fix bug in OutgoingPacketAckHandler entropy calculation for simple ACKs

ackhandler/outgoing_packet_ack_handler.go

@@ -65,13 +65,6 @@ func (h *outgoingPacketAckHandler) calculateExpectedEntropy(ackFrame *frames.Ack
 
 	var expectedEntropy EntropyAccumulator
 
-	if !ackFrame.HasNACK() { // if the packet doesn't have NACKs, the correct entropy value is the entropy we sent the LargestObserved packet with. Just look it up in the map
-		packet, ok := h.packetHistory[ackFrame.LargestObserved]
-		if !ok {
-			return 0, errMapAccess
-		}
-		expectedEntropy = packet.Entropy
-	} else { // if the packet has NACKs, the entropy value has to be calculated
 	// get the entropy for the highestInOrderAckedPacketNumber
 	// There are two cases:
 	// 1. the packet with highestInOrderAckedPacketNumber has already been ACKed, then it doesn't exist in the packetHistory map anymore, but the value was saved as h.highestInOrderAckedEntropy
@@ -86,6 +79,7 @@ func (h *outgoingPacketAckHandler) calculateExpectedEntropy(ackFrame *frames.Ack
 		expectedEntropy = packet.Entropy
 	}
 
+	if ackFrame.HasNACK() { // if the packet has NACKs, the entropy value has to be calculated
 		nackRangeIndex := len(ackFrame.NackRanges) - 1
 		nackRange := ackFrame.NackRanges[nackRangeIndex]
 		for i := highestInOrderAckedPacketNumber + 1; i <= ackFrame.LargestObserved; i++ {

ackhandler/outgoing_packet_ack_handler_test.go

@@ -160,6 +160,25 @@ var _ = Describe("AckHandler", func() {
 				err = handler.ReceivedAck(&ack)
 				Expect(err).ToNot(HaveOccurred())
 			})
+
+			It("checks the entropy of an ACK after a previous ACK was already received", func() {
+				expectedEntropy := EntropyAccumulator(0)
+				expectedEntropy.Add(1, packets[0].EntropyBit)
+				ack := frames.AckFrame{
+					LargestObserved: 1,
+					Entropy:         byte(expectedEntropy),
+				}
+				err := handler.ReceivedAck(&ack)
+				Expect(err).ToNot(HaveOccurred())
+				expectedEntropy.Add(2, packets[1].EntropyBit)
+				expectedEntropy.Add(3, packets[2].EntropyBit)
+				ack = frames.AckFrame{
+					LargestObserved: 3,
+					Entropy:         byte(expectedEntropy),
+				}
+				err = handler.ReceivedAck(&ack)
+				Expect(err).ToNot(HaveOccurred())
+			})
 		})
 
 		Context("ACKs with NACK ranges", func() {
@@ -244,7 +263,7 @@ var _ = Describe("AckHandler", func() {
 				Expect(handler.packetHistory).To(HaveKey(protocol.PacketNumber(2)))
 			})
 
-			It("checks the entropy of an ACK with a NACK after an previous ACK was already received", func() {
+			It("checks the entropy of an ACK with a NACK after a previous ACK was already received", func() {
 				expectedEntropy := EntropyAccumulator(0)
 				expectedEntropy.Add(1, packets[0].EntropyBit)
 				ack := frames.AckFrame{