initialize the MTU discoverer immediately

connection.go

@@ -523,6 +523,14 @@ func (s *connection) preSetup() {
 
 	s.windowUpdateQueue = newWindowUpdateQueue(s.streamsMap, s.connFlowController, s.framer.QueueControlFrame)
 	s.datagramQueue = newDatagramQueue(s.scheduleSending, s.logger)
+	s.mtuDiscoverer = newMTUDiscoverer(
+		s.rttStats,
+		getMaxPacketSize(s.conn.RemoteAddr()),
+		func(size protocol.ByteCount) {
+			s.sentPacketHandler.SetMaxDatagramSize(size)
+			s.packer.SetMaxPacketSize(size)
+		},
+	)
 	s.connState.Version = s.version
 }
 
@@ -805,16 +813,7 @@ func (s *connection) handleHandshakeConfirmed() {
 		if maxPacketSize == 0 {
 			maxPacketSize = protocol.MaxByteCount
 		}
-		maxPacketSize = utils.Min(maxPacketSize, protocol.MaxPacketBufferSize)
-		s.mtuDiscoverer = newMTUDiscoverer(
-			s.rttStats,
-			getMaxPacketSize(s.conn.RemoteAddr()),
-			maxPacketSize,
-			func(size protocol.ByteCount) {
-				s.sentPacketHandler.SetMaxDatagramSize(size)
-				s.packer.SetMaxPacketSize(size)
-			},
-		)
+		s.mtuDiscoverer.Start(utils.Min(maxPacketSize, protocol.MaxPacketBufferSize))
 	}
 }
 

mock_mtu_discoverer_test.go

@@ -36,6 +36,20 @@ func (m *MockMTUDiscoverer) EXPECT() *MockMTUDiscovererMockRecorder {
 	return m.recorder
 }
 
+// CurrentSize mocks base method.
+func (m *MockMTUDiscoverer) CurrentSize() protocol.ByteCount {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "CurrentSize")
+	ret0, _ := ret[0].(protocol.ByteCount)
+	return ret0
+}
+
+// CurrentSize indicates an expected call of CurrentSize.
+func (mr *MockMTUDiscovererMockRecorder) CurrentSize() *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "CurrentSize", reflect.TypeOf((*MockMTUDiscoverer)(nil).CurrentSize))
+}
+
 // GetPing mocks base method.
 func (m *MockMTUDiscoverer) GetPing() (ackhandler.Frame, protocol.ByteCount) {
 	m.ctrl.T.Helper()
@@ -64,3 +78,15 @@ func (mr *MockMTUDiscovererMockRecorder) ShouldSendProbe(arg0 interface{}) *gomo
 	mr.mock.ctrl.T.Helper()
 	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "ShouldSendProbe", reflect.TypeOf((*MockMTUDiscoverer)(nil).ShouldSendProbe), arg0)
 }
+
+// Start mocks base method.
+func (m *MockMTUDiscoverer) Start(arg0 protocol.ByteCount) {
+	m.ctrl.T.Helper()
+	m.ctrl.Call(m, "Start", arg0)
+}
+
+// Start indicates an expected call of Start.
+func (mr *MockMTUDiscovererMockRecorder) Start(arg0 interface{}) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "Start", reflect.TypeOf((*MockMTUDiscoverer)(nil).Start), arg0)
+}

mtu_discoverer.go

@@ -10,7 +10,11 @@ import (
 )
 
 type mtuDiscoverer interface {
+	// Start starts the MTU discovery process.
+	// It's unnecessary to call ShouldSendProbe before that.
+	Start(maxPacketSize protocol.ByteCount)
 	ShouldSendProbe(now time.Time) bool
+	CurrentSize() protocol.ByteCount
 	GetPing() (ping ackhandler.Frame, datagramSize protocol.ByteCount)
 }
 
