved version and unlock it.
;;;    SCCS: will fail with error (ad19).
;;; 
;;;    Either of these consequences is acceptable.
;;; 
;;; 3. File looked unregistered but is actually locked by caller, changed.
;;; 
;;;    Potential cause: self-race during window P.
;;; 
;;;    RCS: will register the caller's workfile as a delta with a
;;; null change comment (the -t- switch will be ignored).
;;;    SCCS: will fail with error (ad19).
;;; 
;;; 4. File looked unregistered but is locked by someone else.
;;; 
;;;    Potential cause: someone else's admin during window P, with
;;; caller's admin happening *after* their checkout.
;;; 
;;;    RCS: will fail with a "no lock set by <user>" message.
;;;    SCCS: will fail with error (ad19).
;;; 
;;;    We can let these errors be passed up to the user.
;;; 
;;; Apparent state B ---
;;;
;;; 5. File looked registered and not locked, but is actually unregistered.
;;; 
;;;    Potential cause: master file got nuked during window P.
;;; 
;;;    RCS: will fail with "RCS/<file>: No such file or directory"
;;;    SCCS: will fail with error ut4.
;;; 
;;;    We can let these errors be passed up to the user.
;;; 
;;; 6. File looked registered and not locked, but is actually locked by the
;;; calling user and unchanged.
;;; 
;;;    Potential cause: self-race during window T.
;;; 
;;;    RCS: in the same directory as the previous workfile, co -l will fail
;;; with "co error: writable foo exists; checkout aborted".  In any other
;;; directory, checkout will succeed.
;;;    SCCS: will fail with ge17.
;;; 
;;;    Either of these consequences is acceptable.
;;; 
;;; 7. File looked registered and not locked, but is actually locked by the
;;; calling user and changed.
;;; 
;;;    As case 6.
;;; 
;;; 8. File looked registered and not locked, but is actually locked by another
;;; user.
;;; 
;;;    Potential cause: someone else checks it out during window T.
;;; 
;;;    RCS: co error: revision 1.3 already locked by <user>
;;;    SCCS: fails with ge4 (in directory) or ut7 (outside it).
;;; 
;;;    We can let these errors be passed up to the user.
;;; 
;;; Apparent state C ---
;;;
;;; 9. File looks locked by calling user and unchanged, but is unregistered.
;;; 
;;;    As case 5.
;;; 
;;; 10. File looks locked by calling user and unchanged, but is actually not
;;; locked.
;;; 
;;;    Potential cause: a self-race in window U, or by the revert's
;;; landing during window X of some other user's steal-lock or window S
;;; of another user's revert.
;;; 
;;;    RCS: succeeds, refreshing the file from the identical version in
;;; the master.
;;;    SCCS: fails with error ut4 (p file nonexistent).
;;;
;;;    Either of these consequences is acceptable.
;;; 
;;; 11. File is locked by calling user.  It looks unchanged, but is actually
;;; changed.
;;; 
;;;    Potential cause: the file would have to be touched by a self-race
;;; during window Q.
;;; 
;;;    The revert will succeed, removing whatever changes came with
;;; the touch.  It is theoretically possible that work could be lost.
;;; 
;;; 12. File looks like it's locked by the calling user and unchanged, but
;;; it's actually locked by someone else.
;;; 
;;;    Potential cause: a steal-lock in window V.
;;; 
;;;    RCS: co error: revision <rev> locked by <user>; use co -r or rcs -u
;;;    SCCS: fails with error un2
;;; 
;;;    We can pass these errors up to the user.
;;; 
;;; Apparent state D ---
;;;
;;; 13. File looks like it's locked by the calling user and changed, but it's
;;; actually unregistered.
;;; 
;;;    Potential cause: master file got nuked during window P.
;;; 
;;;    RCS: Checks in the user's version as an initial delta.
;;;    SCCS: will fail with error ut4.
;;;
;;;    This case is kind of nasty.  It means VC may fail to detect the
;;; loss of previous version information.
;;; 
;;; 14. File looks like it's locked by the calling user and changed, but it's
;;; actually unlocked.
;;; 
;;;    Potential cause: self-race in window V, or the checkin happening
;;; during the window X of someone else's steal-lock or window S of
;;; someone else's revert.
;;; 
;;;    RCS: ci will fail with "no lock set by <user>".
;;;    SCCS: delta will fail with error ut4.
;;; 
;;; 15. File looks like it's locked by the calling user and changed, but it's
;;; actually locked by the calling user and unchanged.
;;; 
;;;    Potential cause: another self-race --- a whole checkin/checkout
;;; sequence by the calling user would have to land in window R.
;;; 
;;;    SCCS: checks in a redundant delta and leaves the file unlocked as usual.
;;;    RCS: reverts to the file state as of the second user's checkin, leaving
;;; the file unlocked.
;;;
;;;    It is theoretically possible that work could be lost under RCS.
;;; 
;;; 16. File looks like it's locked by the calling user and changed, but it's
;;; actually locked by a different user.
;;; 
;;;    RCS: ci error: no lock set by <user>
;;;    SCCS: unget will fail with error un2
;;; 
;;;    We can pass these errors up to the user.
;;; 
;;; Apparent state E ---
;;;
;;; 17. File looks like it's locked by some other user, but it's actually
;;; unregistered.
;;; 
;;;    As case 13.
;;; 
;;; 18. File looks like it's locked by some other user, but it's actually
;;; unlocked.
;;; 
;;;    Potential cause: someone released a lock during window W.
;;; 
;;;    RCS: The calling user will get the lock on the file.
;;;    SCCS: unget -n will fail with cm4.
;;; 
;;;    Either of these consequences will be OK.
;;; 
;;; 19. File looks like it's locked by some other user, but it's actually
;;; locked by the calling user and unchanged.
;;; 
;;;    Potential cause: the other user relinquishing a lock followed by
;;; a self-race, both in window W.
;;; 
;;;     Under both RCS and SCCS, both unlock and lock will succeed, making
;;; the sequence a no-op.
;;; 
;;; 20. File looks like it's locked by some other user, but it's actually
;;; locked by the calling user and changed.
;;; 
;;;     As case 19.
;;; 
;;; PROBLEM CASES:
;;; 
;;;    In order of decreasing severity:
;;; 
;;;    Cases 11 and 15 under RCS are the only one that potentially lose work.
;;; They would require a self-race for this to happen.
;;; 
;;;    Case 13 in RCS loses information about previous deltas, retaining
;;; only the information in the current workfile.  This can only happen
;;; if the master file gets nuked in window P.
;;; 
;;;    Case 3 in RCS and case 15 under SCCS insert a redundant delta with
;;; no change comment in the master.  This would require a self-race in
;;; window P or R respectively.
;;; 
;;;    Cases 2, 10, 19 and 20 do extra work, but make no changes.
;;; 
;;;    Unfortunately, it appears to me that no recovery is possible in these
;;; cases.  They don't yield error messages, so there's no way to tell that
;;; a race condition has occurred.
;;; 
;;;    All other cases don't change either the workfile or the master, and
;;; trigger command errors which the user will see.
;;; 
;;;    Thus, there is no explicit recovery code.

;;; vc.el ends here