Do all BaseChannel operations within a single Thread::Invoke.

Instead of doing a separate Invoke for each channel, this CL first
gathers a list of operations to be performed on the signaling thread,
then does a single Invoke on the worker thread (and nested Invoke
on the network thread) to update all channels at once.

This includes the methods:
* Enable
* SetLocalContent/SetRemoteContent
* RegisterRtpDemuxerSink
* UpdateRtpHeaderExtensionMap

Also, removed the need for a network thread Invoke in
IsReadyToSendMedia_w by moving ownership of was_ever_writable_ to the
worker thread.

Bug: webrtc:12266
Change-Id: I31e61fe0758aeb053b09db84f234deb58dfb3d05
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/194181
Commit-Queue: Taylor <deadbeef@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32817}
diff --git a/pc/channel.cc b/pc/channel.cc
index aad7c54..34269a1 100644
--- a/pc/channel.cc
+++ b/pc/channel.cc
@@ -170,7 +170,9 @@
 bool BaseChannel::ConnectToRtpTransport() {
   RTC_DCHECK_RUN_ON(network_thread());
   RTC_DCHECK(rtp_transport_);
-  if (!RegisterRtpDemuxerSink_n()) {
+  // TODO(bugs.webrtc.org/12230): This accesses demuxer_criteria_ on the
+  // networking thread.
+  if (!rtp_transport_->RegisterRtpDemuxerSink(demuxer_criteria_, this)) {
     RTC_LOG(LS_ERROR) << "Failed to set up demuxing for " << ToString();
     return false;
   }
@@ -291,13 +293,42 @@
       Bind(&BaseChannel::SetRemoteContent_w, this, content, type, error_desc));
 }
 
-bool BaseChannel::SetPayloadTypeDemuxingEnabled(bool enabled) {
+void BaseChannel::SetPayloadTypeDemuxingEnabled(bool enabled) {
   TRACE_EVENT0("webrtc", "BaseChannel::SetPayloadTypeDemuxingEnabled");
-  return InvokeOnWorker<bool>(
+  InvokeOnWorker<void>(
       RTC_FROM_HERE,
       Bind(&BaseChannel::SetPayloadTypeDemuxingEnabled_w, this, enabled));
 }
 
+bool BaseChannel::UpdateRtpTransport(std::string* error_desc) {
+  return network_thread_->Invoke<bool>(RTC_FROM_HERE, [this, error_desc] {
+    RTC_DCHECK_RUN_ON(network_thread());
+    RTC_DCHECK(rtp_transport_);
+    // TODO(bugs.webrtc.org/12230): This accesses demuxer_criteria_ on the
+    // networking thread.
+    if (!rtp_transport_->RegisterRtpDemuxerSink(demuxer_criteria_, this)) {
+      RTC_LOG(LS_ERROR) << "Failed to set up demuxing for " << ToString();
+      rtc::StringBuilder desc;
+      desc << "Failed to set up demuxing for m-section with mid='"
+           << content_name() << "'.";
+      SafeSetError(desc.str(), error_desc);
+      return false;
+    }
+    // NOTE: This doesn't take the BUNDLE case in account meaning the RTP header
+    // extension maps are not merged when BUNDLE is enabled. This is fine
+    // because the ID for MID should be consistent among all the RTP transports,
+    // and that's all RtpTransport uses this map for.
+    //
+    // TODO(deadbeef): Move this call to JsepTransport, there is no reason
+    // BaseChannel needs to be involved here.
+    if (media_type() != cricket::MEDIA_TYPE_DATA) {
+      rtp_transport_->UpdateRtpHeaderExtensionMap(
+          receive_rtp_header_extensions_);
+    }
+    return true;
+  });
+}
+
 bool BaseChannel::IsReadyToReceiveMedia_w() const {
   // Receive data if we are enabled and have local content,
   return enabled() &&
@@ -305,12 +336,6 @@
 }
 
 bool BaseChannel::IsReadyToSendMedia_w() const {
-  // Need to access some state updated on the network thread.
-  return network_thread_->Invoke<bool>(
-      RTC_FROM_HERE, Bind(&BaseChannel::IsReadyToSendMedia_n, this));
-}
-
-bool BaseChannel::IsReadyToSendMedia_n() const {
   // Send outgoing data if we are enabled, have local and remote content,
   // and we have had some form of connectivity.
   return enabled() &&
@@ -508,38 +533,6 @@
       });
 }
 
