usb: do not allocate raw packets without size

Allocating a packet in the packet stream without a payload is not
allowed. Therefore we have to allocate CTRL message packets, that do
not have a payload, with a bogus length instead.

repos/libports/src/lib/qemu-usb/host.cc

@@ -228,10 +228,19 @@ struct Usb_host_device : List<Usb_host_device>::Element
 
 	Usb::Packet_descriptor alloc_packet(int length)
 	{
-
 		if (!usb_raw.source()->ready_to_submit())
 			throw -1;
 
+		if (length <= 0) {
+			/*
+			 * XXX Packets without payload are not supported by the packet stream
+			 *     currently. Therefore, we use a (small) bogus length
+			 *     here and depend on the USB driver to handle this case
+			 *     correctly.
+			 */
+			length = 4;
+		}
+
 		Usb::Packet_descriptor packet = usb_raw.source()->alloc_packet(length);
 		packet.completion = alloc_completion();
 		return packet;

repos/os/include/usb/packet_handler.h

@@ -85,6 +85,16 @@ class Usb::Packet_handler
 				throw Usb::Session::Tx::Source::Packet_alloc_failed();
 			}
 
+			if (size == 0) {
+				/*
+				 * XXX Packets without payload are not supported by the packet
+				 *     stream currently. Therefore, we use a (small) bogus
+				 *     length here and depend on the USB driver to handle this
+				 *     case correctly.
+				 */
+				size = 4;
+			}
+
 			while (true) {
 				try {
 					Packet_descriptor p = _connection.source()->alloc_packet(size);