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xbackupfile.c

/* xbackupfile.c -- Backup helping routines
   Copyright (C) 1990-1997, 1998 Free Software Foundation, Inc.

   This program 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.

   This program 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 this program; see the file COPYING.
   If not, write to the Free Software Foundation,
   59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.  */

/* Written by Akim Demaille <demaille@inf.enst.fr> */

#include "xbackupfile.h"

#include <assert.h>

#if HAVE_SYS_STAT_H
# include <sys/stat.h>
#endif

#ifdef STAT_MACROS_BROKEN
# undef S_ISREG
#endif /* STAT_MACROS_BROKEN.  */

#if !defined(S_ISREG) && defined(S_IFREG)
# define S_ISREG(m) (((m) & S_IFMT) == S_IFREG)
#endif

#if HAVE_UNISTD_H
# include <unistd.h>
#endif

#ifdef HAVE_ERRNO_H
# include <errno.h>
#endif
#ifndef errno
extern int errno;
#endif

#if HAVE_STDLIB_H
# define getopt system_getopt
# include <stdlib.h>
# undef getopt
#endif

/* The following test is to work around the gross typo in
   systems like Sony NEWS-OS Release 4.0C, whereby EXIT_FAILURE
   is defined to 0, not 1.  */
#if !EXIT_FAILURE
# undef EXIT_FAILURE
# define EXIT_FAILURE 1
#endif

#ifndef EXIT_SUCCESS
# define EXIT_SUCCESS 0
#endif

#ifdef HAVE_FCNTL_H
# include <fcntl.h>
#else
# include <sys/file.h>
#endif

#if !defined (SEEK_SET)
# define SEEK_SET 0
# define SEEK_CUR 1
# define SEEK_END 2
#endif
#ifndef F_OK
# define F_OK 0
# define X_OK 1
# define W_OK 2
# define R_OK 4
#endif

#include "error.h"
#include "quotearg.h"

#if ENABLE_NLS
# include <libintl.h>
# define _(Text) gettext (Text)
#else
# define _(Text) Text
#endif

#define STREQ(s1, s2)            (strcmp ((s1), (s2)) == 0)


/* Non failing call to find_backup_file_name */
char *
xfind_backup_file_name (const char * file, enum backup_type backup_type)
{
  char * res;

  res = find_backup_file_name (file, backup_type);
  if (!res)
    error (1, 0, _("Memory exhausted"));

  return res;
}

/*
  The following approach has been suggested by Miles Bader and
  Paul Eggert.

From: Paul Eggert <eggert@twinsun.com>
Date: Wed, 16 Sep 1998 14:07:47 -0700 (PDT)

   From: Miles Bader <miles@ccs.mt.nec.co.jp>
   Date: Wed, 16 Sep 1998 10:15:34 +0900 (JST)

   In general, if you can get away with it, it's better *not* to stat files
   and then abort based on information you find; instead, try to simply
   structure the algorithm so that the actual operation which makes the
   change fails in an abortable way, and report *that* failure.  This
   avoids race conditions.

This is a good suggestion.  Here's an idea for doing this.  When you
first open the file F for writing, use O_EXCL.  This will work
correctly if F is a directory, since the initial open call will fail
with errno==EISDIR in that case.  Some older systems don't have
O_EXCL; on those systems, you'll have to approximate it with `stat',
but that's the best that you can do.

I believe that demaille wants to back up only regular files (e.g. he
doesn't want to back up /dev/null or /dev/tty, or back up
directories).  In that case, there's an unavoidable race condition,
since there's no way to say `rename this file only if it's a regular
file' atomically.  But it's still better to use O_EXCL; this will
avoid some of the race conditions.

