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

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

;; Author: Eric S. Raymond <esr@snark.thyrsus.com>
;; Version: 4.0

;;	$Id: vc.el,v 1.14 1992/10/26 05:33:20 rms Exp rms $	

;; 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 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.
;;
;; This code depends on call-process passing back the subprocess exit
;; status.  Thus, you need Emacs 18.58 or later to run it.
;;
;; 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)

;; 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-diff-options '("-a" "-c1")
  "*The command/flags list to be used in constructing diff commands.")
(defvar vc-suppress-confirm nil
  "*If non-nil, reat 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].")

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

;; Header-insertion hair

(defvar vc-header-alist
  '((SCCS "\%W\%") (RCS "\$Id\$"))
  "*Header keywords to be inserted when vc-insert-header is executed.")
(defconst 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
verson-control type in vc-header-strings.")
(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-log-file)
(defvar vc-log-version)

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

(defmacro vc-error-occurred (&rest body)
  (list 'condition-case nil (cons 'progn (append body '(nil))) '(error t)))

;; File property caching

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

;; Random helper functions

(defun vc-name (file)
  "Return the master name of a file, nil if it is not registered"
  (or (vc-file-getprop file 'vc-name)
      (vc-file-setprop file 'vc-name
		       (let ((name-and-type (vc-registered file)))
			 (and name-and-type (car name-and-type))))))

(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))
      (let ((full nil))
	(catch 'found
	  (mapcar
	   (function (lambda (s)
	      (if (and s (file-exists-p (setq full (concat s "/" name))))
		  (throw 'found nil))))
	  exec-path))
	(if full
	    (setq vc-binary-assoc (cons (cons name full) vc-binary-assoc)))
	full)))

(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 (format "Running %s on %s..." command file)))
  (let ((obuf (current-buffer))
	(squeezed nil)
	(vc-file (and file (vc-name file)))
	status)
    (set-buffer (get-buffer-create "*vc*"))
    (erase-buffer)
    (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 "./"))))
      (setq status (apply 'call-process command nil t nil squeezed))
      )
    (goto-char (point-max))
    (previous-line 1)
    (if (or (not (integerp status)) (< okstatus status))
	(progn
	  (previous-line 1)
	  (print (cons command squeezed))
	  (next-line 1)
	  (pop-to-buffer "*vc*")
	  (vc-shrink-to-fit)
	  (goto-char (point-min))
	  (error (format "Running %s...FAILED (%s)" command
			 (if (integerp status)
			     (format "status %d" status)
			   status)))
	  )
      (if vc-command-messages
	  (message (format "Running %s...OK" command)))
      )
    (set-buffer obuf)
    status)
  )

(defun vc-revert-buffer1 (&optional arg no-confirm)
  ;; This code was shamelessly lifted from Sebastian Kremer's rcs.el mode.
  ;; Revert buffer, try to keep point where user expects it 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* ((opoint (point))
	 (osize (buffer-size))
	 diff
	 (context 100)
	 (ostring (buffer-substring (point)
				    (min (point-max)
					 (+ (point) context))))
	 (l (length ostring)))
    (revert-buffer arg no-confirm)
    (setq diff (- osize (buffer-size)))
    (if (< diff 0) (setq diff (- diff)))
    (goto-char opoint)
    (cond ((equal "" ostring)
	   (goto-char (point-max)))
	  ((or (search-forward ostring nil t)
	       ;; Can't use search-backward since the match may continue
	       ;; after point.
	       (progn (goto-char (- (point) diff l))
		      ;; goto-char doesn't signal an error at
		      ;; beginning of buffer like backward-char would
		      (search-forward ostring nil t)))
	   ;; to beginning of OSTRING
	   (backward-char l)))))

(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
	 ))
     )))

;; 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 writeable, locked copy for editing.
   If the file is registered and not locked by anyone, this checks out
a writeable 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 creation 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."
  (interactive "P")
  (if buffer-file-name
      (let
	  (do-update owner version
		     (file buffer-file-name)
		     (vc-file (vc-name buffer-file-name))
		     (err-msg nil)
		     owner)

	(cond

	 ;; if there is no master file corresponding, create one
	 ((not vc-file)
	  (vc-register verbose)
	  (vc-next-action verbose))

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

	 ;; a checked-out version exists, but the user may not own the lock
	 ((not (string-equal owner (user-login-name)))
	  (vc-steal-lock
	   file
	   (and verbose (read-string "Version to steal: "))
	   owner))