-void BaseChannel::UpdateRtpHeaderExtensionMap(
-    const RtpHeaderExtensions& header_extensions) {
-  // Update the header extension map on network thread in case there is data
-  // race.
-  //
-  // NOTE: This doesn't take the BUNDLE case in account meaning the RTP header
-  // extension maps are not merged when BUNDLE is enabled. This is fine because
-  // the ID for MID should be consistent among all the RTP transports.
-  network_thread_->Invoke<void>(RTC_FROM_HERE, [this, &header_extensions] {
-    RTC_DCHECK_RUN_ON(network_thread());
-    rtp_transport_->UpdateRtpHeaderExtensionMap(header_extensions);
-  });
-}
-
-bool BaseChannel::RegisterRtpDemuxerSink_w() {
-  // Copy demuxer criteria, since they're a worker-thread variable
-  // and we want to pass them to the network thread
-  return network_thread_->Invoke<bool>(
-      RTC_FROM_HERE, [this, demuxer_criteria = demuxer_criteria_] {
-        RTC_DCHECK_RUN_ON(network_thread());
-        RTC_DCHECK(rtp_transport_);
-        return rtp_transport_->RegisterRtpDemuxerSink(demuxer_criteria, this);
-      });
-}
-
-bool BaseChannel::RegisterRtpDemuxerSink_n() {
-  RTC_DCHECK(rtp_transport_);
-  // TODO(bugs.webrtc.org/12230): This accesses demuxer_criteria_ on the
-  // networking thread.
-  return rtp_transport_->RegisterRtpDemuxerSink(demuxer_criteria_, this);
-}
-
 void BaseChannel::EnableMedia_w() {
   RTC_DCHECK(worker_thread_ == rtc::Thread::Current());
   if (enabled_)
@@ -573,22 +566,28 @@
   if (writable_) {
     return;
   }
+  writable_ = true;
 
   RTC_LOG(LS_INFO) << "Channel writable (" << ToString() << ")"
-                   << (was_ever_writable_ ? "" : " for the first time");
-
-  was_ever_writable_ = true;
-  writable_ = true;
-  UpdateMediaSendRecvState();
+                   << (was_ever_writable_n_ ? "" : " for the first time");
+  // We only have to do this AsyncInvoke once, when first transitioning to
+  // writable.
+  if (!was_ever_writable_n_) {
+    invoker_.AsyncInvoke<void>(RTC_FROM_HERE, worker_thread_, [this] {
+      RTC_DCHECK_RUN_ON(worker_thread());
+      was_ever_writable_ = true;
+      UpdateMediaSendRecvState_w();
+    });
+  }
+  was_ever_writable_n_ = true;
 }
 
 void BaseChannel::ChannelNotWritable_n() {
-  if (!writable_)
+  if (!writable_) {
     return;
-
-  RTC_LOG(LS_INFO) << "Channel not writable (" << ToString() << ")";
+  }
   writable_ = false;
-  UpdateMediaSendRecvState();
+  RTC_LOG(LS_INFO) << "Channel not writable (" << ToString() << ")";
 }
 
 bool BaseChannel::AddRecvStream_w(const StreamParams& sp) {
@@ -604,9 +603,9 @@
   media_channel()->ResetUnsignaledRecvStream();
 }
 
-bool BaseChannel::SetPayloadTypeDemuxingEnabled_w(bool enabled) {
+void BaseChannel::SetPayloadTypeDemuxingEnabled_w(bool enabled) {
   if (enabled == payload_type_demuxing_enabled_) {
-    return true;
+    return;
   }
   payload_type_demuxing_enabled_ = enabled;
   if (!enabled) {
@@ -617,21 +616,10 @@
     // there is no straightforward way to identify those streams.
     media_channel()->ResetUnsignaledRecvStream();
     demuxer_criteria_.payload_types.clear();
-    if (!RegisterRtpDemuxerSink_w()) {
-      RTC_LOG(LS_ERROR) << "Failed to disable payload type demuxing for "
-                        << ToString();
-      return false;
-    }
   } else if (!payload_types_.empty()) {
     demuxer_criteria_.payload_types.insert(payload_types_.begin(),
                                            payload_types_.end());
-    if (!RegisterRtpDemuxerSink_w()) {
-      RTC_LOG(LS_ERROR) << "Failed to enable payload type demuxing for "
-                        << ToString();
-      return false;
-    }
   }
-  return true;
 }
 
 bool BaseChannel::UpdateLocalStreams_w(const std::vector<StreamParams>& streams,
@@ -772,11 +760,6 @@
     demuxer_criteria_.ssrcs.insert(new_stream.ssrcs.begin(),
                                    new_stream.ssrcs.end());
   }
-  // Re-register the sink to update the receiving ssrcs.
-  if (!RegisterRtpDemuxerSink_w()) {
-    RTC_LOG(LS_ERROR) << "Failed to set up demuxing for " << ToString();
-    ret = false;
-  }
   remote_streams_ = streams;
   return ret;
 }
@@ -795,6 +778,10 @@
   return webrtc::RtpExtension::FilterDuplicateNonEncrypted(extensions);
 }
 
+void BaseChannel::SetReceiveExtensions(const RtpHeaderExtensions& extensions) {
+  receive_rtp_header_extensions_ = extensions;
+}
+
 void BaseChannel::OnMessage(rtc::Message* pmsg) {
   TRACE_EVENT0("webrtc", "BaseChannel::OnMessage");
   switch (pmsg->message_id) {
@@ -873,12 +860,6 @@
   Deinit();
 }
 
-void BaseChannel::UpdateMediaSendRecvState() {
-  RTC_DCHECK_RUN_ON(network_thread());
-  invoker_.AsyncInvoke<void>(RTC_FROM_HERE, worker_thread_,
-                             [this] { UpdateMediaSendRecvState_w(); });
-}
-
 void VoiceChannel::Init_w(webrtc::RtpTransportInternal* rtp_transport) {
   BaseChannel::Init_w(rtp_transport);
 }
@@ -916,7 +897,7 @@
 