  The following code was written by Paul Eggert.
*/

/* Create FILE for output, using open flags OFLAG and default mode MODE.
   But before creating FILE, back up its previous contents if necessary.
   OFLAG must include the O_CREAT flag.  */
int
create_file_for_backup (char const *file, int oflag, mode_t mode,
                  enum backup_type backup_type)
{
  int fd;
  struct stat st;

  assert (oflag & O_CREAT);

  if (backup_type == none)
    return open (file, oflag, mode);

#ifdef O_EXCL
  fd = open (file, oflag | O_EXCL, mode);
#else
  {
    /* This substitute for O_EXCL allows races between `stat' and `open'.  */
    if (stat (file, &st) == 0)
      {
      errno = EEXIST;
      fd = -1;
      }
    else if (errno == ENOENT)
      fd = open (file, oflag, mode);
    else
      return -1;
  }
#endif

  if (fd < 0 && errno == EEXIST)
    {
      /* We want to back up only regular files, even though this
         allows races between `stat' and `open'.  */
      if (stat (file, &st) == 0 && S_ISREG (st.st_mode))
      {
        char *backup = find_backup_file_name (file, backup_type);
        if (rename (file, backup) != 0)
          return -1;
        free (backup);
      }

      fd = open (file, oflag, mode);
    }

  return fd;
}

/* Since there were two different approaches, and one may fear
 * portability problems, the old version is here provided by disabled.
 * Follows the solution based on Miles Bader and Paul Eggert's
 * approach.  */

/*======================
 * I could not make it behave the way I want (it works for
 * a mode 000, which I don't want), so this code the old code will
 * be used instead
 ========================*/
#define USE_OLD_FOPEN_BACKUP 1

#ifdef USE_OLD_FOPEN_BACKUP
/* Open a stream for writing on the file FILENAME, making, if
   necessary and adequate, a backup according the policy BACKUP_TYPE */
FILE *
fopen_backup (const char * filename, enum backup_type backup_type)
{
  char * backup_name = NULL;
  struct stat filestat;
  FILE * res;

  /* No backup upon non existing files */
  if (stat (filename, &filestat))
    {
      if ((errno == ENOENT)
        || (errno == ENOTDIR))
      /* the file does not exist: return */
      backup_type = none ;
      else
      /* Another kind of error occured: exit */
      error (1, errno, _("cannot get informations on file `%s'"),
             quotearg (filename));
    }

  /* If the file is special (/dev/null etc.), don't backup.
     Or if we don't have the rights to open the file for writing, don't
     backup, so that the forthcoming fopen does complain on the rights*/
  if (!S_ISREG (filestat.st_mode)
      || access (filename, W_OK))
    backup_type = none ;

  /* Definitely, make a backup */
  if (backup_type != none)
    {
      backup_name = xfind_backup_file_name (filename, backup_type);
      if (rename (filename, backup_name))
      error (1, errno, _("cannot rename file `%s' as `%s'"),
             quotearg (filename), quotearg (backup_name));
    }

  /* Open the file for reading */
  res = fopen (filename, "w");
  if (!res)
    {
      error (0, errno, _("cannot create file `%s'"), quotearg (filename));
      if (backup_name)
      {
        if (rename (filename, backup_name))
          error (0, errno, _("cannot rename file `%s' as `%s'"),
               quotearg (filename), quotearg (backup_name));
        else
          fprintf (stderr, _("restored file `%s'"), quotearg (filename));
      }
      exit (EXIT_FAILURE);
    }

  if (backup_name)
    free (backup_name);

  return res;
}
#else /* !USE_OLD_FOPEN_BACKUP */
FILE *
fopen_backup (const char * filename, enum backup_type backup_type)
{
  FILE * res;
  int fd;

  fd = create_file_for_backup (filename, O_CREAT, 0666, backup_type);
  if (fd < 0)
    {
      if (backup_type == none)
      error (1, errno, _("cannot create file `%s'"), quotearg (filename));
      else
      error (1, errno, ("cannot backup and create file `%s'"),
             quotearg (filename));
    }

  res = fdopen (fd, "w");
  if (!res)
    error (1, errno, _("cannot create file `%s'"), quotearg (filename));

  return res;
}
#endif /* !USE_OLD_FOPEN_BACKUP */

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