1 files changed, 113 insertions, 0 deletions
diff --git a/lib/readlinkat.c b/lib/readlinkat.c
new file mode 100644
index 0000000..ab45e14
--- /dev/null
+++ b/lib/readlinkat.c
@@ -0,0 +1,113 @@
+/* Read a symlink relative to an open directory.
+   Copyright (C) 2009-2022 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/>.  */
+
+/* written by Eric Blake */
+
+#include <config.h>
+
+/* Specification.  */
+#include <unistd.h>
+
+#include <errno.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/stat.h>
+
+#if HAVE_READLINKAT
+
+# undef fstatat
+# undef readlinkat
+
+ssize_t
+rpl_readlinkat (int fd, char const *file, char *buf, size_t bufsize)
+{
+# if READLINK_TRAILING_SLASH_BUG
+  size_t file_len = strlen (file);
+  if (file_len && file[file_len - 1] == '/')
+    {
+      /* Even if FILE without the slash is a symlink to a directory,
+         both lstat() and stat() must resolve the trailing slash to
+         the directory rather than the symlink.  We can therefore
+         safely use fstatat(..., 0) to distinguish between EINVAL and
+         ENOTDIR/ENOENT, avoiding extra overhead of rpl_fstatat().  */
+      struct stat st;
+      if (fstatat (fd, file, &st, 0) == 0 || errno == EOVERFLOW)
+        errno = EINVAL;
+      return -1;
+    }
+# endif /* READLINK_TRAILING_SLASH_BUG */
+
+  ssize_t r = readlinkat (fd, file, buf, bufsize);
+
+# if READLINK_TRUNCATE_BUG
+  if (r < 0 && errno == ERANGE)
+    {
+      /* Try again with a bigger buffer.  This is just for test cases;
+         real code invariably discards short reads.  */
+      char stackbuf[4032];
+      r = readlinkat (fd, file, stackbuf, sizeof stackbuf);
+      if (r < 0)
+        {
+          if (errno == ERANGE)
+            {
+              /* Clear the buffer, which is good enough for real code.
+                 Thankfully, no test cases try short reads of enormous
+                 symlinks and what would be the point anyway?  */
+              r = bufsize;
+              memset (buf, 0, r);
+            }
+        }
+      else
+        {
+          if (bufsize < r)
+            r = bufsize;
+          memcpy (buf, stackbuf, r);
+        }
+    }
+# endif
+
+  return r;
+}
+
+#else
+
+/* Gnulib provides a readlink stub for mingw; use it for distinction
+   between EINVAL and ENOENT, rather than always failing with ENOSYS.  */
+
+/* POSIX 2008 says that unlike readlink, readlinkat returns 0 for
+   success instead of the buffer length.  But this would render
+   readlinkat worthless since readlink does not guarantee a
+   NUL-terminated buffer.  Assume this was a bug in POSIX.  */
+
+/* Read the contents of symlink FILE into buffer BUF of size BUFSIZE, in the
+   directory open on descriptor FD.  If possible, do it without changing
+   the working directory.  Otherwise, resort to using save_cwd/fchdir,
+   then readlink/restore_cwd.  If either the save_cwd or the restore_cwd
+   fails, then give a diagnostic and exit nonzero.  */
+
+# define AT_FUNC_NAME readlinkat
+# define AT_FUNC_F1 readlink
+# define AT_FUNC_POST_FILE_PARAM_DECLS , char *buf, size_t bufsize
+# define AT_FUNC_POST_FILE_ARGS        , buf, bufsize
+# define AT_FUNC_RESULT ssize_t
+# include "at-func.c"
+# undef AT_FUNC_NAME
+# undef AT_FUNC_F1
+# undef AT_FUNC_POST_FILE_PARAM_DECLS
+# undef AT_FUNC_POST_FILE_ARGS
+# undef AT_FUNC_RESULT
+
+#endif