Don't prevent random file systems from being unmounted.

This fix relies on having the 'fchdir' function, and on having
"." be searchable (or at least readable, on platforms lacking O_SEARCH),
but that's good enough to handle the vast majority of cases and the
remaining folks can just live with the annoyance of file systems
that occasionally can't be unmounted.
* configure.ac (fchdir): New function to check for.
* lib/save-cwd.c: Copy from gnulib, except omit the part that
allocates memory, since that can cause problems in Emacs.
* lib/save-cwd.h: Copy from gnulib.

Fixes: debbugs:18232

lib/save-cwd.c

@@ -1,3 +1,91 @@
+/* save-cwd.c -- Save and restore current working directory.
+
+   Copyright (C) 1995, 1997-1998, 2003-2006, 2009-2014 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/>.  */
+
+/* Gnulib needs to save and restore the current working directory to
+   fully emulate functions like fstatat.  But Emacs doesn't care what
+   the current working directory is; it always uses absolute file
+   names.  This module replaces the Gnulib module by omitting the code
+   that Emacs does not need.  */
+
 #include <config.h>
-#define SAVE_CWD_INLINE _GL_EXTERN_INLINE
+
 #include "save-cwd.h"
+
+#include <fcntl.h>
+#include <unistd.h>
+
+/* Record the location of the current working directory in CWD so that
+   the program may change to other directories and later use restore_cwd
+   to return to the recorded location.  This function may allocate
+   space using malloc (via getcwd) or leave a file descriptor open;
+   use free_cwd to perform the necessary free or close.  Upon failure,
+   no memory is allocated, any locally opened file descriptors are
+   closed;  return non-zero -- in that case, free_cwd need not be
+   called, but doing so is ok.  Otherwise, return zero.
+
+   The _raison d'etre_ for this interface is that the working directory
+   is sometimes inaccessible, and getcwd is not robust or as efficient.
+   So, we prefer to use the open/fchdir approach, but fall back on
+   getcwd if necessary.  This module works for most cases with just
+   the getcwd-lgpl module, but to be truly robust, use the getcwd module.
+
+   Some systems lack fchdir altogether: e.g., OS/2, pre-2001 Cygwin,
+   SCO Xenix.  Also, SunOS 4 and Irix 5.3 provide the function, yet it
+   doesn't work for partitions on which auditing is enabled.  If
+   you're still using an obsolete system with these problems, please
+   send email to the maintainer of this code.  */
+
+int
+save_cwd (struct saved_cwd *cwd)
+{
+#ifdef HAVE_FCHDIR
+  cwd->desc = open (".", O_SEARCH | O_CLOEXEC);
+#else
+  cwd->desc = -1;
+#endif
+  /* The 'name' member is present only to minimize differences from
+     gnulib.  Initialize it to zero, if only to simplify debugging.  */
+  cwd->name = 0;
+  return 0;
+}
+
+/* Change to recorded location, CWD, in directory hierarchy.
+   Upon failure, return -1 (errno is set by chdir or fchdir).
+   Upon success, return zero.  */
+
+int
+restore_cwd (const struct saved_cwd *cwd)
+{
+#ifdef HAVE_FCHDIR
+  /* Restore the previous directory if possible, to avoid tying down
+     the file system of the new directory (Bug#18232).  */
+  if (fchdir (cwd->desc) == 0)
+    return 0;
+
+  /* Don't worry if fchdir fails, as Emacs doesn't care what the
+     working directory is.  The fchdir call is inside an 'if' merely to
+     pacify compilers that complain if fchdir's return value is ignored.  */
+#endif
+  return 0;
+}
+
+void
+free_cwd (struct saved_cwd *cwd)
+{
+  close (cwd->desc);
+}