	 ;; OK, user owns the lock on the file
	 (t (progn

	      ;; 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)
		    (vc-resynch-window file t))

		;; 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))))))
    (error "There is no file associated with buffer %s" (buffer-name))))

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

;;;###autoload
(defun vc-register (&optional override)
  "Register the current file into your version-control system."
  (interactive "P")
  (if (vc-name buffer-file-name)
      (error "This file is already registered."))
  (vc-buffer-sync)
  (vc-admin
   buffer-file-name
   (and override (read-string "Initial version level: ")))
  )

(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 nil noquery)
	     (vc-mode-line buffer-file-name))
	 (progn
	   (delete-window)
	   (kill-buffer (current-buffer))))))


(defun vc-admin (file rev)
  "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."
  (if vc-initial-comment
      (progn
	(pop-to-buffer (get-buffer-create "*VC-log*"))
	(vc-log-mode)
	(narrow-to-region (point-max) (point-max))
	(vc-mode-line file (file-name-nondirectory file))
	(setq vc-log-operation 'vc-backend-admin)
	(setq vc-log-file file)
	(setq vc-log-version rev)
	(message "Enter initial comment.  Type C-c C-c when done."))
    (progn
      (vc-backend-admin file rev)
      ;; Inhibit query here, since otherwise we always get asked.
      (vc-resynch-window file vc-keep-workfiles 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."))
  (require 'sendmail)
  (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-checkout (file &optional writeable)
  "Retrieve a copy of the latest version of the given file."
  (vc-backend-checkout file writeable)
  (if (string-equal file buffer-file-name)
      (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."
  (pop-to-buffer (get-buffer-create "*VC-log*"))
  (vc-log-mode)
  (narrow-to-region (point-max) (point-max))
  (vc-mode-line file (file-name-nondirectory file))
  (setq vc-log-operation 'vc-backend-checkin
	vc-log-file file
	vc-log-version rev
	vc-log-after-operation-hook 'vc-checkin-hook)
  (message "Enter log message.  Type C-c C-c when done.")
  (if comment
      (progn
	(insert comment)
	(vc-finish-logentry))))

(defun vc-finish-logentry ()
  "Complete the operation implied by the current log entry."
  (interactive)
  (goto-char (point-max))
  (if (not (bolp)) (newline))
  ;; delimit current page
  (save-excursion
    (widen)
    (goto-char (point-max))
    (if (and (not (bobp)) (not (= (char-after (1- (point))) ?\f)))
	(insert-char ?\f 1)))
  (if (not (bobp))
      (forward-char -1))
  (mark-page)
  ;; Check for errors
  (vc-backend-logentry-check vc-log-file)
  ;; OK, do it to it
  (if vc-log-operation
      (funcall vc-log-operation 
	       vc-log-file
	       vc-log-version
	       (buffer-substring (region-beginning) (1- (region-end))))
    (error "No log operation is pending."))
  ;; Return to "parent" buffer of this checkin and remove checkin window
  (pop-to-buffer (get-file-buffer vc-log-file))
  (delete-window (get-buffer-window "*VC-log*"))
  (bury-buffer "*VC-log*")
  ;; Now make sure we see the expanded headers
  (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-next-comment ()
  "Fill the log buffer with the next message in the msg ring."
  (interactive)
  (widen)
  (forward-page)
  (if (= (point) (point-max))
      (goto-char (point-min)))
  (mark-page)
  (narrow-to-page))

(defun vc-previous-comment ()
  "Fill the log buffer with the previous message in the msg ring."
  (interactive)
  (widen)
  (if (= (point) (point-min))
      (goto-char (point-max)))
  (backward-page)
  (mark-page)
  (narrow-to-page))

(defun vc-comment-search-backward (regexp)
  "Fill the log buffer with the last message in the msg ring matching REGEXP."
  (interactive "sSearch backward for: ")
  (widen)
  (if (= (point) (point-min))
      (goto-char (point-max)))
  (re-search-backward regexp nil t)
  (mark-page)
  (narrow-to-page))

(defun vc-comment-search-forward (regexp)
  "Fill the log buffer with the next message in the msg ring matching REGEXP."
  (interactive "sSearch forward for: ")
  (widen)
  (if (= (point) (point-min))
      (goto-char (point-max)))
  (re-search-forward regexp nil t)
  (mark-page)
  (narrow-to-page))