   RtpHeaderExtensions rtp_header_extensions =
       GetFilteredRtpHeaderExtensions(audio->rtp_header_extensions());
-  UpdateRtpHeaderExtensionMap(rtp_header_extensions);
+  SetReceiveExtensions(rtp_header_extensions);
   media_channel()->SetExtmapAllowMixed(audio->extmap_allow_mixed());
 
   AudioRecvParameters recv_params = last_recv_params_;
@@ -936,11 +917,6 @@
     for (const AudioCodec& codec : audio->codecs()) {
       MaybeAddHandledPayloadType(codec.id);
     }
-    // Need to re-register the sink to update the handled payload.
-    if (!RegisterRtpDemuxerSink_w()) {
-      RTC_LOG(LS_ERROR) << "Failed to set up audio demuxing for " << ToString();
-      return false;
-    }
   }
 
   last_recv_params_ = recv_params;
@@ -1003,10 +979,6 @@
                             "disable payload type demuxing for "
                          << ToString();
     ClearHandledPayloadTypes();
-    if (!RegisterRtpDemuxerSink_w()) {
-      RTC_LOG(LS_ERROR) << "Failed to update audio demuxing for " << ToString();
-      return false;
-    }
   }
 
   // TODO(pthatcher): Move remote streams into AudioRecvParameters,
@@ -1087,7 +1059,7 @@
 
   RtpHeaderExtensions rtp_header_extensions =
       GetFilteredRtpHeaderExtensions(video->rtp_header_extensions());
-  UpdateRtpHeaderExtensionMap(rtp_header_extensions);
+  SetReceiveExtensions(rtp_header_extensions);
   media_channel()->SetExtmapAllowMixed(video->extmap_allow_mixed());
 
   VideoRecvParameters recv_params = last_recv_params_;
@@ -1130,11 +1102,6 @@
     for (const VideoCodec& codec : video->codecs()) {
       MaybeAddHandledPayloadType(codec.id);
     }
-    // Need to re-register the sink to update the handled payload.
-    if (!RegisterRtpDemuxerSink_w()) {
-      RTC_LOG(LS_ERROR) << "Failed to set up video demuxing for " << ToString();
-      return false;
-    }
   }
 
   last_recv_params_ = recv_params;
@@ -1241,10 +1208,6 @@
                             "disable payload type demuxing for "
                          << ToString();
     ClearHandledPayloadTypes();
-    if (!RegisterRtpDemuxerSink_w()) {
-      RTC_LOG(LS_ERROR) << "Failed to update video demuxing for " << ToString();
-      return false;
-    }
   }
 
   // TODO(pthatcher): Move remote streams into VideoRecvParameters,
@@ -1355,11 +1318,6 @@
   for (const DataCodec& codec : data->codecs()) {
     MaybeAddHandledPayloadType(codec.id);
   }
-  // Need to re-register the sink to update the handled payload.
-  if (!RegisterRtpDemuxerSink_w()) {
-    RTC_LOG(LS_ERROR) << "Failed to set up data demuxing for " << ToString();
-    return false;
-  }
 
   last_recv_params_ = recv_params;
 
diff --git a/pc/channel.h b/pc/channel.h
index 1fb2a39..0ba23eb 100644
--- a/pc/channel.h
+++ b/pc/channel.h
@@ -119,9 +119,6 @@
     RTC_DCHECK_RUN_ON(network_thread());
     return srtp_active();
   }
-
-  bool writable() const { return writable_; }
-
   // Set an RTP level transport which could be an RtpTransport without
   // encryption, an SrtpTransport for SDES or a DtlsSrtpTransport for DTLS-SRTP.
   // This can be called from any thread and it hops to the network thread
@@ -143,7 +140,8 @@
     return rtp_transport();
   }
 
-  // Channel control
+  // Channel control. Must call UpdateRtpTransport afterwards to apply any
+  // changes to the RtpTransport on the network thread.
   bool SetLocalContent(const MediaContentDescription* content,
                        webrtc::SdpType type,
                        std::string* error_desc) override;
@@ -158,7 +156,11 @@
   // This method will also remove any existing streams that were bound to this
   // channel on the basis of payload type, since one of these streams might
   // actually belong to a new channel. See: crbug.com/webrtc/11477
-  bool SetPayloadTypeDemuxingEnabled(bool enabled) override;
+  //
+  // As with SetLocalContent/SetRemoteContent, must call UpdateRtpTransport
+  // afterwards to apply changes to the RtpTransport on the network thread.
+  void SetPayloadTypeDemuxingEnabled(bool enabled) override;
+  bool UpdateRtpTransport(std::string* error_desc) override;
 
   bool Enable(bool enable) override;
 
