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fchdir.c

/* fchdir replacement.
   Copyright (C) 2006-2009 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 3 of the License, 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.  If not, see <http://www.gnu.org/licenses/>.  */

#include <config.h>

/* Specification.  */
#include <unistd.h>

#include <assert.h>
#include <dirent.h>
#include <errno.h>
#include <fcntl.h>
#include <stdbool.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>

#ifndef REPLACE_OPEN_DIRECTORY
# define REPLACE_OPEN_DIRECTORY 0
#endif

#ifndef HAVE_CANONICALIZE_FILE_NAME
# if GNULIB_CANONICALIZE || GNULIB_CANONICALIZE_LGPL
#  define HAVE_CANONICALIZE_FILE_NAME 1
# else
#  define HAVE_CANONICALIZE_FILE_NAME 0
#  define canonicalize_file_name(name) NULL
# endif
#endif

/* This replacement assumes that a directory is not renamed while opened
   through a file descriptor.

   FIXME: On mingw, this would be possible to enforce if we were to
   also open a HANDLE to each directory currently visited by a file
   descriptor, since mingw refuses to rename any in-use file system
   object.  */

/* Array of file descriptors opened.  If it points to a directory, it stores
   info about this directory.  */
typedef struct
{
  char *name;       /* Absolute name of the directory, or NULL.  */
  /* FIXME - add a DIR* member to make dirfd possible on mingw?  */
} dir_info_t;
static dir_info_t *dirs;
static size_t dirs_allocated;

/* Try to ensure dirs has enough room for a slot at index fd.  Return
   false and set errno to ENOMEM on allocation failure.  */
static bool
ensure_dirs_slot (size_t fd)
{
  if (fd >= dirs_allocated)
    {
      size_t new_allocated;
      dir_info_t *new_dirs;

      new_allocated = 2 * dirs_allocated + 1;
      if (new_allocated <= fd)
        new_allocated = fd + 1;
      new_dirs =
        (dirs != NULL
         ? (dir_info_t *) realloc (dirs, new_allocated * sizeof *dirs)
         : (dir_info_t *) malloc (new_allocated * sizeof *dirs));
      if (new_dirs == NULL)
        return false;
      memset (new_dirs + dirs_allocated, 0,
              (new_allocated - dirs_allocated) * sizeof *dirs);
      dirs = new_dirs;
      dirs_allocated = new_allocated;
    }
  return true;
}

/* Return the canonical name of DIR in malloc'd storage.  */
static char *
get_name (char const *dir)
{
  char *result;
  if (REPLACE_OPEN_DIRECTORY || !HAVE_CANONICALIZE_FILE_NAME)
    {
      /* The function canonicalize_file_name has not yet been ported
         to mingw, with all its drive letter and backslash quirks.
         Fortunately, getcwd is reliable in this case, but we ensure
         we can get back to where we started before using it.  Treat
         "." as a special case, as it is frequently encountered.  */
      char *cwd = getcwd (NULL, 0);
      int saved_errno;
      if (dir[0] == '.' && dir[1] == '\0')
        return cwd;
      if (chdir (cwd))
        return NULL;
      result = chdir (dir) ? NULL : getcwd (NULL, 0);
      saved_errno = errno;
      if (chdir (cwd))
        abort ();
      free (cwd);
      errno = saved_errno;
    }
  else
    {
      /* Avoid changing the directory.  */
      result = canonicalize_file_name (dir);
    }
  return result;
}

/* Hook into the gnulib replacements for open() and close() to keep track
   of the open file descriptors.  */

/* Close FD, cleaning up any fd to name mapping if fd was visiting a
   directory.  */
void
_gl_unregister_fd (int fd)
{
  if (fd >= 0 && fd < dirs_allocated)
    {
      free (dirs[fd].name);
      dirs[fd].name = NULL;
    }
}

