fix rtx payload type remapping issue

If we reference a remote codec that was offered without rtx we need
to change the reference of our local rtx codec in a reoffer.
This only happens with stopped transceivers.

BUG=chromium:1201441

Change-Id: I60dc4908ba7ba2c0249a2a05b03a7b35af504e18
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/238382
Commit-Queue: Philipp Hancke <philipp.hancke@googlemail.com>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35487}

pc/media_session.cc

@@ -2381,6 +2381,22 @@ bool MediaSessionDescriptionFactory::AddVideoContentForOffer(
                                          filtered_codecs, codec, nullptr)) {
         // Use the `found_codec` from `video_codecs` because it has the
         // correctly mapped payload type.
+        if (IsRtxCodec(codec)) {
+          // For RTX we might need to adjust the apt parameter if we got a
+          // remote offer without RTX for a codec for which we support RTX.
+          auto referenced_codec =
+              GetAssociatedCodecForRtx(supported_video_codecs, codec);
+          RTC_DCHECK(referenced_codec);
+
+          // Find the codec we should be referencing and point to it.
+          VideoCodec changed_referenced_codec;
+          if (FindMatchingCodec<VideoCodec>(supported_video_codecs,
+                                            filtered_codecs, *referenced_codec,
+                                            &changed_referenced_codec)) {
+            found_codec.SetParam(kCodecParamAssociatedPayloadType,
+                                 changed_referenced_codec.id);
+          }
+        }
         filtered_codecs.push_back(found_codec);
       }
     }

pc/peer_connection_signaling_unittest.cc

@@ -1266,4 +1266,60 @@ TEST_F(PeerConnectionSignalingUnifiedPlanTest,
       callee->observer()->latest_negotiation_needed_event()));
 }
 
+TEST_F(PeerConnectionSignalingUnifiedPlanTest, RtxReofferApt) {
+  auto callee = CreatePeerConnection();
+
+  std::string sdp =
+      "v=0\r\n"
+      "o=- 8403615332048243445 2 IN IP4 127.0.0.1\r\n"
+      "s=-\r\n"
+      "t=0 0\r\n"
+      "m=video 9 UDP/TLS/RTP/SAVPF 102\r\n"
+      "c=IN IP4 0.0.0.0\r\n"
+      "a=rtcp:9 IN IP4 0.0.0.0\r\n"
+      "a=ice-ufrag:IZeV\r\n"
+      "a=ice-pwd:uaZhQD4rYM/Tta2qWBT1Bbt4\r\n"
+      "a=ice-options:trickle\r\n"
+      "a=fingerprint:sha-256 "
+      "D8:6C:3D:FA:23:E2:2C:63:11:2D:D0:86:BE:C4:D0:65:F9:42:F7:1C:06:04:27:E6:"
+      "1C:2C:74:01:8D:50:67:23\r\n"
+      "a=setup:actpass\r\n"
+      "a=mid:0\r\n"
+      "a=sendrecv\r\n"
+      "a=msid:stream track\r\n"
+      "a=rtcp-mux\r\n"
+      "a=rtcp-rsize\r\n"
+      "a=rtpmap:102 VP8/90000\r\n"
+      "a=rtcp-fb:102 goog-remb\r\n"
+      "a=rtcp-fb:102 transport-cc\r\n"
+      "a=rtcp-fb:102 ccm fir\r\n"
+      "a=rtcp-fb:102 nack\r\n"
+      "a=rtcp-fb:102 nack pli\r\n"
+      "a=ssrc:1224551896 cname:/exJcmhSLpyu9FgV\r\n";
+  std::unique_ptr<webrtc::SessionDescriptionInterface> remote_description =
+      webrtc::CreateSessionDescription(SdpType::kOffer, sdp, nullptr);
+
+  EXPECT_TRUE(callee->SetRemoteDescription(std::move(remote_description)));
+
+  auto answer = callee->CreateAnswer(RTCOfferAnswerOptions());
+  EXPECT_TRUE(
+      callee->SetLocalDescription(CloneSessionDescription(answer.get())));
+
+  callee->pc()->GetTransceivers()[0]->Stop();
+  auto reoffer = callee->CreateOffer(RTCOfferAnswerOptions());
+  auto codecs = reoffer->description()
+                    ->contents()[0]
+                    .media_description()
+                    ->as_video()
+                    ->codecs();
+  ASSERT_GT(codecs.size(), 2u);
+  EXPECT_EQ(codecs[0].name, "VP8");
+  EXPECT_EQ(codecs[1].name, "rtx");
+  auto apt_it = codecs[1].params.find("apt");
+  ASSERT_NE(apt_it, codecs[1].params.end());
+  // The apt should match the id from the remote offer.
+  EXPECT_EQ(apt_it->second, rtc::ToString(codecs[0].id));
+  EXPECT_EQ(apt_it->second, "102");
+}
+
 }  // namespace webrtc