areadlinkat: new module

* lib/at-func.c (FUNC_FAIL): New define.
(AT_FUNC_NAME, VALIDATE_FLAG): Use it rather than raw -1.
* modules/areadlinkat: New module.
* lib/linkat.c (areadlinkat): Move...
* lib/areadlinkat.c (areadlinkat): ...to new file.
* lib/areadlink.h (areadlinkat): Declare it.
* modules/linkat (Depends-on): Add areadlinkat.
* MODULES.html.sh (File system functions): Mention it.
* modules/areadlinkat-tests: New test.
* tests/test-areadlinkat.c: New file.

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

1 files changed, 143 insertions, 0 deletions

diff --git a/lib/areadlinkat.c b/lib/areadlinkat.c
new file mode 100644
index 0000000000..5e8bf9d2ab
--- /dev/null
+++ b/lib/areadlinkat.c
@@ -0,0 +1,143 @@
+/* areadlink.c -- readlink wrapper to return the link name in malloc'd storage
+   Unlike xreadlinkat, only call exit on failure to change directory.
+
+   Copyright (C) 2001, 2003-2007, 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 Jim Meyering <jim@meyering.net>,
+   and Bruno Haible <bruno@clisp.org>,
+   and Eric Blake <ebb9@byu.net>.  */
+
+#include <config.h>
+
+/* Specification.  */
+#include "areadlink.h"
+
+#include <string.h>
+#include <errno.h>
+#include <limits.h>
+#include <sys/types.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+#ifndef SSIZE_MAX
+# define SSIZE_MAX ((ssize_t) (SIZE_MAX / 2))
+#endif
+
+#if HAVE_READLINKAT
+
+/* Call readlinkat to get the symbolic link value of FILENAME relative to FD.
+   Return a pointer to that NUL-terminated string in malloc'd storage.
+   If readlinkat fails, return NULL and set errno (although failure to
+   change directory will issue a diagnostic and exit).
+   If realloc fails, or if the link value is longer than SIZE_MAX :-),
+   return NULL and set errno to ENOMEM.  */
+
+char *
+areadlinkat (int fd, char const *filename)
+{
+  /* The initial buffer size for the link value.  A power of 2
+     detects arithmetic overflow earlier, but is not required.  */
+# define INITIAL_BUF_SIZE 1024
+
+  /* Allocate the initial buffer on the stack.  This way, in the common
+     case of a symlink of small size, we get away with a single small malloc()
+     instead of a big malloc() followed by a shrinking realloc().  */
+  char initial_buf[INITIAL_BUF_SIZE];
+
+  char *buffer = initial_buf;
+  size_t buf_size = sizeof (initial_buf);
+
+  while (1)
+    {
+      /* Attempt to read the link into the current buffer.  */
+       ssize_t link_length = readlinkat (fd, filename, buffer, buf_size);
+
+      /* On AIX 5L v5.3 and HP-UX 11i v2 04/09, readlink returns -1
+         with errno == ERANGE if the buffer is too small.  */
+      if (link_length < 0 && errno != ERANGE)
+        {
+          if (buffer != initial_buf)
+            {
+              int saved_errno = errno;
+              free (buffer);
+              errno = saved_errno;
+            }
+          return NULL;
+        }
+
+      if ((size_t) link_length < buf_size)
+        {
+          buffer[link_length++] = '\0';
+
+          /* Return it in a chunk of memory as small as possible.  */
+          if (buffer == initial_buf)
+            {
+              buffer = (char *) malloc (link_length);
+              if (buffer == NULL)
+                /* errno is ENOMEM.  */
+                return NULL;
+              memcpy (buffer, initial_buf, link_length);
+            }
+          else
+            {
+              /* Shrink buffer before returning it.  */
+              if ((size_t) link_length < buf_size)
+                {
+                  char *smaller_buffer = (char *) realloc (buffer, link_length);
+
+                  if (smaller_buffer != NULL)
+                    buffer = smaller_buffer;
+                }
+            }
+          return buffer;
+        }
+
+      if (buffer != initial_buf)
+        free (buffer);
+      buf_size *= 2;
+      if (SSIZE_MAX < buf_size || (SIZE_MAX / 2 < SSIZE_MAX && buf_size == 0))
+        {
+          errno = ENOMEM;
+          return NULL;
+        }
+      buffer = (char *) malloc (buf_size);
+      if (buffer == NULL)
+        /* errno is ENOMEM.  */
+        return NULL;
+    }
+}
+
+#else /* !HAVE_READLINKAT */
+
+/* It is more efficient to change directories only once and call
+   areadlink, rather than repeatedly call the replacement
+   readlinkat.  */
+
+# define AT_FUNC_NAME areadlinkat
+# define AT_FUNC_F1 areadlink
+# define AT_FUNC_POST_FILE_PARAM_DECLS /* empty */
+# define AT_FUNC_POST_FILE_ARGS        /* empty */
+# define AT_FUNC_RESULT char *
+# define AT_FUNC_FAIL NULL
+# 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
+# undef AT_FUNC_FAIL
+
+#endif /* !HAVE_READLINKAT */