[glib] gsocket: Fix g_socket_get_available_bytes() on Windows and OS X

[Dan Winship <danw src gnome org>]

commit 9fd0927e5888458c9b1061f3cdf3a1aaa4924e36
Author: Dan Winship <danw gnome org>
Date:   Sat Aug 24 13:55:06 2013 -0400

    gsocket: Fix g_socket_get_available_bytes() on Windows and OS X
    
    On Windows and OS X, FIONREAD on a UDP socket gets the total number of
    bytes available, not the number of bytes available in the next packet,
    which is the more useful number (and how the function always behaved
    on Linux).
    
    On OS X, fix this by using SO_NREAD. On Windows, fix this by doing a
    MSG_PEEK recv() into a giant buffer, since there is apparently no
    other way to get the information.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=686786

 gio/gsocket.c |   32 ++++++++++++++++++++++++++------
 1 files changed, 26 insertions(+), 6 deletions(-)
---
diff --git a/gio/gsocket.c b/gio/gsocket.c
index cc02784..8fcbce4 100644
--- a/gio/gsocket.c
+++ b/gio/gsocket.c
@@ -2457,24 +2457,44 @@ g_socket_check_connect_result (GSocket  *socket,
  *
  * Get the amount of data pending in the OS input buffer.
  *
+ * If @socket is a UDP or SCTP socket, this will return the size of
+ * just the next packet, even if additional packets are buffered after
+ * that one.
+ *
+ * Note that on Windows, this function is rather inefficient in the
+ * UDP case, and so if you know any plausible upper bound on the size
+ * of the incoming packet, it is better to just do a
+ * g_socket_receive() with a buffer of that size, rather than calling
+ * g_socket_get_available_bytes() first and then doing a receive of
+ * exactly the right size.
+ *
  * Returns: the number of bytes that can be read from the socket
- * without blocking or -1 on error.
+ * without blocking or truncating, or -1 on error.
  *
  * Since: 2.32
  */
 gssize
 g_socket_get_available_bytes (GSocket *socket)
 {
-  gulong avail = 0;
+#ifdef G_OS_WIN32
+  const gint bufsize = 64 * 1024;
+  static guchar *buf = NULL;
+#endif
+  gint avail;
 
   g_return_val_if_fail (G_IS_SOCKET (socket), -1);
 
-#ifndef G_OS_WIN32
+#if defined (SO_NREAD)
+  if (!g_socket_get_option (socket, SOL_SOCKET, SO_NREAD, &avail, NULL))
+      return -1;
+#elif !defined (G_OS_WIN32)
   if (ioctl (socket->priv->fd, FIONREAD, &avail) < 0)
-    return -1;
+    avail = -1;
 #else
-  if (ioctlsocket (socket->priv->fd, FIONREAD, &avail) == SOCKET_ERROR)
-    return -1;
+  if (G_UNLIKELY (g_once_init_enter (&buf)))
+    g_once_init_leave (&buf, g_malloc (bufsize));
+
+  avail = recv (socket->priv->fd, buf, bufsize, MSG_PEEK);
 #endif
 
   return avail;