src/find-mount-point.c

   1 /* find-mount-point.c -- find the root mount point for a file.
   2    Copyright (C) 2010-2024 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 "save-cwd.h"
  22 #include "xgetcwd.h"
  23 #include "find-mount-point.h"
  24
  25 /* Return the root mountpoint of the file system on which FILE exists, in
  26    malloced storage.  FILE_STAT should be the result of stating FILE.
  27    Give a diagnostic and return nullptr if unable to determine the mount point.
  28    Exit if unable to restore current working directory.  */
  29 extern char *
  30 find_mount_point (char const *file, struct stat const *file_stat)
  31 {
  32   struct saved_cwd cwd;
  33   struct stat last_stat;
  34   char *mp = nullptr;           /* The malloc'd mount point.  */
  35
  36   if (save_cwd (&cwd) != 0)
  37     {
  38       error (0, errno, _("cannot get current directory"));
  39       return nullptr;
  40     }
  41
  42   if (S_ISDIR (file_stat->st_mode))
  43     /* FILE is a directory, so just chdir there directly.  */
  44     {
  45       last_stat = *file_stat;
  46       if (chdir (file) < 0)
  47         {
  48           error (0, errno, _("cannot change to directory %s"), quoteaf (file));
  49           return nullptr;
  50         }
  51     }
  52   else
  53     /* FILE is some other kind of file; use its directory.  */
  54     {
  55       char *xdir = dir_name (file);
  56       char *dir;
  57       ASSIGN_STRDUPA (dir, xdir);
  58       free (xdir);
  59
  60       if (chdir (dir) < 0)
  61         {
  62           error (0, errno, _("cannot change to directory %s"), quoteaf (dir));
  63           return nullptr;
  64         }
  65
  66       if (stat (".", &last_stat) < 0)
  67         {
  68           error (0, errno, _("cannot stat current directory (now %s)"),
  69                  quoteaf (dir));
  70           goto done;
  71         }
  72     }
  73
  74   /* Now walk up FILE's parents until we find another file system or /,
  75      chdiring as we go.  LAST_STAT holds stat information for the last place
  76      we visited.  */
  77   while (true)
  78     {
  79       struct stat st;
  80       if (stat ("..", &st) < 0)
  81         {
  82           error (0, errno, _("cannot stat %s"), quoteaf (".."));
  83           goto done;
  84         }
  85       if (st.st_dev != last_stat.st_dev || st.st_ino == last_stat.st_ino)
  86         /* cwd is the mount point.  */
  87         break;
  88       if (chdir ("..") < 0)
  89         {
  90           error (0, errno, _("cannot change to directory %s"), quoteaf (".."));
  91           goto done;
  92         }
  93       last_stat = st;
  94     }
  95
  96   /* Finally reached a mount point, see what it's called.  */
  97   mp = xgetcwd ();
  98
  99 done:
 100   /* Restore the original cwd.  */
 101   {
 102     int save_errno = errno;
 103     if (restore_cwd (&cwd) != 0)
 104       error (EXIT_FAILURE, errno,
 105              _("failed to return to initial working directory"));
 106     free_cwd (&cwd);
 107     errno = save_errno;
 108   }
 109
 110   return mp;
 111 }