/* Mark FD as visiting FILENAME.  FD must be non-negative, and refer
   to an open file descriptor.  If REPLACE_OPEN_DIRECTORY is non-zero,
   this should only be called if FD is visiting a directory.  Close FD
   and return -1 if there is insufficient memory to track the
   directory name; otherwise return FD.  */
int
_gl_register_fd (int fd, const char *filename)
{
  struct stat statbuf;

  assert (0 <= fd);
  if (REPLACE_OPEN_DIRECTORY
      || (fstat (fd, &statbuf) == 0 && S_ISDIR (statbuf.st_mode)))
    {
      if (!ensure_dirs_slot (fd)
          || (dirs[fd].name = get_name (filename)) == NULL)
        {
          int saved_errno = errno;
          close (fd);
          errno = saved_errno;
          return -1;
        }
    }
  return fd;
}

/* Mark NEWFD as a duplicate of OLDFD; useful from dup, dup2, dup3,
   and fcntl.  Both arguments must be valid and distinct file
   descriptors.  Close NEWFD and return -1 if OLDFD is tracking a
   directory, but there is insufficient memory to track the same
   directory in NEWFD; otherwise return NEWFD.

   FIXME: Need to implement rpl_fcntl in gnulib, and have it call
   this.  */
int
_gl_register_dup (int oldfd, int newfd)
{
  assert (0 <= oldfd && 0 <= newfd && oldfd != newfd);
  if (oldfd < dirs_allocated && dirs[oldfd].name)
    {
      /* Duplicated a directory; must ensure newfd is allocated.  */
      if (!ensure_dirs_slot (newfd)
          || (dirs[newfd].name = strdup (dirs[oldfd].name)) == NULL)
        {
          int saved_errno = errno;
          close (newfd);
          errno = saved_errno;
          newfd = -1;
        }
    }
  else if (newfd < dirs_allocated)
    {
      /* Duplicated a non-directory; ensure newfd is cleared.  */
      free (dirs[newfd].name);
      dirs[newfd].name = NULL;
    }
  return newfd;
}

/* If FD is currently visiting a directory, then return the name of
   that directory.  Otherwise, return NULL and set errno.  */
const char *
_gl_directory_name (int fd)
{
  if (0 <= fd && fd < dirs_allocated && dirs[fd].name != NULL)
    return dirs[fd].name;
  /* At this point, fd is either invalid, or open but not a directory.
     If dup2 fails, errno is correctly EBADF.  */
  if (0 <= fd)
    {
      if (dup2 (fd, fd) == fd)
        errno = ENOTDIR;
    }
  else
    errno = EBADF;
  return NULL;
}

/* Return stat information about FD in STATBUF.  Needed when
   rpl_open() used a dummy file to work around an open() that can't
   normally visit directories.  */
#if REPLACE_OPEN_DIRECTORY
int
rpl_fstat (int fd, struct stat *statbuf)
{
  if (0 <= fd && fd < dirs_allocated && dirs[fd].name != NULL)
    return stat (dirs[fd].name, statbuf);
  return fstat (fd, statbuf);
}
#endif

/* Override opendir() and closedir(), to keep track of the open file
   descriptors.  Needed because there is a function dirfd().  */

int
rpl_closedir (DIR *dp)
#undef closedir
{
  int fd = dirfd (dp);
  int retval = closedir (dp);

  if (retval >= 0)
    _gl_unregister_fd (fd);
  return retval;
}

DIR *
rpl_opendir (const char *filename)
#undef opendir
{
  DIR *dp;

  dp = opendir (filename);
  if (dp != NULL)
    {
      int fd = dirfd (dp);
      if (0 <= fd && _gl_register_fd (fd, filename) != fd)
        {
          int saved_errno = errno;
          closedir (dp);
          errno = saved_errno;
          return NULL;
        }
    }
  return dp;
}

/* Override dup(), to keep track of open file descriptors.  */

int
rpl_dup (int oldfd)
#undef dup
{
  int newfd = dup (oldfd);

  if (0 <= newfd)
    newfd = _gl_register_dup (oldfd, newfd);
  return newfd;
}


/* Implement fchdir() in terms of chdir().  */

int
fchdir (int fd)
{
  const char *name = _gl_directory_name (fd);
  return name ? chdir (name) : -1;
}

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