@@ -198,7 +200,7 @@
 
  protected:
   bool was_ever_writable() const {
-    RTC_DCHECK_RUN_ON(network_thread());
+    RTC_DCHECK_RUN_ON(worker_thread());
     return was_ever_writable_;
   }
   void set_local_content_direction(webrtc::RtpTransceiverDirection direction) {
@@ -256,7 +258,7 @@
   bool AddRecvStream_w(const StreamParams& sp) RTC_RUN_ON(worker_thread());
   bool RemoveRecvStream_w(uint32_t ssrc) RTC_RUN_ON(worker_thread());
   void ResetUnsignaledRecvStream_w() RTC_RUN_ON(worker_thread());
-  bool SetPayloadTypeDemuxingEnabled_w(bool enabled)
+  void SetPayloadTypeDemuxingEnabled_w(bool enabled)
       RTC_RUN_ON(worker_thread());
   bool AddSendStream_w(const StreamParams& sp) RTC_RUN_ON(worker_thread());
   bool RemoveSendStream_w(uint32_t ssrc) RTC_RUN_ON(worker_thread());
@@ -264,7 +266,6 @@
   // Should be called whenever the conditions for
   // IsReadyToReceiveMedia/IsReadyToSendMedia are satisfied (or unsatisfied).
   // Updates the send/recv state of the media channel.
-  void UpdateMediaSendRecvState();
   virtual void UpdateMediaSendRecvState_w() = 0;
 
   bool UpdateLocalStreams_w(const std::vector<StreamParams>& streams,
@@ -286,6 +287,9 @@
   // non-encrypted and encrypted extension is present for the same URI.
   RtpHeaderExtensions GetFilteredRtpHeaderExtensions(
       const RtpHeaderExtensions& extensions);
+  // Set a list of RTP extensions we should prepare to receive on the next
+  // UpdateRtpTransport call.
+  void SetReceiveExtensions(const RtpHeaderExtensions& extensions);
 
   // From MessageHandler
   void OnMessage(rtc::Message* pmsg) override;
@@ -302,13 +306,6 @@
   void MaybeAddHandledPayloadType(int payload_type) RTC_RUN_ON(worker_thread());
 
   void ClearHandledPayloadTypes() RTC_RUN_ON(worker_thread());
-
-  void UpdateRtpHeaderExtensionMap(
-      const RtpHeaderExtensions& header_extensions);
-
-  bool RegisterRtpDemuxerSink_w() RTC_RUN_ON(worker_thread());
-  bool RegisterRtpDemuxerSink_n() RTC_RUN_ON(network_thread());
-
   // Return description of media channel to facilitate logging
   std::string ToString() const;
 
@@ -319,7 +316,6 @@
   void DisconnectFromRtpTransport();
   void SignalSentPacket_n(const rtc::SentPacket& sent_packet)
       RTC_RUN_ON(network_thread());
-  bool IsReadyToSendMedia_n() const RTC_RUN_ON(network_thread());
 
   rtc::Thread* const worker_thread_;
   rtc::Thread* const network_thread_;
@@ -344,10 +340,9 @@
       RTC_GUARDED_BY(network_thread());
   std::vector<std::pair<rtc::Socket::Option, int> > rtcp_socket_options_
       RTC_GUARDED_BY(network_thread());
-  // TODO(bugs.webrtc.org/12230): writable_ is accessed in tests
-  // outside of the network thread.
-  bool writable_ = false;
-  bool was_ever_writable_ RTC_GUARDED_BY(network_thread()) = false;
+  bool writable_ RTC_GUARDED_BY(network_thread()) = false;
+  bool was_ever_writable_n_ RTC_GUARDED_BY(network_thread()) = false;
+  bool was_ever_writable_ RTC_GUARDED_BY(worker_thread()) = false;
   const bool srtp_required_ = true;
   const webrtc::CryptoOptions crypto_options_;
 
@@ -371,9 +366,10 @@
 
   // Cached list of payload types, used if payload type demuxing is re-enabled.
   std::set<uint8_t> payload_types_ RTC_GUARDED_BY(worker_thread());
-  // TODO(bugs.webrtc.org/12239): Modified on worker thread, accessed
-  // on network thread in RegisterRtpDemuxerSink_n (called from Init_w)
+  // TODO(bugs.webrtc.org/12239): These two variables are modified on the worker
+  // thread, accessed on the network thread in UpdateRtpTransport.
   webrtc::RtpDemuxerCriteria demuxer_criteria_;
+  RtpHeaderExtensions receive_rtp_header_extensions_;
   // This generator is used to generate SSRCs for local streams.
   // This is needed in cases where SSRCs are not negotiated or set explicitly
   // like in Simulcast.
diff --git a/pc/channel_interface.h b/pc/channel_interface.h
index 68b6486..4580a2f 100644
--- a/pc/channel_interface.h
+++ b/pc/channel_interface.h
@@ -52,7 +52,8 @@
   virtual bool SetRemoteContent(const MediaContentDescription* content,
                                 webrtc::SdpType type,
                                 std::string* error_desc) = 0;
-  virtual bool SetPayloadTypeDemuxingEnabled(bool enabled) = 0;
+  virtual void SetPayloadTypeDemuxingEnabled(bool enabled) = 0;
+  virtual bool UpdateRtpTransport(std::string* error_desc) = 0;
 
