simplify pacing logic by introducing a SendPacingLimited send mode

internal/ackhandler/interfaces.go

@@ -24,8 +24,6 @@ type SentPacketHandler interface {
 	// TimeUntilSend is the time when the next packet should be sent.
 	// It is used for pacing packets.
 	TimeUntilSend() time.Time
-	// HasPacingBudget says if the pacer allows sending of a (full size) packet at this moment.
-	HasPacingBudget() bool
 	SetMaxDatagramSize(count protocol.ByteCount)
 
 	// only to be called once the handshake is complete

internal/ackhandler/send_mode.go

@@ -16,6 +16,10 @@ const (
 	SendPTOHandshake
 	// SendPTOAppData means that an Application data probe packet should be sent
 	SendPTOAppData
+	// SendPacingLimited means that the pacer doesn't allow sending of a packet right now,
+	// but will do in a little while.
+	// The timestamp when sending is allowed again can be obtained via the SentPacketHandler.TimeUntilSend.
+	SendPacingLimited
 	// SendAny means that any packet should be sent
 	SendAny
 )
@@ -34,6 +38,8 @@ func (s SendMode) String() string {
 		return "pto (Application Data)"
 	case SendAny:
 		return "any"
+	case SendPacingLimited:
+		return "pacing limited"
 	default:
 		return fmt.Sprintf("invalid send mode: %d", s)
 	}

internal/ackhandler/send_mode_test.go

@@ -9,6 +9,7 @@ var _ = Describe("Send Mode", func() {
 	It("has a string representation", func() {
 		Expect(SendNone.String()).To(Equal("none"))
 		Expect(SendAny.String()).To(Equal("any"))
+		Expect(SendPacingLimited.String()).To(Equal("pacing limited"))
 		Expect(SendAck.String()).To(Equal("ack"))
 		Expect(SendPTOInitial.String()).To(Equal("pto (Initial)"))
 		Expect(SendPTOHandshake.String()).To(Equal("pto (Handshake)"))

internal/ackhandler/sent_packet_handler.go

@@ -756,6 +756,9 @@ func (h *sentPacketHandler) SendMode() SendMode {
 		}
 		return SendAck
 	}
+	if !h.congestion.HasPacingBudget() {
+		return SendPacingLimited
+	}
 	return SendAny
 }
 

internal/ackhandler/sent_packet_handler_test.go

@@ -599,12 +599,14 @@ var _ = Describe("SentPacketHandler", func() {
 				SendTime:        time.Now(),
 			})
 			cong.EXPECT().CanSend(protocol.ByteCount(42)).Return(true)
+			cong.EXPECT().HasPacingBudget().Return(true)
 			handler.SendMode()
 		})
 
 		It("allows sending of ACKs when congestion limited", func() {
 			handler.ReceivedPacket(protocol.EncryptionHandshake)
 			cong.EXPECT().CanSend(gomock.Any()).Return(true)
+			cong.EXPECT().HasPacingBudget().Return(true)
 			Expect(handler.SendMode()).To(Equal(SendAny))
 			cong.EXPECT().CanSend(gomock.Any()).Return(false)
 			Expect(handler.SendMode()).To(Equal(SendAck))
@@ -613,6 +615,7 @@ var _ = Describe("SentPacketHandler", func() {
 		It("allows sending of ACKs when we're keeping track of MaxOutstandingSentPackets packets", func() {
 			handler.ReceivedPacket(protocol.EncryptionHandshake)
 			cong.EXPECT().CanSend(gomock.Any()).Return(true).AnyTimes()
+			cong.EXPECT().HasPacingBudget().Return(true).AnyTimes()
 			cong.EXPECT().OnPacketSent(gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any()).AnyTimes()
 			for i := protocol.PacketNumber(0); i < protocol.MaxOutstandingSentPackets; i++ {
 				Expect(handler.SendMode()).To(Equal(SendAny))
@@ -889,6 +892,7 @@ var _ = Describe("SentPacketHandler", func() {
 
 	Context("amplification limit, for the server", func() {
 		It("limits the window to 3x the bytes received, to avoid amplification attacks", func() {
+			now := time.Now()
 			handler.ReceivedPacket(protocol.EncryptionInitial) // receiving an Initial packet doesn't validate the client's address
 			handler.ReceivedBytes(200)
 			handler.SentPacket(&Packet{
@@ -896,7 +900,7 @@ var _ = Describe("SentPacketHandler", func() {
 				Length:          599,
 				EncryptionLevel: protocol.EncryptionInitial,
 				Frames:          []Frame{{Frame: &wire.PingFrame{}}},
-				SendTime:        time.Now(),
+				SendTime:        now,
 			})
 			Expect(handler.SendMode()).To(Equal(SendAny))
 			handler.SentPacket(&Packet{
@@ -904,7 +908,7 @@ var _ = Describe("SentPacketHandler", func() {
 				Length:          1,
 				EncryptionLevel: protocol.EncryptionInitial,
 				Frames:          []Frame{{Frame: &wire.PingFrame{}}},
-				SendTime:        time.Now(),
+				SendTime:        now,
 			})
 			Expect(handler.SendMode()).To(Equal(SendNone))
 		})