udpsrc: sanity check size of available packet data for reading to avoid memory waste

On Windows and OS/X, _get_available_bytes() may not return the size
of the next pending packet, but the size of all pending packets in
the kernel-side buffer, which might be rather large depending on
configuration. Sanity-check the size returned by _get_available_bytes()
to make sure we never allocate more memory than the max. size for
a packet, if it's an IPv4 socket.

https://bugzilla.gnome.org/show_bug.cgi?id=610364

gst/udp/gstudpsrc.c

@@ -116,6 +116,9 @@
 #include <sys/socket.h>
 #endif
 
+/* not 100% correct, but a good upper bound for memory allocation purposes */
+#define MAX_IPV4_UDP_PACKET_SIZE (65536 - 8)
+
 GST_DEBUG_CATEGORY_STATIC (udpsrc_debug);
 #define GST_CAT_DEFAULT (udpsrc_debug)
 
@@ -442,6 +445,11 @@ retry:
 no_select:
   GST_LOG_OBJECT (udpsrc, "ioctl says %d bytes available", (int) readsize);
 
+  /* sanity check value from _get_available_bytes(), which might be as
+   * large as the kernel-side buffer on some operating systems */
+  if (g_socket_get_family (udpsrc->used_socket) == G_SOCKET_FAMILY_IPV4)
+    readsize = MIN (MAX_IPV4_UDP_PACKET_SIZE, readsize);
+
   ret = GST_BASE_SRC_CLASS (parent_class)->alloc (GST_BASE_SRC_CAST (udpsrc),
       -1, readsize, &outbuf);
   if (ret != GST_FLOW_OK)