Separate reading remote_ssrc from using the rtp_config() getter.

`remote_ssrc` can be considered const while some other state represented
by rtp_config() can not and also is tied to a specific thread.
Separating access to these variables, makes moving things around easier.

Bug: webrtc:11993
Change-Id: I70aa000daab6174a401e01dca163213174e8f284
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/261316
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#36818}
diff --git a/call/audio_receive_stream.h b/call/audio_receive_stream.h
index 17691f7..b18e076 100644
--- a/call/audio_receive_stream.h
+++ b/call/audio_receive_stream.h
@@ -194,6 +194,11 @@
   // Returns current value of base minimum delay in milliseconds.
   virtual int GetBaseMinimumPlayoutDelayMs() const = 0;
 
+  // Synchronization source (stream identifier) to be received.
+  // This member will not change mid-stream and can be assumed to be const
+  // post initialization.
+  virtual uint32_t remote_ssrc() const = 0;
+
  protected:
   virtual ~AudioReceiveStream() {}
 };
diff --git a/call/call.cc b/call/call.cc
index 1a0357f..0703fd4 100644
--- a/call/call.cc
+++ b/call/call.cc
@@ -1245,16 +1245,17 @@
   // TODO(bugs.webrtc.org/11993): Unregister on the network thread.
   receive_stream_impl->UnregisterFromTransport();
 
-  RTC_DCHECK(receive_stream != nullptr);
-  const FlexfecReceiveStream::RtpConfig& rtp = receive_stream->rtp_config();
-  UnregisterReceiveStream(rtp.remote_ssrc);
+  auto ssrc = receive_stream_impl->remote_ssrc();
+  UnregisterReceiveStream(ssrc);
 
   // Remove all SSRCs pointing to the FlexfecReceiveStreamImpl to be
   // destroyed.
-  receive_side_cc_.GetRemoteBitrateEstimator(UseSendSideBwe(rtp))
-      ->RemoveStream(rtp.remote_ssrc);
+  receive_side_cc_
+      .GetRemoteBitrateEstimator(
+          UseSendSideBwe(receive_stream_impl->rtp_config()))
+      ->RemoveStream(ssrc);
 
-  delete receive_stream;
+  delete receive_stream_impl;
 }
 
 void Call::AddAdaptationResource(rtc::scoped_refptr<Resource> resource) {
diff --git a/call/flexfec_receive_stream_impl.cc b/call/flexfec_receive_stream_impl.cc
index a784481..6c8378d 100644
--- a/call/flexfec_receive_stream_impl.cc
+++ b/call/flexfec_receive_stream_impl.cc
@@ -204,7 +204,10 @@
 void FlexfecReceiveStreamImpl::SetRtpExtensions(
     std::vector<RtpExtension> extensions) {
   RTC_DCHECK_RUN_ON(&packet_sequence_checker_);
-  config_.rtp.extensions = std::move(extensions);
+  // TODO(tommi): Remove this cast once header extensions are managed outside
+  // of the config struct.
+  const_cast<std::vector<RtpExtension>&>(config_.rtp.extensions) =
+      std::move(extensions);
 }
 
 }  // namespace webrtc
diff --git a/call/flexfec_receive_stream_impl.h b/call/flexfec_receive_stream_impl.h
index c2407cd..c640ac6 100644
--- a/call/flexfec_receive_stream_impl.h
+++ b/call/flexfec_receive_stream_impl.h
@@ -61,12 +61,15 @@
   // ReceiveStream impl.
   void SetRtpExtensions(std::vector<RtpExtension> extensions) override;
   const RtpConfig& rtp_config() const override { return config_.rtp; }
+  uint32_t remote_ssrc() const { return config_.rtp.remote_ssrc; }
 
  private:
   RTC_NO_UNIQUE_ADDRESS SequenceChecker packet_sequence_checker_;
 
-  // Config. Mostly const, header extensions may change.
-  Config config_ RTC_GUARDED_BY(packet_sequence_checker_);
+  // Config. Mostly const, header extensions may change, which is an exception
+  // case that's specifically handled in `SetRtpExtensions`, which must be
+  // called on the `packet_sequence_checker` thread.
+  const Config config_;
 
   // Erasure code interfacing.
   const std::unique_ptr<FlexfecReceiver> receiver_;