From dda9c27d0759b43bb49766e569a38d538cf65c8d Mon Sep 17 00:00:00 2001
From: Marten Seemann <martenseemann@gmail.com>
Date: Mon, 1 Jun 2020 12:39:36 +0700
Subject: [PATCH] only send a single packet to unblock the server during the
 handshake

---
 internal/ackhandler/sent_packet_handler.go    | 51 ++++++++++++-------
 .../ackhandler/sent_packet_handler_test.go    |  6 ++-
 2 files changed, 38 insertions(+), 19 deletions(-)

diff --git a/internal/ackhandler/sent_packet_handler.go b/internal/ackhandler/sent_packet_handler.go
index beb5c2ec..5dc57bdd 100644
--- a/internal/ackhandler/sent_packet_handler.go
+++ b/internal/ackhandler/sent_packet_handler.go
@@ -1,6 +1,7 @@
 package ackhandler
 
 import (
+	"errors"
 	"fmt"
 	"math"
 	"time"
@@ -617,25 +618,39 @@ func (h *sentPacketHandler) onVerifiedLossDetectionTimeout() error {
 	}
 
 	// PTO
-	_, encLevel = h.getPTOTimeAndSpace()
-	if h.logger.Debug() {
-		h.logger.Debugf("Loss detection alarm for %s fired in PTO mode. PTO count: %d", encLevel, h.ptoCount)
-	}
 	h.ptoCount++
-	if h.qlogger != nil {
-		h.qlogger.LossTimerExpired(qlog.TimerTypePTO, encLevel)
-		h.qlogger.UpdatedPTOCount(h.ptoCount)
-	}
-	h.numProbesToSend += 2
-	switch encLevel {
-	case protocol.EncryptionInitial:
-		h.ptoMode = SendPTOInitial
-	case protocol.EncryptionHandshake:
-		h.ptoMode = SendPTOHandshake
-	case protocol.Encryption1RTT:
-		h.ptoMode = SendPTOAppData
-	default:
-		return fmt.Errorf("TPO timer in unexpected encryption level: %s", encLevel)
+	if h.bytesInFlight > 0 {
+		_, encLevel = h.getPTOTimeAndSpace()
+		if h.logger.Debug() {
+			h.logger.Debugf("Loss detection alarm for %s fired in PTO mode. PTO count: %d", encLevel, h.ptoCount)
+		}
+		if h.qlogger != nil {
+			h.qlogger.LossTimerExpired(qlog.TimerTypePTO, encLevel)
+			h.qlogger.UpdatedPTOCount(h.ptoCount)
+		}
+		h.numProbesToSend += 2
+		switch encLevel {
+		case protocol.EncryptionInitial:
+			h.ptoMode = SendPTOInitial
+		case protocol.EncryptionHandshake:
+			h.ptoMode = SendPTOHandshake
+		case protocol.Encryption1RTT:
+			h.ptoMode = SendPTOAppData
+		default:
+			return fmt.Errorf("TPO timer in unexpected encryption level: %s", encLevel)
+		}
+	} else {
+		if h.perspective == protocol.PerspectiveServer {
+			return errors.New("sentPacketHandler BUG: PTO fired, but bytes_in_flight is 0")
+		}
+		h.numProbesToSend++
+		if h.initialPackets != nil {
+			h.ptoMode = SendPTOInitial
+		} else if h.handshakePackets != nil {
+			h.ptoMode = SendPTOHandshake
+		} else {
+			return errors.New("sentPacketHandler BUG: PTO fired, but bytes_in_flight is 0 and Initial and Handshake already dropped")
+		}
 	}
 	return nil
 }
diff --git a/internal/ackhandler/sent_packet_handler_test.go b/internal/ackhandler/sent_packet_handler_test.go
index 3bc2704d..38ec87dd 100644
--- a/internal/ackhandler/sent_packet_handler_test.go
+++ b/internal/ackhandler/sent_packet_handler_test.go
@@ -813,6 +813,10 @@ var _ = Describe("SentPacketHandler", func() {
 			Expect(handler.OnLossDetectionTimeout()).To(Succeed())
 			Expect(handler.SendMode()).To(Equal(SendPTOInitial))
 
+			// send a single packet to unblock the server
+			handler.SentPacket(initialPacket(&Packet{PacketNumber: 2}))
+			Expect(handler.SendMode()).To(Equal(SendAny))
+
 			// Now receive an ACK for a Handshake packet.
 			// This tells the client that the server completed address validation.
 			handler.SentPacket(handshakePacket(&Packet{PacketNumber: 1}))
@@ -825,7 +829,7 @@ var _ = Describe("SentPacketHandler", func() {
 			Expect(handler.GetLossDetectionTimeout()).To(BeZero())
 		})
 
-		It("sends an Handshake packet to unblock the server, if Initial keys were already dropped", func() {
+		It("sends a Handshake packet to unblock the server, if Initial keys were already dropped", func() {
 			handler.SentPacket(initialPacket(&Packet{PacketNumber: 1}))
 			Expect(handler.ReceivedAck(
 				&wire.AckFrame{AckRanges: []wire.AckRange{{Smallest: 1, Largest: 1}}},