Move calls to GetWindowUpdate out of the send loop

session.go

@@ -562,19 +562,19 @@ func (s *session) handleCloseError(closeErr closeError) error {
 }
 
 func (s *session) sendPacket() error {
+	// Get WindowUpdate frames
+	// this call triggers the flow controller to increase the flow control windows, if necessary
+	windowUpdateFrames := s.getWindowUpdateFrames()
+	for _, wuf := range windowUpdateFrames {
+		s.packer.QueueControlFrameForNextPacket(wuf)
+	}
+
 	// Repeatedly try sending until we don't have any more data, or run out of the congestion window
 	for {
 		if !s.sentPacketHandler.SendingAllowed() {
 			return nil
 		}
 
-		// get WindowUpdate frames
-		// this call triggers the flow controller to increase the flow control windows, if necessary
-		windowUpdateFrames := s.getWindowUpdateFrames()
-		for _, wuf := range windowUpdateFrames {
-			s.packer.QueueControlFrameForNextPacket(wuf)
-		}
-
 		// check for retransmissions first
 		for {
 			retransmitPacket := s.sentPacketHandler.DequeuePacketForRetransmission()
@@ -646,6 +646,7 @@ func (s *session) sendPacket() error {
 		for _, f := range windowUpdateFrames {
 			s.packer.QueueControlFrameForNextPacket(f)
 		}
+		windowUpdateFrames = nil
 
 		err = s.sendPackedPacket(packet)
 		if err != nil {

session_test.go

@@ -1069,14 +1069,13 @@ var _ = Describe("Session", func() {
 				Expect(ok).To(BeTrue())
 			})
 
-			It("retransmits a WindowUpdates if it hasn't already sent a WindowUpdate with a higher ByteOffset", func() {
+			It("retransmits a WindowUpdate if it hasn't already sent a WindowUpdate with a higher ByteOffset", func() {
 				_, err := sess.GetOrOpenStream(5)
 				Expect(err).ToNot(HaveOccurred())
 				fcm := mocks_fc.NewMockFlowControlManager(mockCtrl)
 				sess.flowControlManager = fcm
 				fcm.EXPECT().GetWindowUpdates()
 				fcm.EXPECT().GetReceiveWindow(protocol.StreamID(5)).Return(protocol.ByteCount(0x1000), nil)
-				fcm.EXPECT().GetWindowUpdates()
 				wuf := &frames.WindowUpdateFrame{
 					StreamID:   5,
 					ByteOffset: 0x1000,