   // Access to the local and remote streams that were set on the channel.
   virtual const std::vector<StreamParams>& local_streams() const = 0;
diff --git a/pc/channel_unittest.cc b/pc/channel_unittest.cc
index c407147..fb62b08 100644
--- a/pc/channel_unittest.cc
+++ b/pc/channel_unittest.cc
@@ -323,19 +323,26 @@
             fake_rtcp_packet_transport2_.get(), asymmetric);
       }
     });
+    // The transport becoming writable will asynchronously update the send state
+    // on the worker thread; since this test uses the main thread as the worker
+    // thread, we must process the message queue for this to occur.
+    WaitForThreads();
   }
 
   bool SendInitiate() {
     bool result = channel1_->SetLocalContent(&local_media_content1_,
-                                             SdpType::kOffer, NULL);
+                                             SdpType::kOffer, NULL) &&
+                  channel1_->UpdateRtpTransport(nullptr);
     if (result) {
       channel1_->Enable(true);
       result = channel2_->SetRemoteContent(&remote_media_content1_,
-                                           SdpType::kOffer, NULL);
+                                           SdpType::kOffer, NULL) &&
+               channel2_->UpdateRtpTransport(nullptr);
       if (result) {
         ConnectFakeTransports();
         result = channel2_->SetLocalContent(&local_media_content2_,
-                                            SdpType::kAnswer, NULL);
+                                            SdpType::kAnswer, NULL) &&
+                 channel2_->UpdateRtpTransport(nullptr);
       }
     }
     return result;
@@ -344,27 +351,32 @@
   bool SendAccept() {
     channel2_->Enable(true);
     return channel1_->SetRemoteContent(&remote_media_content2_,
-                                       SdpType::kAnswer, NULL);
+                                       SdpType::kAnswer, NULL) &&
+           channel1_->UpdateRtpTransport(nullptr);
   }
 
   bool SendOffer() {
     bool result = channel1_->SetLocalContent(&local_media_content1_,
-                                             SdpType::kOffer, NULL);
+                                             SdpType::kOffer, NULL) &&
+                  channel1_->UpdateRtpTransport(nullptr);
     if (result) {
       channel1_->Enable(true);
       result = channel2_->SetRemoteContent(&remote_media_content1_,
-                                           SdpType::kOffer, NULL);
+                                           SdpType::kOffer, NULL) &&
+               channel2_->UpdateRtpTransport(nullptr);
     }
     return result;
   }
 
   bool SendProvisionalAnswer() {
     bool result = channel2_->SetLocalContent(&local_media_content2_,
-                                             SdpType::kPrAnswer, NULL);
+                                             SdpType::kPrAnswer, NULL) &&
+                  channel2_->UpdateRtpTransport(nullptr);
     if (result) {
       channel2_->Enable(true);
       result = channel1_->SetRemoteContent(&remote_media_content2_,
-                                           SdpType::kPrAnswer, NULL);
+                                           SdpType::kPrAnswer, NULL) &&
+               channel1_->UpdateRtpTransport(nullptr);
       ConnectFakeTransports();
     }
     return result;
@@ -372,10 +384,12 @@
 
   bool SendFinalAnswer() {
     bool result = channel2_->SetLocalContent(&local_media_content2_,
-                                             SdpType::kAnswer, NULL);
+                                             SdpType::kAnswer, NULL) &&
+                  channel2_->UpdateRtpTransport(nullptr);
     if (result)
       result = channel1_->SetRemoteContent(&remote_media_content2_,
-                                           SdpType::kAnswer, NULL);
+                                           SdpType::kAnswer, NULL) &&
+               channel1_->UpdateRtpTransport(nullptr);
     return result;
   }
 
@@ -608,10 +622,12 @@
     CreateContent(0, kPcmuCodec, kH264Codec, &content1);
     content1.AddStream(stream1);
     EXPECT_TRUE(channel1_->SetLocalContent(&content1, SdpType::kOffer, NULL));
+    EXPECT_TRUE(channel1_->UpdateRtpTransport(nullptr));
     EXPECT_TRUE(channel1_->Enable(true));
     EXPECT_EQ(1u, media_channel1_->send_streams().size());
 
     EXPECT_TRUE(channel2_->SetRemoteContent(&content1, SdpType::kOffer, NULL));
+    EXPECT_TRUE(channel2_->UpdateRtpTransport(nullptr));
     EXPECT_EQ(1u, media_channel2_->recv_streams().size());
     ConnectFakeTransports();
 
@@ -619,8 +635,10 @@
     typename T::Content content2;
     CreateContent(0, kPcmuCodec, kH264Codec, &content2);
     EXPECT_TRUE(channel1_->SetRemoteContent(&content2, SdpType::kAnswer, NULL));
+    EXPECT_TRUE(channel1_->UpdateRtpTransport(nullptr));
     EXPECT_EQ(0u, media_channel1_->recv_streams().size());
     EXPECT_TRUE(channel2_->SetLocalContent(&content2, SdpType::kAnswer, NULL));
+    EXPECT_TRUE(channel2_->UpdateRtpTransport(nullptr));
     EXPECT_TRUE(channel2_->Enable(true));
     EXPECT_EQ(0u, media_channel2_->send_streams().size());
 
@@ -633,10 +651,12 @@
     CreateContent(0, kPcmuCodec, kH264Codec, &content3);
     content3.AddStream(stream2);
     EXPECT_TRUE(channel2_->SetLocalContent(&content3, SdpType::kOffer, NULL));
+    EXPECT_TRUE(channel2_->UpdateRtpTransport(nullptr));
     ASSERT_EQ(1u, media_channel2_->send_streams().size());
     EXPECT_EQ(stream2, media_channel2_->send_streams()[0]);
 
