mdw@git.distorted.org.uk Git - lisp/blob - safely.lisp

   1 ;;; -*-lisp-*-
   2 ;;;
   3 ;;; $Id$
   4 ;;;
   5 ;;; Safely modify collections of files
   6 ;;;
   7 ;;; (c) 2005 Straylight/Edgeware
   8 ;;;
   9
  10 ;;;----- Licensing notice ---------------------------------------------------
  11 ;;;
  12 ;;; This program is free software; you can redistribute it and/or modify
  13 ;;; it under the terms of the GNU General Public License as published by
  14 ;;; the Free Software Foundation; either version 2 of the License, or
  15 ;;; (at your option) any later version.
  16 ;;;
  17 ;;; This program is distributed in the hope that it will be useful,
  18 ;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
  19 ;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  20 ;;; GNU General Public License for more details.
  21 ;;;
  22 ;;; You should have received a copy of the GNU General Public License
  23 ;;; along with this program; if not, write to the Free Software Foundation,
  24 ;;; Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
  25
  26 (defpackage #:safely
  27   (:use #:common-lisp #:mdw.base)
  28   (:export #:safely #:safely-close #:safely-delete-file
  29            #:safely-open-output-stream #:safely-bail #:safely-commit
  30            #:safely-writing))
  31 (in-package #:safely)
  32
  33 #+(or cmu sbcl)
  34 (eval-when (:compile-toplevel :execute)
  35   (import #+cmu '(ext:unix-namestring unix:unix-link)
  36           #+sbcl '(sb-int:unix-namestring)))
  37
  38 (defstruct (safely (:predicate safelyp))
  39   "Stores information about how to commit or undo safe writes."
  40   (streams nil)
  41   (trail nil))
  42
  43 (defun safely-close (safe stream)
  44   "Make sure that STREAM is closed when SAFE is finished."
  45   (push stream (safely-streams safe)))
  46
  47 (defun safely-delete-file (safe file)
  48   "Delete FILE when SAFE is committed."
  49   (push `(:delete ,file) (safely-trail safe)))
  50
  51 (defun generate-fresh-file-name (base tag &optional func)
  52   "Return a fresh file name constructed from BASE (a filespec) and TAG (some
  53    short descriptive string).  The generated name has the same directory and
  54    type as the BASE name, but a different name.
  55
  56    If FUNC is non-nil, then it is a function to call on the generated file
  57    name: generate-fresh-file-name runs in a loop, calling FUNC with generated
  58    file names until FUNC returns non-nil, at which point generate-fresh-
  59    file-name returns two values: the generated name, and the result of FUNC.
  60    generate-fresh-file-name catches errors of type file-error from FUNC, and
  61    just tries again with a new name.
  62
  63    If FUNC is nil, it's treated the same as a function which always returns
  64    t.
  65
  66    This is inspired by a similar facility in scsh."
  67   (let ((base (pathname base)))
  68     (dotimes (i 256
  69               (error "Gave up trying to find a temporary ~A file for ~S."
  70                      tag base))
  71       (let* ((new (merge-pathnames
  72                    (make-pathname
  73                     :name (format nil "~A-~A-~X"
  74                                   (pathname-name base)
  75                                   tag
  76                                   (random most-positive-fixnum)))
  77                    base))
  78              (ret (and (not (probe-file new))
  79                        (if func
  80                            (handler-case (funcall func new)
  81                              (file-error (cond)
  82                                (unless (pathname-match-p
  83                                         (file-error-pathname cond)
  84                                         new)
  85                                  (error cond))
  86                                nil))
  87                            t))))
  88         (when ret
  89           (return (values new ret)))))))
  90
  91 (defun safely-open-output-stream (safe file &rest open-args)
  92   "Create an output stream which will be named FILE when SAFE is committed.
  93    Other OPEN-ARGS are passed to open."
  94   (multiple-value-bind
  95       (name stream)
  96       (generate-fresh-file-name file "new"
  97                                 (lambda (name)
  98                                   (apply #'open name
  99                                          :direction :output
 100                                          :if-exists nil
 101                                          open-args)))
 102     (safely-close safe stream)
 103     (push `(:shunt ,name ,file)
 104           (safely-trail safe))
 105     stream))
 106
 107 (defun delete-file-without-moaning (file)
 108   "Delete the FILE, ignoring errors."
 109   (handler-case (delete-file file)
 110     (file-error () nil)))
 111
 112 (defun rename-file-without-moaning (old new)
 113   "Rename OLD to NEW, ignoring errors, and without doing any stupid name
 114    mangling."
 115   (handler-case (rename-file old new)
 116     (file-error () nil)))
 117
 118 (defun safely-unwind (trail)
 119   "Roll back the TRAIL of operations."
