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

   1 ;;; -*-lisp-*-
   2 ;;;
   3 ;;; Safely modify collections of files
   4 ;;;
   5 ;;; (c) 2005 Straylight/Edgeware
   6 ;;;
   7
   8 ;;;----- Licensing notice ---------------------------------------------------
   9 ;;;
  10 ;;; This program is free software; you can redistribute it and/or modify
  11 ;;; it under the terms of the GNU General Public License as published by
  12 ;;; the Free Software Foundation; either version 2 of the License, or
  13 ;;; (at your option) any later version.
  14 ;;;
  15 ;;; This program is distributed in the hope that it will be useful,
  16 ;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
  17 ;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  18 ;;; GNU General Public License for more details.
  19 ;;;
  20 ;;; You should have received a copy of the GNU General Public License
  21 ;;; along with this program; if not, write to the Free Software Foundation,
  22 ;;; Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
  23
  24 (defpackage #:safely
  25   (:use #:common-lisp #:mdw.base))
  26 (in-package #:safely)
  27
  28 #+(or cmu sbcl)
  29 (eval-when (:compile-toplevel :execute)
  30   (import #+cmu '(ext:unix-namestring unix:unix-link)
  31           #+sbcl '(sb-ext:native-namestring)))
  32 #+cmu
  33 (progn
  34   (declaim (inline unix-namestring))
  35   (defun native-namestring (pathname &key as-file)
  36     (declare (ignore as-file))
  37     (unix-namestring pathname nil)))
  38
  39 (export '(safely safelyp make-safely))
  40 (defstruct (safely (:predicate safelyp) (:constructor make-safely ()))
  41   "Stores information about how to commit or undo safe writes."
  42   (streams nil)
  43   (trail nil))
  44
  45 (export 'safely-close)
  46 (defun safely-close (safe stream)
  47   "Make sure that STREAM is closed when SAFE is finished."
  48   (push stream (safely-streams safe)))
  49
  50 (export 'safely-delete-file)
  51 (defun safely-delete-file (safe file)
  52   "Delete FILE when SAFE is committed."
  53   (push `(:delete ,file) (safely-trail safe)))
  54
  55 (defun generate-fresh-file-name (base tag &optional func)
  56   "Return a fresh file name constructed from BASE (a filespec) and TAG (some
  57    short descriptive string).  The generated name has the same directory and
  58    type as the BASE name, but a different name.
  59
  60    If FUNC is non-nil, then it is a function to call on the generated file
  61    name: generate-fresh-file-name runs in a loop, calling FUNC with generated
  62    file names until FUNC returns non-nil, at which point generate-fresh-
  63    file-name returns two values: the generated name, and the result of FUNC.
  64    generate-fresh-file-name catches errors of type file-error from FUNC, and
  65    just tries again with a new name.
  66
  67    If FUNC is nil, it's treated the same as a function which always returns
  68    t.
  69
  70    This is inspired by a similar facility in scsh."
  71   (let ((base (pathname base)))
  72     (dotimes (i 256
  73               (error "Gave up trying to find a temporary ~A file for ~S."
  74                      tag base))
  75       (let* ((new (merge-pathnames
  76                    (make-pathname
  77                     :name (format nil "~A-~A-~X"
  78                                   (pathname-name base)
  79                                   tag
  80                                   (random most-positive-fixnum)))
  81                    base))
  82              (ret (and (not (probe-file new))
  83                        (if func
  84                            (handler-case (funcall func new)
  85                              (file-error (cond)
  86                                (unless (pathname-match-p
  87                                         (file-error-pathname cond)
  88                                         new)
  89                                  (error cond))
  90                                nil))
  91                            t))))
  92         (when ret
  93           (return (values new ret)))))))
  94
  95 (export 'safely-open-output-stream)
  96 (defun safely-open-output-stream (safe file &rest open-args)
  97   "Create an output stream which will be named FILE when SAFE is committed.
  98    Other OPEN-ARGS are passed to open."
  99   (multiple-value-bind
 100       (name stream)
 101       (generate-fresh-file-name file "new"
 102                                 (lambda (name)
 103                                   (apply #'open name
 104                                          :direction :output
 105                                          :if-exists nil
 106                                          open-args)))
 107     (safely-close safe stream)
 108     (push `(:shunt ,name ,file)
 109           (safely-trail safe))
 110     stream))
 111
 112 #+clisp
 113 (progn
 114   (ffi:def-call-out %rename (:name "rename")
 115     (:language :stdc)
 116     (:library "libc.so.6")
 117     (:arguments (from ffi:c-string)
 118                 (to ffi:c-string))
 119     (:return-type ffi:int)))
 120
 121 (declaim (inline rename))
 122 (defun rename (old new)
 123   #-clisp
 124   (let ((target (make-pathname :directory '(:relative)
 125                                :defaults new)))
 126     (rename-file old target))
 127
 128   #+clisp
 129   (let ((rc (%rename (namestring old) (namestring new))))
 130     (when (= rc -1)
 131       (error "Failed to rename ~S to ~S: ~A" old new (posix:strerror)))))
 132
 133 (defun delete-file-without-moaning (file)
 134   "Delete the FILE, ignoring errors."
 135   (handler-case (delete-file file)
 136     (file-error () nil)))
 137
 138 (defun rename-file-without-moaning (old new)
 139   "Rename OLD to NEW, ignoring errors, and without doing any stupid name
 140    mangling."
