Do not create receiver for ealy media in offerer

For offerer, if the remote side sends early media
before the remote description (answer) is received,
the undeclared SSRC processor can create a receiver
and that receiver could be left dangling as
transceiver `mid` is not updated from remote
description answer.

Still leaving the simulcast probe path and only
avoiding creating a receiver for non-simulcast path.

Add a flag `handleUndeclaredSSRCWithoutAnswer` to control handling
of early media without SDP answer for non-simulcast tracks.
The default behaviour is to not process early media without SDP answer.

errors.go

@@ -231,6 +231,10 @@ var (
 	errPeerConnSetIdentityProviderNotImplemented = errors.New("TODO SetIdentityProvider")
 	errPeerConnWriteRTCPOpenWriteStream          = errors.New("WriteRTCP failed to open WriteStream")
 	errPeerConnTranscieverMidNil                 = errors.New("cannot find transceiver with mid")
+	errPeerConnEarlyMediaWithoutAnswer           = errors.New(
+		"cannot process early media without SDP answer," +
+			"use SettingEngine.SetHandleUndeclaredSSRCWithoutAnswer(true) to process without answer",
+	)
 
 	errRTPReceiverDTLSTransportNil            = errors.New("DTLSTransport must not be nil")
 	errRTPReceiverReceiveAlreadyCalled        = errors.New("Receive has already been called")

peerconnection.go

@@ -1719,10 +1719,12 @@ func (pc *PeerConnection) handleIncomingSSRC(rtpStream io.Reader, ssrc SSRC) err
 	// (urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id and urn:ietf:params:rtp-hdrext:sdes:mid)
 	// and even if the RTP stream contains an incorrect MID or RID.
 	// while this can be incorrect, this is done to maintain compatibility with older behavior.
-	if len(remoteDescription.parsed.MediaDescriptions) == 1 {
-		mediaSection := remoteDescription.parsed.MediaDescriptions[0]
-		if handled, err := pc.handleUndeclaredSSRC(ssrc, mediaSection); handled || err != nil {
-			return err
+	if remoteDescription.Type != SDPTypeAnswer || pc.api.settingEngine.handleUndeclaredSSRCWithoutAnswer {
+		if len(remoteDescription.parsed.MediaDescriptions) == 1 {
+			mediaSection := remoteDescription.parsed.MediaDescriptions[0]
+			if handled, err := pc.handleUndeclaredSSRC(ssrc, mediaSection); handled || err != nil {
+				return err
+			}
 		}
 	}
 
@@ -1748,6 +1750,11 @@ func (pc *PeerConnection) handleIncomingSSRC(rtpStream io.Reader, ssrc SSRC) err
 		RTPHeaderExtensionCapability{sdp.SDESMidURI},
 	)
 	if !audioSupported && !videoSupported {
+		if remoteDescription.Type == SDPTypeAnswer && !pc.api.settingEngine.handleUndeclaredSSRCWithoutAnswer {
+			// if we are offerer, wait for answer with media setion to process this SSRC
+			return errPeerConnEarlyMediaWithoutAnswer
+		}
+
 		// try to find media section by payload type as a last resort for legacy clients.
 		mediaSection, ok := pc.findMediaSectionByPayloadType(payloadType, remoteDescription)
 		if ok {

settingengine.go

@@ -109,6 +109,7 @@ type SettingEngine struct {
 	fireOnTrackBeforeFirstRTP                 bool
 	disableCloseByDTLS                        bool
 	dataChannelBlockWrite                     bool
+	handleUndeclaredSSRCWithoutAnswer         bool
 }
 
 func (e *SettingEngine) getSCTPMaxMessageSize() uint32 {
@@ -570,3 +571,9 @@ func (e *SettingEngine) SetFireOnTrackBeforeFirstRTP(fireOnTrackBeforeFirstRTP b
 func (e *SettingEngine) DisableCloseByDTLS(isEnabled bool) {
 	e.disableCloseByDTLS = isEnabled
 }
+
+// SetHandleUndeclaredSSRCWithoutAnswer controls if an SDP answer is required for
+// processing early media of non-simulcast tracks.
+func (e *SettingEngine) SetHandleUndeclaredSSRCWithoutAnswer(handleUndeclaredSSRCWithoutAnswer bool) {
+	e.handleUndeclaredSSRCWithoutAnswer = handleUndeclaredSSRCWithoutAnswer
+}