1 files changed, 113 insertions, 0 deletions
diff --git a/src/find-mount-point.c b/src/find-mount-point.c
new file mode 100644
index 0000000..b3dff48
--- /dev/null
+++ b/src/find-mount-point.c
@@ -0,0 +1,113 @@
+/* find-mount-point.c -- find the root mount point for a file.
+   Copyright (C) 2010-2022 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 <https://www.gnu.org/licenses/>.  */
+
+#include <config.h>
+#include <sys/types.h>
+
+#include "system.h"
+#include "die.h"
+#include "error.h"
+#include "save-cwd.h"
+#include "xgetcwd.h"
+#include "find-mount-point.h"
+
+/* Return the root mountpoint of the file system on which FILE exists, in
+   malloced storage.  FILE_STAT should be the result of stating FILE.
+   Give a diagnostic and return NULL if unable to determine the mount point.
+   Exit if unable to restore current working directory.  */
+extern char *
+find_mount_point (char const *file, struct stat const *file_stat)
+{
+  struct saved_cwd cwd;
+  struct stat last_stat;
+  char *mp = NULL;		/* The malloc'd mount point.  */
+
+  if (save_cwd (&cwd) != 0)
+    {
+      error (0, errno, _("cannot get current directory"));
+      return NULL;
+    }
+
+  if (S_ISDIR (file_stat->st_mode))
+    /* FILE is a directory, so just chdir there directly.  */
+    {
+      last_stat = *file_stat;
+      if (chdir (file) < 0)
+        {
+          error (0, errno, _("cannot change to directory %s"), quoteaf (file));
+          return NULL;
+        }
+    }
+  else
+    /* FILE is some other kind of file; use its directory.  */
+    {
+      char *xdir = dir_name (file);
+      char *dir;
+      ASSIGN_STRDUPA (dir, xdir);
+      free (xdir);
+
+      if (chdir (dir) < 0)
+        {
+          error (0, errno, _("cannot change to directory %s"), quoteaf (dir));
+          return NULL;
+        }
+
+      if (stat (".", &last_stat) < 0)
+        {
+          error (0, errno, _("cannot stat current directory (now %s)"),
+                 quoteaf (dir));
+          goto done;
+        }
+    }
+
+  /* Now walk up FILE's parents until we find another file system or /,
+     chdiring as we go.  LAST_STAT holds stat information for the last place
+     we visited.  */
+  while (true)
+    {
+      struct stat st;
+      if (stat ("..", &st) < 0)
+        {
+          error (0, errno, _("cannot stat %s"), quoteaf (".."));
+          goto done;
+        }
+      if (st.st_dev != last_stat.st_dev || st.st_ino == last_stat.st_ino)
+        /* cwd is the mount point.  */
+        break;
+      if (chdir ("..") < 0)
+        {
+          error (0, errno, _("cannot change to directory %s"), quoteaf (".."));
+          goto done;
+        }
+      last_stat = st;
+    }
+
+  /* Finally reached a mount point, see what it's called.  */
+  mp = xgetcwd ();
+
+done:
+  /* Restore the original cwd.  */
+  {
+    int save_errno = errno;
+    if (restore_cwd (&cwd) != 0)
+      die (EXIT_FAILURE, errno,
+           _("failed to return to initial working directory"));
+    free_cwd (&cwd);
+    errno = save_errno;
+  }
+
+  return mp;
+}