 120   (dolist (job trail)
 121     (ecase (car job)
 122       (:shunt (destructuring-bind (new file) (cdr job)
 123                 (declare (ignore file))
 124                 (delete-file-without-moaning new)))
 125       (:delete)
 126       (:rmtmp (destructuring-bind (file) (cdr job)
 127                 (delete-file-without-moaning file)))
 128       (:revert (destructuring-bind (old new) (cdr job)
 129                  (rename-file-without-moaning old new))))))
 130
 131 (defun safely-reset (safe)
 132   "Reset SAFE to its initial state."
 133   (setf (safely-streams safe) nil)
 134   (setf (safely-trail safe) nil))
 135
 136 (defun safely-bail (safe)
 137   "Abort the operations in SAFE, unwinding all the things that have been
 138    done.  Streams are closed, new files are removed."
 139   (dolist (stream (safely-streams safe))
 140     (close stream :abort t))
 141   (safely-unwind (safely-trail safe))
 142   (safely-reset safe))
 143
 144 #+sbcl
 145 (defun unix-link (from to)
 146   (sb-unix::int-syscall ("link" sb-alien:c-string sb-alien:c-string)
 147                         from to))
 148
 149 (defun safe-copy (file tag)
 150   "Make a copy of the FILE.  Return the new name."
 151
 152   #+(or cmu sbcl)
 153   ;; Use link(2) where available.
 154   (generate-fresh-file-name file tag
 155                             (lambda (name)
 156                               (let ((from (unix-namestring file t))
 157                                     (to (unix-namestring name nil)))
 158                                 (and from to
 159                                      (unix-link from to)))))
 160
 161   #-(or cmu sbcl)
 162   ;; Otherwise just copy the file contents and hope for the best.
 163   (with-open-file (input file :element-type :default)
 164     (multiple-value-bind
 165         (copy output)
 166         (generate-fresh-file-name file tag
 167                                   (lambda (name)
 168                                     (open name
 169                                           :direction :output
 170                                           :if-exists nil
 171                                           :element-type :default)))
 172       (unwind-protect
 173            (progn
 174              (let ((buffer (make-array 8192
 175                                        :element-type (stream-element-type
 176                                                       input))))
 177                (loop
 178                  (let ((read (read-sequence buffer input)))
 179                    (when (plusp read)
 180                      (write-sequence buffer output :end read))
 181                    (when (< read (length buffer))
 182                      (return copy))))))
 183         (close output)))))
 184
 185 (defun safely-commit (safe)
 186   "Commit SAFE.  The files deleted by safely-delete-file are deleted; the
 187    files created by safely-open-output-stream are renamed over the old
 188    versions, if any.  If a problem occurs during this stage, everything is
 189    rewound and no changes are made."
 190   (let ((trail (safely-trail safe))
 191         (revert nil)
 192         (cleanup nil))
 193     (unwind-protect
 194         (progn
 195           (dolist (stream (safely-streams safe))
 196             (close stream))
 197           (loop
 198             (unless trail
 199               (return))
 200               (let ((job (pop trail)))
 201                 (ecase (car job)
 202                   (:shunt (destructuring-bind (tag new file) job
 203                             (declare (ignore tag))
 204                             (push `(:rmtmp ,new) revert)
 205                             (if (probe-file file)
 206                                 (let ((old (safe-copy file "old")))
 207                                   (push `(:rmtmp ,old) cleanup)
 208                                   (push `(:revert ,old ,file) revert))
 209                                 (push `(:rmtmp ,file) revert))
 210                             (rename-file new file)))
 211                   (:delete (destructuring-bind (tag file) job
 212                              (declare (ignore tag))
 213                              (let ((old (safe-copy file "delete")))
 214                                (push `(:revert ,old ,file) revert)
 215                                (push `(:rmtmp ,old) cleanup)
 216                                (delete-file file)))))))
 217           (setf revert nil))
 218       (safely-unwind trail)
 219       (safely-unwind revert)
 220       (safely-unwind cleanup)
 221       (safely-reset safe))))
 222
 223 (defmacro safely ((safe &key) &body body)
 224   "Do stuff within the BODY safely.  If BODY completes without errors, the
 225    SAFE is committed; otherwise it's bailed."
 226   `(let ((,safe (make-safely)))
 227      (unwind-protect
 228          (progn
 229            ,@body
 230            (safely-commit ,safe)
 231            (setf ,safe nil))
 232        (when ,safe
 233          (safely-bail ,safe)))))
 234
 235 (defmacro safely-writing ((stream file &rest open-args) &body body)
 236   "Simple macro for writing a single file safely.  STREAM is opened onto a
 237    temporary file, and if BODY completes, it is renamed to FILE."
 238   (with-gensyms safe
 239     `(safely (,safe)
 240        (let ((,stream (apply #'safely-open-output-stream
 241                              ,safe ,file ,open-args)))
 242          ,@body))))
 243
 244 ;;;----- That's all, folks --------------------------------------------------