     EXPECT_TRUE(channel1_->SetRemoteContent(&content3, SdpType::kOffer, NULL));
+    EXPECT_TRUE(channel1_->UpdateRtpTransport(nullptr));
     ASSERT_EQ(1u, media_channel1_->recv_streams().size());
     EXPECT_EQ(stream2, media_channel1_->recv_streams()[0]);
 
@@ -644,9 +664,11 @@
     typename T::Content content4;
     CreateContent(0, kPcmuCodec, kH264Codec, &content4);
     EXPECT_TRUE(channel1_->SetLocalContent(&content4, SdpType::kAnswer, NULL));
+    EXPECT_TRUE(channel1_->UpdateRtpTransport(nullptr));
     EXPECT_EQ(0u, media_channel1_->send_streams().size());
 
     EXPECT_TRUE(channel2_->SetRemoteContent(&content4, SdpType::kAnswer, NULL));
+    EXPECT_TRUE(channel2_->UpdateRtpTransport(nullptr));
     EXPECT_EQ(0u, media_channel2_->recv_streams().size());
 
     SendCustomRtp2(kSsrc2, 0);
@@ -915,8 +937,6 @@
     EXPECT_FALSE(channel2_->SrtpActiveForTesting());
     EXPECT_TRUE(SendInitiate());
     WaitForThreads();
-    EXPECT_TRUE(channel1_->writable());
-    EXPECT_TRUE(channel2_->writable());
     EXPECT_TRUE(SendAccept());
     EXPECT_TRUE(channel1_->SrtpActiveForTesting());
     EXPECT_TRUE(channel2_->SrtpActiveForTesting());
diff --git a/pc/sdp_offer_answer.cc b/pc/sdp_offer_answer.cc
index fd697ce..f924c40 100644
--- a/pc/sdp_offer_answer.cc
+++ b/pc/sdp_offer_answer.cc
@@ -2473,11 +2473,6 @@
   // But all call-sites should be verifying this before calling us!
   RTC_DCHECK(session_error() == SessionError::kNone);
 
-  // If this is answer-ish we're ready to let media flow.
-  if (type == SdpType::kPrAnswer || type == SdpType::kAnswer) {
-    EnableSending();
-  }
-
   // Update the signaling state according to the specified state machine (see
   // https://w3c.github.io/webrtc-pc/#rtcsignalingstate-enum).
   if (type == SdpType::kOffer) {
@@ -4201,21 +4196,6 @@
   }
 }
 
-void SdpOfferAnswerHandler::EnableSending() {
-  RTC_DCHECK_RUN_ON(signaling_thread());
-  for (const auto& transceiver : transceivers()->List()) {
-    cricket::ChannelInterface* channel = transceiver->internal()->channel();
-    if (channel && !channel->enabled()) {
-      channel->Enable(true);
-    }
-  }
-
-  if (data_channel_controller()->rtp_data_channel() &&
-      !data_channel_controller()->rtp_data_channel()->enabled()) {
-    data_channel_controller()->rtp_data_channel()->Enable(true);
-  }
-}
-
 RTCError SdpOfferAnswerHandler::PushdownMediaDescription(
     SdpType type,
     cricket::ContentSource source) {
@@ -4225,15 +4205,13 @@
   RTC_DCHECK_RUN_ON(signaling_thread());
   RTC_DCHECK(sdesc);
 
-  if (!UpdatePayloadTypeDemuxingState(source)) {
-    // Note that this is never expected to fail, since RtpDemuxer doesn't return
-    // an error when changing payload type demux criteria, which is all this
-    // does.
-    LOG_AND_RETURN_ERROR(RTCErrorType::INTERNAL_ERROR,
-                         "Failed to update payload type demuxing state.");
-  }
+  // Gather lists of updates to be made on cricket channels on the signaling
+  // thread, before performing them all at once on the worker thread. Necessary
+  // due to threading restrictions.
+  auto payload_type_demuxing_updates = GetPayloadTypeDemuxingUpdates(source);
+  std::vector<ContentUpdate> content_updates;
 
-  // Push down the new SDP media section for each audio/video transceiver.
+  // Collect updates for each audio/video transceiver.
   for (const auto& transceiver : transceivers()->List()) {
     const ContentInfo* content_info =
         FindMediaSectionForTransceiver(transceiver, sdesc);
@@ -4243,19 +4221,12 @@
     }
     const MediaContentDescription* content_desc =
         content_info->media_description();
-    if (!content_desc) {
-      continue;
-    }
-    std::string error;
-    bool success = (source == cricket::CS_LOCAL)
-                       ? channel->SetLocalContent(content_desc, type, &error)
-                       : channel->SetRemoteContent(content_desc, type, &error);
-    if (!success) {
-      LOG_AND_RETURN_ERROR(RTCErrorType::INVALID_PARAMETER, error);
+    if (content_desc) {
+      content_updates.emplace_back(channel, content_desc);
     }
   }
 
