Use fdopendir, fstatat and readlinkat, for efficiency.

On my host, this speeds up directory-files-and-attributes by a
factor of 3, when applied to Emacs's src directory.
These functions are standardized by POSIX and are common these
days; fall back on a (slower) gnulib implementation if the host
is too old to supply them.
* .bzrignore: Add lib/dirent.h.
* lib/Makefile.am (libgnu_a_SOURCES): Add openat-die.c, save-cwd.c.
* lib/careadlinkat.c, lib/careadlinkat.h: Merge from gnulib,
incorporating: 2013-01-29 careadlinkat: do not provide careadlinkatcwd.
* lib/gnulib.mk, m4/gnulib-comp.m4: Regenerate.
* lib/dirent.in.h, lib/fdopendir.c, lib/fstatat.c, lib/openat-priv.h:
* lib/openat-proc.c, lib/openat.h, m4/dirent_h.m4, m4/fdopendir.m4:
* m4/fstatat.m4: New files, from gnulib.
* lib/openat-die.c, lib/save-cwd.c, lib/save-cwd.h: New files.
These last three are specific to Emacs and are not copied from gnulib.
They are simpler than the gnulib versions and are tuned for Emacs.
* admin/merge-gnulib (GNULIB_MODULES): Add fdopendir, fstatat, readlinkat.
(GNULIB_TOOL_FLAGS): Do not avoid at-internal, openat-h.
Avoid dup, open, opendir.
* nt/inc/sys/stat.h (fstatat):
* nt/inc/unistd.h (readlinkat): New decls.
* src/conf_post.h (GNULIB_SUPPORT_ONLY_AT_FDCWD): Remove.
* src/dired.c: Include <fcntl.h>.
(open_directory): New function, which uses open and fdopendir
rather than opendir.  DOS_NT platforms still use opendir, though.
(directory_files_internal, file_name_completion): Use it.
(file_attributes): New function, with most of the old Ffile_attributes.
(directory_files_internal, Ffile_attributes): Use it.
(file_attributes, file_name_completion_stat): First arg is now fd,
not dir name.  All uses changed.  Use fstatat rather than lstat +
stat.
(file_attributes): Use emacs_readlinkat rather than Ffile_symlink_p.
* src/fileio.c: Include <allocator.h>, <careadlinkat.h>.
(emacs_readlinkat): New function, with much of the old
Ffile_symlink_p, but with an fd argument for speed.
It uses readlinkat rather than careadlinkatcwd, so that it
need not assume the working directory.
(Ffile_symlink_p): Use it.
* src/filelock.c (current_lock_owner): Use emacs_readlinkat
rather than emacs_readlink.
* src/lisp.h (emacs_readlinkat): New decl.
(READLINK_BUFSIZE, emacs_readlink): Remove.
* src/sysdep.c: Do not include <allocator.h>, <careadlinkat.h>.
(emacs_norealloc_allocator, emacs_readlink): Remove.
This stuff is moved to fileio.c.
* src/w32.c (fstatat, readlinkat): New functions.
(careadlinkat): Don't check that fd == AT_FDCWD.
(careadlinkatcwd): Remove; no longer needed.

Fixes: debbugs:13539

1 files changed, 110 insertions, 0 deletions

diff --git a/lib/openat-proc.c b/lib/openat-proc.c
new file mode 100644
index 00000000000..d7a68e26d0b
--- /dev/null
+++ b/lib/openat-proc.c
@@ -0,0 +1,110 @@
+/* Create /proc/self/fd-related names for subfiles of open directories.
+
+   Copyright (C) 2006, 2009-2013 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/>.  */
+
+/* Written by Paul Eggert.  */
+
+#include <config.h>
+
+#include "openat-priv.h"
+
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+#include "intprops.h"
+
+/* The results of open() in this file are not used with fchdir,
+   and we do not leak fds to any single-threaded code that could use stdio,
+   therefore save some unnecessary work in fchdir.c.
+   FIXME - if the kernel ever adds support for multi-thread safety for
+   avoiding standard fds, then we should use open_safer.  */
+#undef open
+#undef close
+
+#define PROC_SELF_FD_FORMAT "/proc/self/fd/%d/%s"
+
+#define PROC_SELF_FD_NAME_SIZE_BOUND(len) \
+  (sizeof PROC_SELF_FD_FORMAT - sizeof "%d%s" \
+   + INT_STRLEN_BOUND (int) + (len) + 1)
+
+
+/* Set BUF to the expansion of PROC_SELF_FD_FORMAT, using FD and FILE
+   respectively for %d and %s.  If successful, return BUF if the
+   result fits in BUF, dynamically allocated memory otherwise.  But
+   return NULL if /proc is not reliable, either because the operating
+   system support is lacking or because memory is low.  */
+char *
+openat_proc_name (char buf[OPENAT_BUFFER_SIZE], int fd, char const *file)
+{
+  static int proc_status = 0;
+
+  /* Make sure the caller gets ENOENT when appropriate.  */
+  if (!*file)
+    {
+      buf[0] = '\0';
+      return buf;
+    }
+
+  if (! proc_status)
+    {
+      /* Set PROC_STATUS to a positive value if /proc/self/fd is
+         reliable, and a negative value otherwise.  Solaris 10
+         /proc/self/fd mishandles "..", and any file name might expand
+         to ".." after symbolic link expansion, so avoid /proc/self/fd
+         if it mishandles "..".  Solaris 10 has openat, but this
+         problem is exhibited on code that built on Solaris 8 and
+         running on Solaris 10.  */
+
+      int proc_self_fd = open ("/proc/self/fd",
+                               O_SEARCH | O_DIRECTORY | O_NOCTTY | O_NONBLOCK);
+      if (proc_self_fd < 0)
+        proc_status = -1;
+      else
+        {
+          /* Detect whether /proc/self/fd/%i/../fd exists, where %i is the
+             number of a file descriptor open on /proc/self/fd.  On Linux,
+             that name resolves to /proc/self/fd, which was opened above.
+             However, on Solaris, it may resolve to /proc/self/fd/fd, which
+             cannot exist, since all names in /proc/self/fd are numeric.  */
+          char dotdot_buf[PROC_SELF_FD_NAME_SIZE_BOUND (sizeof "../fd" - 1)];
+          sprintf (dotdot_buf, PROC_SELF_FD_FORMAT, proc_self_fd, "../fd");
+          proc_status = access (dotdot_buf, F_OK) ? -1 : 1;
+          close (proc_self_fd);
+        }
+    }
+
+  if (proc_status < 0)
+    return NULL;
+  else
+    {
+      size_t bufsize = PROC_SELF_FD_NAME_SIZE_BOUND (strlen (file));
+      char *result = buf;
+      if (OPENAT_BUFFER_SIZE < bufsize)
+        {
+          result = malloc (bufsize);
+          if (! result)
+            return NULL;
+        }
+      sprintf (result, PROC_SELF_FD_FORMAT, fd, file);
+      return result;
+    }
+}