don't send the same StopWaitingFrame twice

fixes #229

ackhandler/stop_waiting_manager.go

@@ -0,0 +1,29 @@
+package ackhandler
+
+import (
+	"github.com/lucas-clemente/quic-go/frames"
+	"github.com/lucas-clemente/quic-go/protocol"
+)
+
+// This stopWaitingManager is not supposed to satisfy the StopWaitingManager interface, which is a remnant of the legacy AckHandler, and should be remove once we drop support for QUIC 33
+type stopWaitingManager struct {
+	largestLeastUnackedSent protocol.PacketNumber
+	nextLeastUnacked        protocol.PacketNumber
+}
+
+func (s *stopWaitingManager) GetStopWaitingFrame() *frames.StopWaitingFrame {
+	if s.nextLeastUnacked <= s.largestLeastUnackedSent {
+		return nil
+	}
+
+	s.largestLeastUnackedSent = s.nextLeastUnacked
+	return &frames.StopWaitingFrame{
+		LeastUnacked: s.nextLeastUnacked,
+	}
+}
+
+func (s *stopWaitingManager) ReceivedAck(ack *frames.AckFrame) {
+	if ack.LargestAcked >= s.nextLeastUnacked {
+		s.nextLeastUnacked = ack.LargestAcked + 1
+	}
+}