;; Additional entry points for examining version histories

;;;###autoload
(defun vc-diff (historic)
  "Display diffs between file versions."
  (interactive "P")
  (if historic
      (call-interactively 'vc-version-diff)
    (let ((old
	   (and
	    current-prefix-arg
	    (read-string "Version to compare against: ")))
	  (file buffer-file-name)
	  unchanged)
      (vc-buffer-sync)
      (setq unchanged (vc-workfile-unchanged-p buffer-file-name))
      (if unchanged
	  (message (format "No changes to %s since latest version." file))
	(pop-to-buffer "*vc*")
	(vc-backend-diff file nil)
	(goto-char (point-min))
	)
      (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: \nsOlder version: \nsNewer version: ")
  (if (string-equal rel1 "") (setq rel1 nil))
  (if (string-equal rel2 "") (setq rel2 nil))
  (if (file-directory-p file)
      (progn
	(set-buffer (get-buffer-create "*vc-status*"))
	(erase-buffer)
	(insert "Diffs between " rel1 " and " rel2 ":\n\n")
	(set-buffer (get-buffer-create "*vc*"))
	(vc-file-tree-walk
	 (function (lambda (f)
		     (and
		      (not (file-directory-p f))
		      (vc-name f)
		      (vc-backend-diff f rel1 rel2))
		     (append-to-buffer "*vc-status*" (point-min) (point-max))
		     ))
	 default-directory)
	(pop-to-buffer "*vc-status*")
	(insert "\nEnd of diffs.\n")
	(goto-char (point-min))
	(set-buffer-modified-p nil)
	)
    (progn
      (vc-backend-diff file rel1 rel2)
      (goto-char (point-min))
      (if (equal (point-min) (point-max))
	  (message (format "No changes to %s between %s and %s." file rel1 rel2))
	(pop-to-buffer "*vc*")
	(goto-char (point-min))
	)
      )
    )
  )

;; 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-strings"
  (interactive)
  (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))
	      )
	    )))))

;; Status-checking functions

;;;###autoload
(defun vc-directory (verbose)
  "Show version-control status of all files under the current directory."
  (interactive "P")
  (let ((dir (substring default-directory 0 (1- (length default-directory))))
	nonempty)
    (save-excursion
      (set-buffer (get-buffer-create "*vc-status*"))
      (erase-buffer)
      (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)))))))
       dir)
      (setq nonempty (not (zerop (buffer-size)))))
    (if nonempty
	(progn
	  (pop-to-buffer "*vc-status*" t)
	  (vc-shrink-to-fit)
	  (goto-char (point-min)))
      (message "No files are currently registered under %s" dir))
    ))

;; 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)
  (or
   name
   (let ((firstchar (aref name 0)))
     (and (>= firstchar ?0) (<= firstchar ?9) name))
   (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-quiescent-p ()
  ;; Is the current directory ready to be snapshot?
  (let ((dir (substring default-directory 0 (1- (length default-directory)))))
    (catch 'quiet
      (vc-file-tree-walk
       (function (lambda (f)
		   (if (and (vc-registered f) (vc-locking-user f))
		       (throw 'quiet nil))))
       dir)
      t)))

;;;###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: ")
  (if (not (vc-quiescent-p))
      (error "Can't make a snapshot, locked files are in the way.")
    (vc-file-tree-walk
     (function (lambda (f) (and
		   (not (file-directory-p f))
		   (vc-name f)
		   (vc-backend-assign-name f name))))
     default-directory)
    ))

;;;###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: ")
  (if (not (vc-quiescent-p))
      (error "Can't retrieve a snapshot, locked files are in the way.")
    (vc-file-tree-walk
     (function (lambda (f) (and
		   (not (file-directory-p f))
		   (vc-name f)
		   (vc-error-occurred (vc-backend-checkout f nil name)))))
     default-directory)
    ))

;; Miscellaneous other entry points

;;;###autoload
(defun vc-print-log ()
  "List the change log of the current buffer in a window."
  (interactive)
  (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*"))
	(goto-char (point-min))
	)
    (error "There is no version-control master associated with this buffer")
    )
  )

