src/find-mount-point.c

   1 /* find-mount-point.c -- find the root mount point for a file.
   2    Copyright (C) 2010-2022 Free Software Foundation, Inc.
   3
   4    This program is free software: you can redistribute it and/or modify
   5    it under the terms of the GNU General Public License as published by
   6    the Free Software Foundation, either version 3 of the License, or
   7    (at your option) any later version.
   8
   9    This program is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12    GNU General Public License for more details.
  13
  14    You should have received a copy of the GNU General Public License
  15    along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
  16
  17 #include <config.h>
  18 #include <sys/types.h>
  19
  20 #include "system.h"
  21 #include "die.h"
  22 #include "error.h"
  23 #include "save-cwd.h"
  24 #include "xgetcwd.h"
  25 #include "find-mount-point.h"
  26
  27 /* Return the root mountpoint of the file system on which FILE exists, in
  28    malloced storage.  FILE_STAT should be the result of stating FILE.
  29    Give a diagnostic and return NULL if unable to determine the mount point.
  30    Exit if unable to restore current working directory.  */
  31 extern char *
  32 find_mount_point (char const *file, struct stat const *file_stat)
  33 {
  34   struct saved_cwd cwd;
  35   struct stat last_stat;
  36   char *mp = NULL;              /* The malloc'd mount point.  */
  37
  38   if (save_cwd (&cwd) != 0)
  39     {
  40       error (0, errno, _("cannot get current directory"));
  41       return NULL;
  42     }
  43
  44   if (S_ISDIR (file_stat->st_mode))
  45     /* FILE is a directory, so just chdir there directly.  */
  46     {
  47       last_stat = *file_stat;
  48       if (chdir (file) < 0)
  49         {
  50           error (0, errno, _("cannot change to directory %s"), quoteaf (file));
  51           return NULL;
  52         }
  53     }
  54   else
  55     /* FILE is some other kind of file; use its directory.  */
  56     {
  57       char *xdir = dir_name (file);
  58       char *dir;
  59       ASSIGN_STRDUPA (dir, xdir);
  60       free (xdir);
  61
  62       if (chdir (dir) < 0)
  63         {
  64           error (0, errno, _("cannot change to directory %s"), quoteaf (dir));
  65           return NULL;
  66         }
  67
  68       if (stat (".", &last_stat) < 0)
  69         {
  70           error (0, errno, _("cannot stat current directory (now %s)"),
  71                  quoteaf (dir));
  72           goto done;
  73         }
  74     }
  75
  76   /* Now walk up FILE's parents until we find another file system or /,
  77      chdiring as we go.  LAST_STAT holds stat information for the last place
  78      we visited.  */
  79   while (true)
  80     {
  81       struct stat st;
  82       if (stat ("..", &st) < 0)
  83         {
  84           error (0, errno, _("cannot stat %s"), quoteaf (".."));
  85           goto done;
  86         }
  87       if (st.st_dev != last_stat.st_dev || st.st_ino == last_stat.st_ino)
  88         /* cwd is the mount point.  */
  89         break;
  90       if (chdir ("..") < 0)
  91         {
  92           error (0, errno, _("cannot change to directory %s"), quoteaf (".."));
  93           goto done;
  94         }
  95       last_stat = st;
  96     }
  97
  98   /* Finally reached a mount point, see what it's called.  */
  99   mp = xgetcwd ();
 100
 101 done:
 102   /* Restore the original cwd.  */
 103   {
 104     int save_errno = errno;
 105     if (restore_cwd (&cwd) != 0)
 106       die (EXIT_FAILURE, errno,
 107            _("failed to return to initial working directory"));
 108     free_cwd (&cwd);
 109     errno = save_errno;
 110   }
 111
 112   return mp;
 113 }