 141   (handler-case (rename old new)
 142     (file-error () nil)))
 143
 144 (defun safely-unwind (trail)
 145   "Roll back the TRAIL of operations."
 146   (dolist (job trail)
 147     (ecase (car job)
 148       (:shunt (destructuring-bind (new file) (cdr job)
 149                 (declare (ignore file))
 150                 (delete-file-without-moaning new)))
 151       (:delete)
 152       (:rmtmp (destructuring-bind (file) (cdr job)
 153                 (delete-file-without-moaning file)))
 154       (:revert (destructuring-bind (old new) (cdr job)
 155                  (rename-file-without-moaning old new))))))
 156
 157 (defun safely-reset (safe)
 158   "Reset SAFE to its initial state."
 159   (setf (safely-streams safe) nil)
 160   (setf (safely-trail safe) nil))
 161
 162 (export 'safely-bail)
 163 (defun safely-bail (safe)
 164   "Abort the operations in SAFE, unwinding all the things that have been
 165    done.  Streams are closed, new files are removed."
 166   (dolist (stream (safely-streams safe))
 167     (close stream :abort t))
 168   (safely-unwind (safely-trail safe))
 169   (safely-reset safe))
 170
 171 #+sbcl
 172 (defun unix-link (from to)
 173   (sb-unix::int-syscall ("link" sb-alien:c-string sb-alien:c-string)
 174                         from to))
 175
 176 (defun safe-copy (file tag)
 177   "Make a copy of the FILE.  Return the new name."
 178
 179   #+(or cmu sbcl)
 180   ;; Use link(2) where available.
 181   (generate-fresh-file-name file tag
 182                             (lambda (name)
 183                               (let ((from (native-namestring file
 184                                                              :as-file t))
 185                                     (to (native-namestring name
 186                                                            :as-file t)))
 187                                 (and from to
 188                                      (unix-link from to)))))
 189
 190   #+clisp
 191   (generate-fresh-file-name file tag
 192                             (lambda (name)
 193                               (posix:copy-file (namestring file)
 194                                                (namestring name)
 195                                                :method :hardlink
 196                                                :if-exists nil)))
 197
 198   #-(or cmu sbcl clisp)
 199   ;; Otherwise just copy the file contents and hope for the best.
 200   (with-open-file (input file :element-type :default)
 201     (multiple-value-bind
 202         (copy output)
 203         (generate-fresh-file-name file tag
 204                                   (lambda (name)
 205                                     (open name
 206                                           :direction :output
 207                                           :if-exists nil
 208                                           :element-type :default)))
 209       (unwind-protect
 210            (progn
 211              (let ((buffer (make-array 8192
 212                                        :element-type (stream-element-type
 213                                                       input))))
 214                (loop
 215                  (let ((read (read-sequence buffer input)))
 216                    (when (plusp read)
 217                      (write-sequence buffer output :end read))
 218                    (when (< read (length buffer))
 219                      (return copy))))))
 220         (close output)))))
 221
 222 (export 'safely-commit)
 223 (defun safely-commit (safe)
 224   "Commit SAFE.  The files deleted by safely-delete-file are deleted; the
 225    files created by safely-open-output-stream are renamed over the old
 226    versions, if any.  If a problem occurs during this stage, everything is
 227    rewound and no changes are made."
 228   (let ((trail (safely-trail safe))
 229         (revert nil)
 230         (cleanup nil))
 231     (unwind-protect
 232         (progn
 233           (dolist (stream (safely-streams safe))
 234             (close stream))
 235           (loop
 236             (unless trail
 237               (return))
 238             (let ((job (pop trail)))
 239               (ecase (car job)
 240                 (:shunt (destructuring-bind (tag new file) job
 241                           (declare (ignore tag))
 242                           (push `(:rmtmp ,new) revert)
 243                           (if (probe-file file)
 244                               (let ((old (safe-copy file "old")))
 245                                 (push `(:rmtmp ,old) cleanup)
 246                                 (push `(:revert ,old ,file) revert))
 247                               (push `(:rmtmp ,file) revert))
 248                           (rename new file)))
 249                 (:delete (destructuring-bind (tag file) job
 250                            (declare (ignore tag))
 251                            (let ((old (safe-copy file "delete")))
 252                              (push `(:revert ,old ,file) revert)
 253                              (push `(:rmtmp ,old) cleanup)
 254                              (delete-file file)))))))
 255           (setf revert nil))
 256       (safely-unwind trail)
 257       (safely-unwind revert)
 258       (safely-unwind cleanup)
 259       (safely-reset safe))))
 260
 261 ;; The symbol `safely' is already exported.
 262 (defmacro safely ((safe &key) &body body)
 263   "Do stuff within the BODY safely.  If BODY completes without errors, the
 264    SAFE is committed; otherwise it's bailed."
 265   `(let ((,safe (make-safely)))
 266      (unwind-protect
 267          (progn
 268            ,@body
 269            (safely-commit ,safe)
 270            (setf ,safe nil))
 271        (when ,safe
 272          (safely-bail ,safe)))))
 273
 274 (export 'safely-writing)
 275 (defmacro safely-writing ((stream file &rest open-args) &body body)
 276   "Simple macro for writing a single file safely.  STREAM is opened onto a
 277    temporary file, and if BODY completes, it is renamed to FILE."
 278   (with-gensyms safe
 279     `(safely (,safe)
 280        (let ((,stream (safely-open-output-stream ,safe ,file ,@open-args)))
 281          ,@body))))
 282
 283 ;;;----- That's all, folks --------------------------------------------------