open new stream when receiving a WindowUpdate for an unknown stream

fixes #203

session.go

@@ -349,9 +349,16 @@ func (s *Session) handleWindowUpdateFrame(frame *frames.WindowUpdateFrame) error
 	s.streamsMutex.RLock()
 	defer s.streamsMutex.RUnlock()
 	if frame.StreamID != 0 {
-		if s, ok := s.streams[frame.StreamID]; ok && s == nil {
+		stream, ok := s.streams[frame.StreamID]
+		if ok && stream == nil {
 			return errWindowUpdateOnClosedStream
 		}
+
+		// open new stream when receiving a WindowUpdate for a non-existing stream
+		// this can occur if the client immediately sends a WindowUpdate for a newly opened stream, and packet reordering occurs such that the packet opening the new stream arrives after the WindowUpdate
+		if !ok {
+			s.newStreamImpl(frame.StreamID)
+		}
 	}
 	_, err := s.flowControlManager.UpdateWindow(frame.StreamID, frame.ByteOffset)
 	return err

session_test.go

@@ -316,13 +316,14 @@ var _ = Describe("Session", func() {
 			Expect(err).ToNot(HaveOccurred())
 		})
 
-		It("errors when the stream is not known", func() {
-			// See https://github.com/lucas-clemente/quic-go/issues/203
+		It("opens a new stream when receiving a WINDOW_UPDATE for an unknown stream", func() {
 			err := session.handleWindowUpdateFrame(&frames.WindowUpdateFrame{
 				StreamID:   5,
 				ByteOffset: 1337,
 			})
-			Expect(err).To(HaveOccurred())
+			Expect(err).ToNot(HaveOccurred())
+			Expect(session.streams).To(HaveKey(protocol.StreamID(5)))
+			Expect(session.streams[5]).ToNot(BeNil())
 		})
 
 		It("errors when receiving a WindowUpdateFrame for a closed stream", func() {