Do not use LastTS for dummy offset. (livekit#2700)

* Do not use LastTS for dummy offset.

LastTS could be random when using dummy start. That should not be used
in calculating offsets.

Also, do not push padding into sequence before init. Could have heppened
with dummy start.

* apply dummy offset before comparing to last

* refresh ref TS

* initialize codec munger on catch up forwarding

pkg/sfu/forwarder.go

@@ -1589,6 +1589,7 @@ func (f *Forwarder) processSourceSwitch(extPkt *buffer.ExtPacket, layer int32) e
 		return nil
 	} else if f.referenceLayerSpatial == buffer.InvalidLayerSpatial {
 		f.referenceLayerSpatial = layer
+		f.codecMunger.SetLast(extPkt)
 		f.logger.Debugw(
 			"catch up forwarding",
 			"sequenceNumber", extPkt.Packet.SequenceNumber,
@@ -1641,8 +1642,9 @@ func (f *Forwarder) processSourceSwitch(extPkt *buffer.ExtPacket, layer int32) e
 	}
 
 	// adjust extRefTS to current packet's timestamp mapped to that of reference layer's
-	extRefTS = (extRefTS & 0xFFFF_FFFF_0000_0000) + uint64(refTS)
+	extRefTS = (extRefTS & 0xFFFF_FFFF_0000_0000) + uint64(refTS) + f.dummyStartTSOffset
 	lastTS := uint32(extLastTS)
+	refTS = uint32(extRefTS)
 	if (refTS-lastTS) < 1<<31 && refTS < lastTS {
 		extRefTS += (1 << 32)
 	}
@@ -1660,21 +1662,23 @@ func (f *Forwarder) processSourceSwitch(extPkt *buffer.ExtPacket, layer int32) e
 				rtpDiff := uint64(timeSinceFirst.Nanoseconds() * int64(f.codec.ClockRate) / 1e9)
 				extExpectedTS = f.extFirstTS + rtpDiff
 				if f.dummyStartTSOffset == 0 {
-					f.dummyStartTSOffset = extExpectedTS - extRefTS
+					f.dummyStartTSOffset = extExpectedTS - uint64(refTS)
+					extRefTS = extExpectedTS
 					f.logger.Infow(
 						"calculating dummyStartTSOffset",
 						"preStartTime", f.preStartTime.String(),
 						"extFirstTS", f.extFirstTS,
-						"timeSinceFirst", timeSinceFirst,
+						"timeSinceFirst", timeSinceFirst.String(),
 						"rtpDiff", rtpDiff,
 						"extRefTS", extRefTS,
+						"incomingTS", extPkt.Packet.Timestamp,
+						"referenceLayerSpatial", f.referenceLayerSpatial,
 						"dummyStartTSOffset", f.dummyStartTSOffset,
 					)
 				}
 			}
 		}
 	}
-	extRefTS += f.dummyStartTSOffset
 
 	var extNextTS uint64
 	if f.lastSSRC == 0 {
@@ -1759,6 +1763,7 @@ func (f *Forwarder) processSourceSwitch(extPkt *buffer.ExtPacket, layer int32) e
 		"switchingAt", switchingAt.String(),
 		"layer", layer,
 		"extLastTS", extLastTS,
+		"lastMarker", rtpMungerState.LastMarker,
 		"extRefTS", extRefTS,
 		"dummyStartTSOffset", f.dummyStartTSOffset,
 		"referenceLayerSpatial", f.referenceLayerSpatial,

pkg/sfu/sequencer.go

@@ -237,7 +237,7 @@ func (s *sequencer) pushPadding(extStartSNInclusive uint64, extEndSNInclusive ui
 	s.Lock()
 	defer s.Unlock()
 
-	if s.snRangeMap == nil {
+	if s.snRangeMap == nil || !s.initialized {
 		return
 	}