-  // If using the RtpDataChannel, push down the new SDP section for it too.
+  // If using the RtpDataChannel, add it to the list of updates.
   if (data_channel_controller()->rtp_data_channel()) {
     const ContentInfo* data_content =
         cricket::GetFirstDataContent(sdesc->description());
@@ -4263,21 +4234,21 @@
       const MediaContentDescription* data_desc =
           data_content->media_description();
       if (data_desc) {
-        std::string error;
-        bool success = (source == cricket::CS_LOCAL)
-                           ? data_channel_controller()
-                                 ->rtp_data_channel()
-                                 ->SetLocalContent(data_desc, type, &error)
-                           : data_channel_controller()
-                                 ->rtp_data_channel()
-                                 ->SetRemoteContent(data_desc, type, &error);
-        if (!success) {
-          LOG_AND_RETURN_ERROR(RTCErrorType::INVALID_PARAMETER, error);
-        }
+        content_updates.push_back(
+            {data_channel_controller()->rtp_data_channel(), data_desc});
       }
     }
   }
 
+  RTCError error = pc_->worker_thread()->Invoke<RTCError>(
+      RTC_FROM_HERE,
+      rtc::Bind(&SdpOfferAnswerHandler::ApplyChannelUpdates, this, type, source,
+                std::move(payload_type_demuxing_updates),
+                std::move(content_updates)));
+  if (!error.ok()) {
+    return error;
+  }
+
   // Need complete offer/answer with an SCTP m= section before starting SCTP,
   // according to https://tools.ietf.org/html/draft-ietf-mmusic-sctp-sdp-19
   if (pc_->sctp_mid() && local_description() && remote_description()) {
@@ -4306,6 +4277,49 @@
   return RTCError::OK();
 }
 
+RTCError SdpOfferAnswerHandler::ApplyChannelUpdates(
+    SdpType type,
+    cricket::ContentSource source,
+    std::vector<PayloadTypeDemuxingUpdate> payload_type_demuxing_updates,
+    std::vector<ContentUpdate> content_updates) {
+  RTC_DCHECK_RUN_ON(pc_->worker_thread());
+  // If this is answer-ish we're ready to let media flow.
+  bool enable_sending = type == SdpType::kPrAnswer || type == SdpType::kAnswer;
+  std::set<cricket::ChannelInterface*> modified_channels;
+  for (const auto& update : payload_type_demuxing_updates) {
+    modified_channels.insert(update.channel);
+    update.channel->SetPayloadTypeDemuxingEnabled(update.enabled);
+  }
+  for (const auto& update : content_updates) {
+    modified_channels.insert(update.channel);
+    std::string error;
+    bool success = (source == cricket::CS_LOCAL)
+                       ? update.channel->SetLocalContent(
+                             update.content_description, type, &error)
+                       : update.channel->SetRemoteContent(
+                             update.content_description, type, &error);
+    if (!success) {
+      LOG_AND_RETURN_ERROR(RTCErrorType::INVALID_PARAMETER, error);
+    }
+    if (enable_sending && !update.channel->enabled()) {
+      update.channel->Enable(true);
+    }
+  }
+  // The above calls may have modified properties of the channel (header
+  // extension mappings, demuxer criteria) which still need to be applied to the
+  // RtpTransport.
+  return pc_->network_thread()->Invoke<RTCError>(
+      RTC_FROM_HERE, [modified_channels] {
+        for (auto channel : modified_channels) {
+          std::string error;
+          if (!channel->UpdateRtpTransport(&error)) {
+            LOG_AND_RETURN_ERROR(RTCErrorType::INVALID_PARAMETER, error);
+          }
+        }
+        return RTCError::OK();
+      });
+}
+
 RTCError SdpOfferAnswerHandler::PushdownTransportDescription(
     cricket::ContentSource source,
     SdpType type) {
@@ -4904,7 +4918,8 @@
   return "";
 }
 
-bool SdpOfferAnswerHandler::UpdatePayloadTypeDemuxingState(
+std::vector<SdpOfferAnswerHandler::PayloadTypeDemuxingUpdate>
+SdpOfferAnswerHandler::GetPayloadTypeDemuxingUpdates(
     cricket::ContentSource source) {
   RTC_DCHECK_RUN_ON(signaling_thread());
   // We may need to delete any created default streams and disable creation of
@@ -4976,8 +4991,7 @@
 
