linkat: new module

* modules/linkat: New file.
* lib/at-func2.c (at_func2): Likewise.
* lib/linkat.c (linkat): Likewise.
* m4/linkat.m4 (gl_FUNC_LINKAT): Likewise.
* lib/openat-priv.h (at_func2): Add declaration.
* m4/unistd_h.m4 (gl_UNISTD_H_DEFAULTS): Add witnesses.
* modules/unistd (Makefile.am): Substitute them.
* lib/unistd.in.h (linkat): Declare it.
* MODULES.html.sh (systems lacking POSIX:2008): Mention module.
* doc/posix-functions/linkat.texi (linkat): Likewise.
* doc/posix-functions/link.texi (link): Tweak wording.
* tests/test-link.c (main): Move guts...
* tests/test-link.h (test_link): ...into new file.
* modules/linkat-tests: New test.
* tests/test-linkat.c: Likewise.
* modules/link-tests (Files): Ship new file.
(Depends-on): Add stdbool.

Signed-off-by: Eric Blake <ebb9@byu.net>

1 files changed, 181 insertions, 0 deletions

diff --git a/lib/linkat.c b/lib/linkat.c
new file mode 100644
index 0000000000..bda0627569
--- /dev/null
+++ b/lib/linkat.c
@@ -0,0 +1,181 @@
+/* Create a hard link relative to open directories.
+   Copyright (C) 2009 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 Eric Blake */
+
+#include <config.h>
+
+#include <unistd.h>
+
+#include <errno.h>
+#include <fcntl.h>
+#include <limits.h>
+#include <sys/stat.h>
+
+#include "areadlink.h"
+#include "dirname.h"
+#include "filenamecat.h"
+#include "openat-priv.h"
+
+#if HAVE_SYS_PARAM_H
+# include <sys/param.h>
+#endif
+#ifndef MAXSYMLINKS
+# ifdef SYMLOOP_MAX
+#  define MAXSYMLINKS SYMLOOP_MAX
+# else
+#  define MAXSYMLINKS 20
+# endif
+#endif
+
+/* Create a link.  If FILE1 is a symlink, either create a hardlink to
+   that symlink, or fake it by creating an identical symlink.  */
+#if LINK_FOLLOWS_SYMLINKS == 0
+# define link_immediate link
+#else
+static int
+link_immediate (char const *file1, char const *file2)
+{
+  char *target = areadlink (file1);
+  if (target)
+    {
+      /* A symlink cannot be modified in-place.  Therefore, creating
+         an identical symlink behaves like a hard link to a symlink,
+         except for incorrect st_ino and st_nlink.  However, we must
+         be careful of EXDEV.  */
+      struct stat st1;
+      struct stat st2;
+      char *dir = mdir_name (file2);
+      if (!dir)
+        {
+          free (target);
+          errno = ENOMEM;
+          return -1;
+        }
+      if (lstat (file1, &st1) == 0 && stat (dir, &st2) == 0)
+        {
+          if (st1.st_dev == st2.st_dev)
+            {
+              int result = symlink (target, file2);
+              int saved_errno = errno;
+              free (target);
+              free (dir);
+              errno = saved_errno;
+              return result;
+            }
+          free (target);
+          free (dir);
+          errno = EXDEV;
+          return -1;
+        }
+      free (target);
+      free (dir);
+    }
+  if (errno == ENOMEM)
+    return -1;
+  return link (file1, file2);
+}
+#endif
+
+/* Create a link.  If FILE1 is a symlink, create a hardlink to the
+   canonicalized file.  */
+#if 0 < LINK_FOLLOWS_SYMLINKS
+# define link_follow link
+#else
+static int
+link_follow (char const *file1, char const *file2)
+{
+  char *name = (char *) file1;
+  char *target;
+  int result;
+  int i = MAXSYMLINKS;
+
+  /* Using realpath or canonicalize_file_name is too heavy-handed: we
+     don't need an absolute name, and we don't need to resolve
+     intermediate symlinks, just the basename of each iteration.  */
+  while (i-- && (target = areadlink (name)))
+    {
+      if (IS_ABSOLUTE_FILE_NAME (target))
+        {
+          if (name != file1)
+            free (name);
+          name = target;
+        }
+      else
+        {
+          char *dir = mdir_name (name);
+          if (name != file1)
+            free (name);
+          if (!dir)
+            {
+              free (target);
+              errno = ENOMEM;
+              return -1;
+            }
+          name = mfile_name_concat (dir, target, NULL);
+          free (dir);
+          free (target);
+          if (!name)
+            {
+              errno = ENOMEM;
+              return -1;
+            }
+        }
+    }
+  if (i < 0)
+    {
+      target = NULL;
+      errno = ELOOP;
+    }
+  if (!target && errno != EINVAL)
+    {
+      if (name != file1)
+        {
+          int saved_errno = errno;
+          free (name);
+          errno = saved_errno;
+        }
+      return -1;
+    }
+  result = link (name, file2);
+  if (name != file1)
+    {
+      int saved_errno = errno;
+      free (name);
+      errno = saved_errno;
+    }
+  return result;
+}
+#endif
+
+/* Create a link to FILE1, in the directory open on descriptor FD1, to FILE2,
+   in the directory open on descriptor FD2.  If FILE1 is a symlink, FLAG
+   controls whether to dereference FILE1 first.  If possible, do it without
+   changing the working directory.  Otherwise, resort to using
+   save_cwd/fchdir, then rename/restore_cwd.  If either the save_cwd or
+   the restore_cwd fails, then give a diagnostic and exit nonzero.  */
+
+int
+linkat (int fd1, char const *file1, int fd2, char const *file2, int flag)
+{
+  if (flag & ~AT_SYMLINK_FOLLOW)
+    {
+      errno = EINVAL;
+      return -1;
+    }
+  return at_func2 (fd1, file1, fd2, file2,
+                   flag ? link_follow : link_immediate);
+}