;;;###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)
  (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)
  "Undo your latest checkin."
  (interactive "P")
  (let ((target (vc-your-latest-version (buffer-file-name))))
    (if (null target)
	(error "You didn't check in the last change."))
    (and (y-or-n-p (format "Remove version %s from master? " target))
      (vc-backend-uncheck (buffer-file-name) target)))
    (if norevert
	(vc-mode-line (buffer-file-name))
	(vc-checkout (buffer-file-name) nil))
  )

(defun vc-rename-file (old new)
  "Rename a file, taking its master files with it."
  (interactive "fOld name: \nFNew name: ")
  (let ((oldbuf (get-file-buffer old)))
    (if (buffer-modified-p oldbuf)
	(error "Please save files before moving them."))
    (if (get-file-buffer new)
	(error "Already editing new file name."))
    (let ((oldmaster (vc-name old)))
      (if oldmaster
	(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 (vc-name new)))
      (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.
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-relative-name file) files)))
	      (setq buffers (cdr buffers)))
	    files))))
  (find-file-other-window "ChangeLog")
  (vc-buffer-sync)
  (undo-boundary)
  (goto-char (point-min))
  (message "Computing change log entries...")
  (shell-command (mapconcat 'identity (cons "rcs2log" args) " ") t)
  (message "Computing change log entries... done"))

;; Functions for querying the master and lock files.

(defun 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 (match-substring 2)))
		       (if (string< latest-date date)
			   (progn
			     (setq latest-date date)
			     (setq latest-val
				   (match-substring 1))))))
		   latest-val))
	     (prog1
		 (and (re-search-forward p nil t)
		      (let ((value (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."
  (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 twiddled 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 (in vc-hooks.el) and fill it in in each call.

(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.]+\\).*locked by: "
		  (regexp-quote (user-login-name))
		  ";\ndate: \\([ /0-9:]+\\);"))
		'(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 writeable rev)
  ;; Retrieve a copy of a saved version into a workfile
  (message "Checking out %s..." file)
  (vc-backend-dispatch file
   (progn
     (vc-do-command 0 "get" file	;; SCCS
		    (if writeable "-e")
		    (and rev (concat "-r" (vc-lookup-triple file rev))))
     )
   (vc-do-command 0 "co" file	;; RCS
		  (if writeable "-l")
		  (and rev (concat "-r" rev)))
   )
  (vc-file-setprop file 'vc-checkout-time (nth 5 (file-attributes file)))
  (message "Checking out %s...done" file)
  )

(defun vc-backend-logentry-check (file)
  (vc-backend-dispatch file
   (if (>= (- (region-end) (region-beginning)) 512)	;; SCCS
       (progn
	 (goto-char 512)
	 (error
	  "Log must be less than 512 characters.  Point is now at char 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))
   (progn
     (delete-file file)		;; RCS
     (vc-do-command 0 "co" file "-u")))
  (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)
  (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)))
    )
  (progn
    (vc-do-command 0 "rcs" "-M" (concat "-u" rev) file)
    (delete-file file)
    (vc-do-command 0 "rcs" (concat "-l" rev) file)
    )
  (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
  (apply 'vc-do-command 1
	 (or (vc-backend-dispatch file "vcdiff" "rcsdiff")
	     (error (format "File %s is not under version control." file)))
	 file
	 (and oldvers (concat "-r" oldvers))
	 (and newvers (concat "-r" newvers))
	 vc-diff-options
  ))

(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-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-search-comment-reverse]	search backward for regexp in the comment ring
\\[vc-search-comment-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-diff-options         A list consisting of the flags
				to be used for generating context diffs.

	vc-header-strings	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)
  (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-backward)
  (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-shrink-to-fit ()
  "Shrink a window vertically until it's just large enough to contain its text"
  (let ((minsize (1+ (count-lines (point-min) (point-max)))))
    (if (< minsize (window-height))
	(let ((window-min-height 2))
	  (shrink-window (- (window-height) minsize))))))

(defun vc-file-tree-walk (func dir &rest args)
  "Apply a given function to dir and all files underneath it, recursively."
  (apply 'funcall func dir args)
  (and (file-directory-p dir)
       (mapcar
	(function (lambda (f) (or
		      (string-equal f ".")
		      (string-equal f "..")
		      (file-symlink-p f)	;; Avoid possible loops
		      (apply 'vc-file-tree-walk
			     func
			     (if (= (aref dir (1- (length dir))) ?/)
				 (concat dir f)
			       (concat dir "/" f))
			     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 cloce 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