   // Gather all updates ahead of time so that all channels can be updated in a
   // single Invoke; necessary due to thread guards.
-  std::vector<std::pair<RtpTransceiverDirection, cricket::ChannelInterface*>>
-      channels_to_update;
+  std::vector<PayloadTypeDemuxingUpdate> channel_updates;
   for (const auto& transceiver : transceivers()->List()) {
     cricket::ChannelInterface* channel = transceiver->internal()->channel();
     const ContentInfo* content =
@@ -4990,38 +5004,22 @@
     if (source == cricket::CS_REMOTE) {
       local_direction = RtpTransceiverDirectionReversed(local_direction);
     }
-    channels_to_update.emplace_back(local_direction,
-                                    transceiver->internal()->channel());
+    cricket::MediaType media_type = channel->media_type();
+    bool in_bundle_group =
+        (bundle_group && bundle_group->HasContentName(channel->content_name()));
+    bool payload_type_demuxing_enabled = false;
+    if (media_type == cricket::MediaType::MEDIA_TYPE_AUDIO) {
+      payload_type_demuxing_enabled =
+          (!in_bundle_group || pt_demuxing_enabled_audio) &&
+          RtpTransceiverDirectionHasRecv(local_direction);
+    } else if (media_type == cricket::MediaType::MEDIA_TYPE_VIDEO) {
+      payload_type_demuxing_enabled =
+          (!in_bundle_group || pt_demuxing_enabled_video) &&
+          RtpTransceiverDirectionHasRecv(local_direction);
+    }
+    channel_updates.emplace_back(channel, payload_type_demuxing_enabled);
   }
-
-  if (channels_to_update.empty()) {
-    return true;
-  }
-  return pc_->worker_thread()->Invoke<bool>(
-      RTC_FROM_HERE, [&channels_to_update, bundle_group,
-                      pt_demuxing_enabled_audio, pt_demuxing_enabled_video]() {
-        for (const auto& it : channels_to_update) {
-          RtpTransceiverDirection local_direction = it.first;
-          cricket::ChannelInterface* channel = it.second;
-          cricket::MediaType media_type = channel->media_type();
-          bool in_bundle_group = (bundle_group && bundle_group->HasContentName(
-                                                      channel->content_name()));
-          if (media_type == cricket::MediaType::MEDIA_TYPE_AUDIO) {
-            if (!channel->SetPayloadTypeDemuxingEnabled(
-                    (!in_bundle_group || pt_demuxing_enabled_audio) &&
-                    RtpTransceiverDirectionHasRecv(local_direction))) {
-              return false;
-            }
-          } else if (media_type == cricket::MediaType::MEDIA_TYPE_VIDEO) {
-            if (!channel->SetPayloadTypeDemuxingEnabled(
-                    (!in_bundle_group || pt_demuxing_enabled_video) &&
-                    RtpTransceiverDirectionHasRecv(local_direction))) {
-              return false;
-            }
-          }
-        }
-        return true;
-      });
+  return channel_updates;
 }
 
 }  // namespace webrtc
diff --git a/pc/sdp_offer_answer.h b/pc/sdp_offer_answer.h
index 43a3dbb..4b14f20 100644
--- a/pc/sdp_offer_answer.h
+++ b/pc/sdp_offer_answer.h
@@ -455,15 +455,32 @@
       cricket::MediaType media_type,
       StreamCollection* new_streams);
 
-  // Enables media channels to allow sending of media.
-  // This enables media to flow on all configured audio/video channels and the
-  // RtpDataChannel.
-  void EnableSending();
   // Push the media parts of the local or remote session description
-  // down to all of the channels.
+  // down to all of the channels, and enable sending if applicable.
   RTCError PushdownMediaDescription(SdpType type,
                                     cricket::ContentSource source);
 
+  struct PayloadTypeDemuxingUpdate {
+    PayloadTypeDemuxingUpdate(cricket::ChannelInterface* channel, bool enabled)
+        : channel(channel), enabled(enabled) {}
+    cricket::ChannelInterface* channel;
+    bool enabled;
+  };
+  struct ContentUpdate {
+    ContentUpdate(cricket::ChannelInterface* channel,
+                  const cricket::MediaContentDescription* content_description)
+        : channel(channel), content_description(content_description) {}
+    cricket::ChannelInterface* channel;
+    const cricket::MediaContentDescription* content_description;
+  };
+  // Helper method used by PushdownMediaDescription to apply a batch of updates
+  // to BaseChannels on the worker thread.
+  RTCError ApplyChannelUpdates(
+      SdpType type,
+      cricket::ContentSource source,
+      std::vector<PayloadTypeDemuxingUpdate> payload_type_demuxing_updates,
+      std::vector<ContentUpdate> content_updates);
+
   RTCError PushdownTransportDescription(cricket::ContentSource source,
                                         SdpType type);
   // Helper function to remove stopped transceivers.
@@ -550,9 +567,14 @@
       const std::string& mid) const;
 
   const std::string GetTransportName(const std::string& content_name);
-  // Based on number of transceivers per media type, enabled or disable
-  // payload type based demuxing in the affected channels.
-  bool UpdatePayloadTypeDemuxingState(cricket::ContentSource source);
+
+  // Based on number of transceivers per media type, and their bundle status and
+  // payload types, determine whether payload type based demuxing should be
+  // enabled or disabled. Returns a list of channels and the corresponding
+  // value to be passed into SetPayloadTypeDemuxingEnabled, so that this action
+  // can be combined with other operations on the worker thread.
+  std::vector<PayloadTypeDemuxingUpdate> GetPayloadTypeDemuxingUpdates(
+      cricket::ContentSource source);
 
   // ==================================================================
   // Access to pc_ variables
diff --git a/pc/test/mock_channel_interface.h b/pc/test/mock_channel_interface.h
index 1be4dcb0c..3a73225 100644
--- a/pc/test/mock_channel_interface.h
+++ b/pc/test/mock_channel_interface.h
@@ -46,7 +46,8 @@
                webrtc::SdpType,
                std::string*),
               (override));
-  MOCK_METHOD(bool, SetPayloadTypeDemuxingEnabled, (bool), (override));
+  MOCK_METHOD(void, SetPayloadTypeDemuxingEnabled, (bool), (override));
+  MOCK_METHOD(bool, UpdateRtpTransport, (std::string*), (override));
   MOCK_METHOD(const std::vector<StreamParams>&,
               local_streams,
               (),