fix twcc panic on packet lost (#668)

pkg/sfu/twcc/twcc.go

@@ -96,7 +96,9 @@ func (t *Responder) buildTransportCCPacket() rtcp.RawPacket {
 	sort.Slice(t.extInfo, func(i, j int) bool {
 		return t.extInfo[i].ExtTSN < t.extInfo[j].ExtTSN
 	})
-	tccPkts := make([]rtpExtInfo, 0, int(float64(len(t.extInfo))*1.2))
+	maxTccPktsLen := int(float64(len(t.extInfo)) * 1.2)
+	tccPkts := make([]rtpExtInfo, 0, maxTccPktsLen)
+	var consumedExtInfo int
 	for _, tccExtInfo := range t.extInfo {
 		if tccExtInfo.ExtTSN < t.lastExtSN {
 			continue
@@ -108,8 +110,16 @@ func (t *Responder) buildTransportCCPacket() rtcp.RawPacket {
 		}
 		t.lastExtSN = tccExtInfo.ExtTSN
 		tccPkts = append(tccPkts, tccExtInfo)
+		consumedExtInfo++
+		if len(tccPkts) >= maxTccPktsLen {
+			break
+		}
+	}
+	if consumedExtInfo == len(t.extInfo) {
+		t.extInfo = t.extInfo[:0]
+	} else {
+		t.extInfo = t.extInfo[consumedExtInfo:]
 	}
-	t.extInfo = t.extInfo[:0]
 
 	firstRecv := false
 	same := true

pkg/sfu/twcc/twcc_test.go

@@ -344,3 +344,15 @@ func BenchmarkBuildPacket(b *testing.B) {
 		_ = twcc.buildTransportCCPacket()
 	}
 }
+
+func TestTccWithPacketLost(t *testing.T) {
+	twcc := NewTransportWideCCResponder(123)
+	var fbreceived int
+	twcc.OnFeedback(func(p rtcp.RawPacket) { fbreceived++ })
+
+	for i := 0; i < 200; i++ {
+		twcc.Push(10000+uint16(i*70), time.Now().UnixNano()+int64(i)*1e6, false)
+	}
+
+	assert.Greater(t, fbreceived, 0)
+}