optimize FirstOutstanding in the sent packet history (#3467)

* optimize FirstOutstanding

* fix variable naming

* bug fix

* minor code improvements

* add a test to make sure that `Iterate` iterates in the right order

* add comment

internal/ackhandler/sent_packet_history.go

@@ -9,18 +9,20 @@ import (
 )
 
 type sentPacketHistory struct {
-	rttStats    *utils.RTTStats
-	packetList  *PacketList
-	packetMap   map[protocol.PacketNumber]*PacketElement
-	highestSent protocol.PacketNumber
+	rttStats              *utils.RTTStats
+	outstandingPacketList *PacketList
+	etcPacketList         *PacketList
+	packetMap             map[protocol.PacketNumber]*PacketElement
+	highestSent           protocol.PacketNumber
 }
 
 func newSentPacketHistory(rttStats *utils.RTTStats) *sentPacketHistory {
 	return &sentPacketHistory{
-		rttStats:    rttStats,
-		packetList:  NewPacketList(),
-		packetMap:   make(map[protocol.PacketNumber]*PacketElement),
-		highestSent: protocol.InvalidPacketNumber,
+		rttStats:              rttStats,
+		outstandingPacketList: NewPacketList(),
+		etcPacketList:         NewPacketList(),
+		packetMap:             make(map[protocol.PacketNumber]*PacketElement),
+		highestSent:           protocol.InvalidPacketNumber,
 	}
 }
 
@@ -30,7 +32,7 @@ func (h *sentPacketHistory) SentPacket(p *Packet, isAckEliciting bool) {
 	}
 	// Skipped packet numbers.
 	for pn := h.highestSent + 1; pn < p.PacketNumber; pn++ {
-		el := h.packetList.PushBack(Packet{
+		el := h.etcPacketList.PushBack(Packet{
 			PacketNumber:    pn,
 			EncryptionLevel: p.EncryptionLevel,
 			SendTime:        p.SendTime,
@@ -41,7 +43,12 @@ func (h *sentPacketHistory) SentPacket(p *Packet, isAckEliciting bool) {
 	h.highestSent = p.PacketNumber
 
 	if isAckEliciting {
-		el := h.packetList.PushBack(*p)
+		var el *PacketElement
+		if p.outstanding() {
+			el = h.outstandingPacketList.PushBack(*p)
+		} else {
+			el = h.etcPacketList.PushBack(*p)
+		}
 		h.packetMap[p.PacketNumber] = el
 	}
 }
@@ -49,10 +56,25 @@ func (h *sentPacketHistory) SentPacket(p *Packet, isAckEliciting bool) {
 // Iterate iterates through all packets.
 func (h *sentPacketHistory) Iterate(cb func(*Packet) (cont bool, err error)) error {
 	cont := true
-	var next *PacketElement
-	for el := h.packetList.Front(); cont && el != nil; el = next {
+	outstandingEl := h.outstandingPacketList.Front()
+	etcEl := h.etcPacketList.Front()
+	var el *PacketElement
+	// whichever has the next packet number is returned first
+	for cont {
+		if outstandingEl == nil || (etcEl != nil && etcEl.Value.PacketNumber < outstandingEl.Value.PacketNumber) {
+			el = etcEl
+		} else {
+			el = outstandingEl
+		}
+		if el == nil {
+			return nil
+		}
+		if el == outstandingEl {
+			outstandingEl = outstandingEl.Next()
+		} else {
+			etcEl = etcEl.Next()
+		}
 		var err error
-		next = el.Next()
 		cont, err = cb(&el.Value)
 		if err != nil {
 			return err
@@ -61,15 +83,13 @@ func (h *sentPacketHistory) Iterate(cb func(*Packet) (cont bool, err error)) err
 	return nil
 }
 
-// FirstOutStanding returns the first outstanding packet.
+// FirstOutstanding returns the first outstanding packet.
 func (h *sentPacketHistory) FirstOutstanding() *Packet {
-	for el := h.packetList.Front(); el != nil; el = el.Next() {
-		p := &el.Value
-		if !p.declaredLost && !p.skippedPacket && !p.IsPathMTUProbePacket {
-			return p
-		}
+	el := h.outstandingPacketList.Front()
+	if el == nil {
+		return nil
 	}
-	return nil
+	return &el.Value
 }
 
 func (h *sentPacketHistory) Len() int {
@@ -81,28 +101,53 @@ func (h *sentPacketHistory) Remove(p protocol.PacketNumber) error {
 	if !ok {
 		return fmt.Errorf("packet %d not found in sent packet history", p)
 	}
-	h.packetList.Remove(el)
+	h.outstandingPacketList.Remove(el)
+	h.etcPacketList.Remove(el)
 	delete(h.packetMap, p)
 	return nil
 }
 
 func (h *sentPacketHistory) HasOutstandingPackets() bool {
-	return h.FirstOutstanding() != nil
+	return h.outstandingPacketList.Len() > 0
 }
 
 func (h *sentPacketHistory) DeleteOldPackets(now time.Time) {
 	maxAge := 3 * h.rttStats.PTO(false)
 	var nextEl *PacketElement
-	for el := h.packetList.Front(); el != nil; el = nextEl {
+	// we don't iterate outstandingPacketList, as we should not delete outstanding packets.
+	// being outstanding for more than 3*PTO should only happen in the case of drastic RTT changes.
+	for el := h.etcPacketList.Front(); el != nil; el = nextEl {
 		nextEl = el.Next()
 		p := el.Value
 		if p.SendTime.After(now.Add(-maxAge)) {
 			break
 		}
-		if !p.skippedPacket && !p.declaredLost { // should only happen in the case of drastic RTT changes
-			continue
-		}
 		delete(h.packetMap, p.PacketNumber)
-		h.packetList.Remove(el)
+		h.etcPacketList.Remove(el)
 	}
 }
+
+func (h *sentPacketHistory) DeclareLost(p *Packet) *Packet {
+	el, ok := h.packetMap[p.PacketNumber]
+	if !ok {
+		return nil
+	}
+	// try to remove it from both lists, as we don't know which one it currently belongs to.
+	// Remove is a no-op for elements that are not in the list.
+	h.outstandingPacketList.Remove(el)
+	h.etcPacketList.Remove(el)
+	p.declaredLost = true
+	// move it to the correct position in the etc list (based on the packet number)
+	for el = h.etcPacketList.Back(); el != nil; el = el.Prev() {
+		if el.Value.PacketNumber < p.PacketNumber {
+			break
+		}
+	}
+	if el == nil {
+		el = h.etcPacketList.PushFront(*p)
+	} else {
+		el = h.etcPacketList.InsertAfter(*p, el)
+	}
+	h.packetMap[p.PacketNumber] = el
+	return &el.Value
+}