Add an end-to-end test for the NACK sender

Test that NACKs are negotiated correctly, and that we receive
the expected NACK if we negotiated it.

interceptor_test.go

@@ -9,12 +9,14 @@ package webrtc
 //
 import (
 	"context"
+	"io"
 	"sync/atomic"
 	"testing"
 	"time"
 
 	"github.com/pion/interceptor"
 	mock_interceptor "github.com/pion/interceptor/pkg/mock"
+	"github.com/pion/rtcp"
 	"github.com/pion/rtp"
 	"github.com/pion/transport/v3/test"
 	"github.com/pion/webrtc/v4/pkg/media"
@@ -284,3 +286,155 @@ func Test_Interceptor_ZeroSSRC(t *testing.T) {
 	<-probeReceiverCreated
 	closePairNow(t, offerer, answerer)
 }
+
+// TestInterceptorNack is an end-to-end test for the NACK sender.
+// It test that:
+//   - we get a NACK if we negotiated generic NACks;
+//   - we don't get a NACK if we did not negotiate generick NACKs;
+//   - the NACK corresponds to the missing packet.
+func TestInterceptorNack(t *testing.T) {
+	to := test.TimeOut(time.Second * 20)
+	defer to.Stop()
+
+	t.Run("Nack", func(t *testing.T) { testInterceptorNack(t, true) })
+	t.Run("NoNack", func(t *testing.T) { testInterceptorNack(t, false) })
+}
+
+func testInterceptorNack(t *testing.T, requestNack bool) {
+	const numPackets = 20
+
+	ir := interceptor.Registry{}
+	m := MediaEngine{}
+	var capability []RTCPFeedback
+	if requestNack {
+		capability = append(capability, RTCPFeedback{"nack", ""})
+	}
+	err := m.RegisterCodec(
+		RTPCodecParameters{
+			RTPCodecCapability: RTPCodecCapability{
+				"video/VP8", 90000, 0,
+				"",
+				capability,
+			},
+			PayloadType: 96,
+		},
+		RTPCodecTypeVideo,
+	)
+	assert.NoError(t, err)
+	api := NewAPI(
+		WithMediaEngine(&m),
+		WithInterceptorRegistry(&ir),
+	)
+
+	pc1, err := api.NewPeerConnection(Configuration{})
+	assert.NoError(t, err)
+	defer func() {
+		//nolint
+		err := pc1.Close()
+		assert.NoError(t, err)
+	}()
+
+	track1, err := NewTrackLocalStaticRTP(
+		RTPCodecCapability{MimeType: MimeTypeVP8},
+		"video", "pion",
+	)
+	assert.NoError(t, err)
+	sender, err := pc1.AddTrack(track1)
+	assert.NoError(t, err)
+
+	pc2, err := NewPeerConnection(Configuration{})
+	assert.NoError(t, err)
+	defer func() {
+		// nolint
+		err := pc2.Close()
+		assert.NoError(t, err)
+	}()
+
+	offer, err := pc1.CreateOffer(nil)
+	assert.NoError(t, err)
+	err = pc1.SetLocalDescription(offer)
+	assert.NoError(t, err)
+	<-GatheringCompletePromise(pc1)
+
+	err = pc2.SetRemoteDescription(*pc1.LocalDescription())
+	assert.NoError(t, err)
+	answer, err := pc2.CreateAnswer(nil)
+	assert.NoError(t, err)
+	err = pc2.SetLocalDescription(answer)
+	assert.NoError(t, err)
+	<-GatheringCompletePromise(pc2)
+
+	err = pc1.SetRemoteDescription(*pc2.LocalDescription())
+	assert.NoError(t, err)
+
+	var gotNack atomic.Bool
+	go func() {
+		buf := make([]byte, 1500)
+		for {
+			n, _, err := sender.Read(buf)
+			// nolint
+			if err == io.EOF {
+				break
+			}
+			assert.NoError(t, err)
+			ps, err := rtcp.Unmarshal(buf[:n])
+			assert.NoError(t, err)
+			for _, p := range ps {
+				if pn, ok := p.(*rtcp.TransportLayerNack); ok {
+					assert.Equal(t, len(pn.Nacks), 1)
+					assert.Equal(t,
+						pn.Nacks[0].PacketID, uint16(1),
+					)
+					assert.Equal(t,
+						pn.Nacks[0].LostPackets,
+						rtcp.PacketBitmap(0),
+					)
+					gotNack.Store(true)
+				}
+			}
+		}
+	}()
+
+	done := make(chan struct{})
+	pc2.OnTrack(func(track2 *TrackRemote, _ *RTPReceiver) {
+		for i := 0; i < numPackets; i++ {
+			if i == 1 {
+				continue
+			}
+			p, _, err := track2.ReadRTP()
+			assert.NoError(t, err)
+			assert.Equal(t, p.SequenceNumber, uint16(i))
+		}
+		done <- struct{}{}
+	})
+
+	go func() {
+		for i := 0; i < numPackets; i++ {
+			time.Sleep(20 * time.Millisecond)
+			if i == 1 {
+				continue
+			}
+			var p rtp.Packet
+			p.Version = 2
+			p.Marker = true
+			p.PayloadType = 96
+			p.SequenceNumber = uint16(i)
+			p.Timestamp = uint32(i * 90000 / 50)
+			p.Payload = []byte{42}
+			err := track1.WriteRTP(&p)
+			assert.NoError(t, err)
+		}
+	}()
+
+	<-done
+
+	if requestNack {
+		if !gotNack.Load() {
+			t.Errorf("Expected to get a NACK, got none")
+		}
+	} else {
+		if gotNack.Load() {
+			t.Errorf("Expected to get no NACK, got one")
+		}
+	}
+}