[Unified Plan] If "a=msid" is missing, create default stream.

Prior to this CL, if the "a=msid" attribute was missing it was treated
the same as if "no streams" were explicitly signaled (a=msid:-); the
receivers would not be associated with any streams.

In order to support legacy endpoints that don't recognize "a=msid" that
assume the Plan B behavior of a stream being created anyway, this CL
creates a stream with a random ID in such cases. For background, see
https://github.com/web-platform-tests/wpt/pull/14054.

Bug: chromium:907508
Change-Id: I9d9dd0e4ba8f9941f8652f4d7873adc560777cd9
Reviewed-on: https://webrtc-review.googlesource.com/c/112900
Reviewed-by: Seth Hampson <shampson@webrtc.org>
Commit-Queue: Henrik Boström <hbos@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25901}

api/rtpreceiverinterface.h

@@ -146,6 +146,7 @@ class RtpReceiverInterface : public rtc::RefCountInterface {
 BEGIN_SIGNALING_PROXY_MAP(RtpReceiver)
 PROXY_SIGNALING_THREAD_DESTRUCTOR()
 PROXY_CONSTMETHOD0(rtc::scoped_refptr<MediaStreamTrackInterface>, track)
+PROXY_CONSTMETHOD0(std::vector<std::string>, stream_ids)
 PROXY_CONSTMETHOD0(std::vector<rtc::scoped_refptr<MediaStreamInterface>>,
                    streams)
 PROXY_CONSTMETHOD0(cricket::MediaType, media_type)

pc/peerconnection.cc

@@ -2468,6 +2468,18 @@ RTCError PeerConnection::ApplyRemoteDescription(
           }
           media_streams.push_back(stream);
         }
+        // Special case: "a=msid" missing, use random stream ID.
+        if (media_streams.empty() &&
+            !(remote_description()->description()->msid_signaling() &
+              cricket::kMsidSignalingMediaSection)) {
+          if (!missing_msid_default_stream_) {
+            missing_msid_default_stream_ = MediaStreamProxy::Create(
+                rtc::Thread::Current(),
+                MediaStream::Create(rtc::CreateRandomUuid()));
+            added_streams.push_back(missing_msid_default_stream_);
+          }
+          media_streams.push_back(missing_msid_default_stream_);
+        }
         // This will add the remote track to the streams.
         // TODO(hbos): When we remove remote_streams(), use set_stream_ids()
         // instead. https://crbug.com/webrtc/9480

pc/peerconnection.h

@@ -1033,6 +1033,11 @@ class PeerConnection : public PeerConnectionInternal,
   std::vector<
       rtc::scoped_refptr<RtpTransceiverProxyWithInternal<RtpTransceiver>>>
       transceivers_;
+  // In Unified Plan, if we encounter remote SDP that does not contain an a=msid
+  // line we create and use a stream with a random ID for our receivers. This is
+  // to support legacy endpoints that do not support the a=msid attribute (as
+  // opposed to streamless tracks with "a=msid:-").
+  rtc::scoped_refptr<MediaStreamInterface> missing_msid_default_stream_;
   // MIDs that have been seen either by SetLocalDescription or
   // SetRemoteDescription over the life of the PeerConnection.
   std::set<std::string> seen_mids_;

pc/peerconnection_integrationtest.cc

@@ -151,6 +151,7 @@ void RemoveSsrcsAndMsids(cricket::SessionDescription* desc) {
     content.media_description()->mutable_streams().clear();
   }
   desc->set_msid_supported(false);
+  desc->set_msid_signaling(0);
 }
 
 // Removes all stream information besides the stream ids, simulating an
@@ -2451,6 +2452,37 @@ TEST_F(PeerConnectionIntegrationTestUnifiedPlan,
   EXPECT_TRUE(ExpectNewFrames(media_expectations));
 }
 
+TEST_F(PeerConnectionIntegrationTestUnifiedPlan, NoStreamsMsidLinePresent) {
+  ASSERT_TRUE(CreatePeerConnectionWrappers());
+  ConnectFakeSignaling();
+  caller()->AddAudioTrack();
+  caller()->AddVideoTrack();
+  caller()->CreateAndSetAndSignalOffer();
+  ASSERT_TRUE_WAIT(SignalingStateStable(), kDefaultTimeout);
+  auto callee_receivers = callee()->pc()->GetReceivers();
+  ASSERT_EQ(2u, callee_receivers.size());
+  EXPECT_TRUE(callee_receivers[0]->stream_ids().empty());
+  EXPECT_TRUE(callee_receivers[1]->stream_ids().empty());
+}
+
+TEST_F(PeerConnectionIntegrationTestUnifiedPlan, NoStreamsMsidLineMissing) {
+  ASSERT_TRUE(CreatePeerConnectionWrappers());
+  ConnectFakeSignaling();
+  caller()->AddAudioTrack();
+  caller()->AddVideoTrack();
+  callee()->SetReceivedSdpMunger(RemoveSsrcsAndMsids);
+  caller()->CreateAndSetAndSignalOffer();
+  ASSERT_TRUE_WAIT(SignalingStateStable(), kDefaultTimeout);
+  auto callee_receivers = callee()->pc()->GetReceivers();
+  ASSERT_EQ(2u, callee_receivers.size());
+  ASSERT_EQ(1u, callee_receivers[0]->stream_ids().size());
+  ASSERT_EQ(1u, callee_receivers[1]->stream_ids().size());
+  EXPECT_EQ(callee_receivers[0]->stream_ids()[0],
+            callee_receivers[1]->stream_ids()[0]);
+  EXPECT_EQ(callee_receivers[0]->streams()[0],
+            callee_receivers[1]->streams()[0]);
+}
+
 // Test that if two video tracks are sent (from caller to callee, in this test),
 // they're transmitted correctly end-to-end.
 TEST_P(PeerConnectionIntegrationTest, EndToEndCallWithTwoVideoTracks) {

pc/webrtcsdp_unittest.cc

@@ -2501,6 +2501,8 @@ TEST_F(WebRtcSdpTest, DeserializeSessionDescriptionWithoutMsid) {
   EXPECT_TRUE(SdpDeserialize(sdp_without_msid, &jdesc));
   // Verify
   EXPECT_TRUE(CompareSessionDescription(jdesc_, jdesc));
+  EXPECT_FALSE(jdesc.description()->msid_signaling() &
+               ~cricket::kMsidSignalingSsrcAttribute);
 }
 
 TEST_F(WebRtcSdpTest, DeserializeSessionDescriptionWithExtmapAllowMixed) {
@@ -3497,6 +3499,9 @@ TEST_F(WebRtcSdpTest, DeserializeSessionDescriptionSpecialMsid) {
                              &deserialized_description));
 
   EXPECT_TRUE(CompareSessionDescription(jdesc_, deserialized_description));
+  EXPECT_EQ(cricket::kMsidSignalingMediaSection |
+                cricket::kMsidSignalingSsrcAttribute,
+            deserialized_description.description()->msid_signaling());
 }
 
 // Tests the serialization of a Unified Plan SDP that is compatible for both