@@ -34,13 +38,11 @@ type mtuFinder struct {
 
 var _ mtuDiscoverer = &mtuFinder{}
 
-func newMTUDiscoverer(rttStats *utils.RTTStats, start, max protocol.ByteCount, mtuIncreased func(protocol.ByteCount)) mtuDiscoverer {
+func newMTUDiscoverer(rttStats *utils.RTTStats, start protocol.ByteCount, mtuIncreased func(protocol.ByteCount)) *mtuFinder {
 	return &mtuFinder{
-		current:       start,
-		rttStats:      rttStats,
-		lastProbeTime: time.Now(), // to make sure the first probe packet is not sent immediately
-		mtuIncreased:  mtuIncreased,
-		max:           max,
+		current:      start,
+		rttStats:     rttStats,
+		mtuIncreased: mtuIncreased,
 	}
 }
 
@@ -48,7 +50,15 @@ func (f *mtuFinder) done() bool {
 	return f.max-f.current <= maxMTUDiff+1
 }
 
+func (f *mtuFinder) Start(maxPacketSize protocol.ByteCount) {
+	f.lastProbeTime = time.Now() // makes sure the first probe packet is not sent immediately
+	f.max = maxPacketSize
+}
+
 func (f *mtuFinder) ShouldSendProbe(now time.Time) bool {
+	if f.max == 0 || f.lastProbeTime.IsZero() {
+		return false
+	}
 	if f.probeInFlight || f.done() {
 		return false
 	}
@@ -72,3 +82,7 @@ func (f *mtuFinder) GetPing() (ackhandler.Frame, protocol.ByteCount) {
 		},
 	}, size
 }
+
+func (f *mtuFinder) CurrentSize() protocol.ByteCount {
+	return f.current
+}

mtu_discoverer_test.go

@@ -19,7 +19,7 @@ var _ = Describe("MTU Discoverer", func() {
 	)
 
 	var (
-		d             mtuDiscoverer
+		d             *mtuFinder
 		rttStats      *utils.RTTStats
 		now           time.Time
 		discoveredMTU protocol.ByteCount
@@ -29,9 +29,9 @@ var _ = Describe("MTU Discoverer", func() {
 		rttStats = &utils.RTTStats{}
 		rttStats.SetInitialRTT(rtt)
 		Expect(rttStats.SmoothedRTT()).To(Equal(rtt))
-		d = newMTUDiscoverer(rttStats, startMTU, maxMTU, func(s protocol.ByteCount) { discoveredMTU = s })
+		d = newMTUDiscoverer(rttStats, startMTU, func(s protocol.ByteCount) { discoveredMTU = s })
+		d.Start(maxMTU)
 		now = time.Now()
-		_ = discoveredMTU
 	})
 
 	It("only allows a probe 5 RTTs after the handshake completes", func() {
@@ -77,13 +77,21 @@ var _ = Describe("MTU Discoverer", func() {
 		Expect(d.ShouldSendProbe(t.Add(10 * rtt))).To(BeFalse())
 	})
 
+	It("doesn't do discovery before being started", func() {
+		d := newMTUDiscoverer(rttStats, startMTU, func(s protocol.ByteCount) {})
+		for i := 0; i < 5; i++ {
+			Expect(d.ShouldSendProbe(time.Now())).To(BeFalse())
+		}
+	})
+
 	It("finds the MTU", func() {
 		const rep = 3000
 		var maxDiff protocol.ByteCount
 		for i := 0; i < rep; i++ {
 			max := protocol.ByteCount(rand.Intn(int(3000-startMTU))) + startMTU + 1
 			currentMTU := startMTU
-			d := newMTUDiscoverer(rttStats, startMTU, max, func(s protocol.ByteCount) { currentMTU = s })
+			d := newMTUDiscoverer(rttStats, startMTU, func(s protocol.ByteCount) { currentMTU = s })
+			d.Start(max)
 			now := time.Now()
 			realMTU := protocol.ByteCount(rand.Intn(int(max-startMTU))) + startMTU
 			t := now.Add(mtuProbeDelay * rtt)