From 281e8ca7c0df966a4b52568660f3b46cc4d85600 Mon Sep 17 00:00:00 2001
From: Marten Seemann <martenseemann@gmail.com>
Date: Wed, 20 Jan 2021 12:47:56 +0800
Subject: [PATCH] prioritize receiving packets over sending out more packets

---
 session.go      |  5 +++++
 session_test.go | 26 ++++++++++++++++++++++++++
 2 files changed, 31 insertions(+)

diff --git a/session.go b/session.go
index d035c1c53..519528b33 100644
--- a/session.go
+++ b/session.go
@@ -1563,6 +1563,11 @@ func (s *session) sendPackets() error {
 		default:
 			return fmt.Errorf("BUG: invalid send mode %d", sendMode)
 		}
+		// Prioritize receiving of packets over sending out more packets.
+		if len(s.receivedPackets) > 0 {
+			s.pacingDeadline = deadlineSendImmediately
+			return nil
+		}
 		if s.sendQueue.WouldBlock() {
 			return nil
 		}
diff --git a/session_test.go b/session_test.go
index 23b5377c4..ff3325a2a 100644
--- a/session_test.go
+++ b/session_test.go
@@ -1571,6 +1571,32 @@ var _ = Describe("Session", func() {
 			Eventually(written).Should(BeClosed())
 		})
 
+		It("stops sending when there are new packets to receive", func() {
+			sender.EXPECT().WouldBlock().AnyTimes()
+			go func() {
+				defer GinkgoRecover()
+				cryptoSetup.EXPECT().RunHandshake().MaxTimes(1)
+				sess.run()
+			}()
+
+			written := make(chan struct{})
+			sender.EXPECT().WouldBlock().AnyTimes()
+			sph.EXPECT().SentPacket(gomock.Any()).Do(func(*ackhandler.Packet) {
+				sph.EXPECT().ReceivedBytes(gomock.Any())
+				sess.handlePacket(&receivedPacket{buffer: getPacketBuffer()})
+			})
+			sph.EXPECT().HasPacingBudget().Return(true).AnyTimes()
+			sph.EXPECT().SendMode().Return(ackhandler.SendAny).AnyTimes()
+			packer.EXPECT().PackPacket().Return(getPacket(1000), nil)
+			packer.EXPECT().PackPacket().Return(nil, nil)
+			sender.EXPECT().Send(gomock.Any()).DoAndReturn(func(p *packetBuffer) { close(written) })
+
+			sess.scheduleSending()
+			time.Sleep(scaleDuration(50 * time.Millisecond))
+
+			Eventually(written).Should(BeClosed())
+		})
+
 		It("stops sending when the send queue is full", func() {
 			sph.EXPECT().SentPacket(gomock.Any())
 			sph.EXPECT().HasPacingBudget().Return(true).AnyTimes()