pc/rtp_transceiver.cc

/*
 *  Copyright 2017 The WebRTC project authors. All Rights Reserved.
 *
 *  Use of this source code is governed by a BSD-style license
 *  that can be found in the LICENSE file in the root of the source
 *  tree. An additional intellectual property rights grant can be found
 *  in the file PATENTS.  All contributing project authors may
 *  be found in the AUTHORS file in the root of the source tree.
 */

#include "pc/rtp_transceiver.h"

#include <stdint.h>

#include <iterator>
#include <set>
#include <string>
#include <utility>
#include <vector>

#include "absl/algorithm/container.h"
#include "absl/memory/memory.h"
#include "api/audio_codecs/audio_codec_pair_id.h"
#include "api/field_trials_view.h"
#include "api/rtp_parameters.h"
#include "api/sequence_checker.h"
#include "media/base/codec.h"
#include "media/base/media_channel.h"
#include "media/base/media_channel_impl.h"
#include "media/base/media_constants.h"
#include "media/base/media_engine.h"
#include "pc/channel.h"
#include "pc/rtp_media_utils.h"
#include "pc/session_description.h"
#include "rtc_base/checks.h"
#include "rtc_base/logging.h"
#include "rtc_base/thread.h"

namespace webrtc {
namespace {

RTCError VerifyCodecPreferences(
    const std::vector<RtpCodecCapability>& unfiltered_codecs,
    const std::vector<cricket::Codec>& recv_codecs,
    const FieldTrialsView& field_trials) {
  // If the intersection between codecs and
  // RTCRtpReceiver.getCapabilities(kind).codecs only contains RTX, RED, FEC
  // codecs or Comfort Noise codecs or is an empty set, throw
  // InvalidModificationError.
  // This ensures that we always have something to offer, regardless of
  // transceiver.direction.
  // TODO(fippo): clean up the filtering killswitch
  std::vector<RtpCodecCapability> codecs = unfiltered_codecs;
  if (!absl::c_any_of(codecs, [&recv_codecs](const RtpCodecCapability& codec) {
        return codec.IsMediaCodec() &&
               absl::c_any_of(recv_codecs,
                              [&codec](const cricket::Codec& recv_codec) {
                                return recv_codec.MatchesRtpCodec(codec);
                              });
      })) {
    LOG_AND_RETURN_ERROR(RTCErrorType::INVALID_MODIFICATION,
                         "Invalid codec preferences: Missing codec from recv "
                         "codec capabilities.");
  }

  // Let codecCapabilities RTCRtpReceiver.getCapabilities(kind).codecs.
  // For each codec in codecs, If
  // codec is not in codecCapabilities, throw InvalidModificationError.
  for (const auto& codec_preference : codecs) {
    bool is_recv_codec = absl::c_any_of(
        recv_codecs, [&codec_preference](const cricket::Codec& codec) {
          return codec.MatchesRtpCodec(codec_preference);
        });
    if (!is_recv_codec) {
      if (!field_trials.IsDisabled(
              "WebRTC-SetCodecPreferences-ReceiveOnlyFilterInsteadOfThrow")) {
        LOG_AND_RETURN_ERROR(
            RTCErrorType::INVALID_MODIFICATION,
            std::string(
                "Invalid codec preferences: invalid codec with name \"") +
                codec_preference.name + "\".");
      } else {
        // Killswitch behavior: filter out any codec not in receive codecs.
        codecs.erase(std::remove_if(
            codecs.begin(), codecs.end(),
            [&recv_codecs](const RtpCodecCapability& codec) {
              return codec.IsMediaCodec() &&
                     !absl::c_any_of(
                         recv_codecs,
                         [&codec](const cricket::Codec& recv_codec) {
                           return recv_codec.MatchesRtpCodec(codec);
                         });
            }));
      }
    }
  }

  // Check we have a real codec (not just rtx, red, fec or CN)
  if (absl::c_all_of(codecs, [](const RtpCodecCapability& codec) {
        return !codec.IsMediaCodec();
      })) {
    LOG_AND_RETURN_ERROR(
        RTCErrorType::INVALID_MODIFICATION,
        "Invalid codec preferences: codec list must have a non "
        "RTX, RED or FEC entry.");
  }

  return RTCError::OK();
}

TaskQueueBase* GetCurrentTaskQueueOrThread() {
  TaskQueueBase* current = TaskQueueBase::Current();
  if (!current)
    current = rtc::ThreadManager::Instance()->CurrentThread();
  return current;
}

}  // namespace

RtpTransceiver::RtpTransceiver(cricket::MediaType media_type,
                               ConnectionContext* context)
    : thread_(GetCurrentTaskQueueOrThread()),
      unified_plan_(false),
      media_type_(media_type),
      context_(context) {
  RTC_DCHECK(media_type == cricket::MEDIA_TYPE_AUDIO ||
             media_type == cricket::MEDIA_TYPE_VIDEO);
}

RtpTransceiver::RtpTransceiver(
    rtc::scoped_refptr<RtpSenderProxyWithInternal<RtpSenderInternal>> sender,
    rtc::scoped_refptr<RtpReceiverProxyWithInternal<RtpReceiverInternal>>
        receiver,
    ConnectionContext* context,
    std::vector<RtpHeaderExtensionCapability> header_extensions_to_negotiate,
    std::function<void()> on_negotiation_needed)
    : thread_(GetCurrentTaskQueueOrThread()),
      unified_plan_(true),
      media_type_(sender->media_type()),
      context_(context),
      header_extensions_to_negotiate_(
          std::move(header_extensions_to_negotiate)),
      on_negotiation_needed_(std::move(on_negotiation_needed)) {
  RTC_DCHECK(media_type_ == cricket::MEDIA_TYPE_AUDIO ||
             media_type_ == cricket::MEDIA_TYPE_VIDEO);
  RTC_DCHECK_EQ(sender->media_type(), receiver->media_type());
  sender->internal()->SetSendCodecs(
      sender->media_type() == cricket::MEDIA_TYPE_VIDEO
          ? media_engine()->video().send_codecs(false)
          : media_engine()->voice().send_codecs());
  senders_.push_back(sender);
  receivers_.push_back(receiver);

  // Set default header extensions depending on whether simulcast/SVC is used.
  RtpParameters parameters = sender->internal()->GetParametersInternal();
  bool uses_simulcast = parameters.encodings.size() > 1;
  bool uses_svc = !parameters.encodings.empty() &&
                  parameters.encodings[0].scalability_mode.has_value() &&
                  parameters.encodings[0].scalability_mode !=
                      ScalabilityModeToString(ScalabilityMode::kL1T1);
  if (uses_simulcast || uses_svc) {
    // Enable DD and VLA extensions, can be deactivated by the API.
    // Skip this if the GFD extension was enabled via field trial
    // for backward compability reasons.
    bool uses_gfd =
        absl::c_find_if(
            header_extensions_to_negotiate_,
            [](const RtpHeaderExtensionCapability& ext) {
              return ext.uri == RtpExtension::kGenericFrameDescriptorUri00 &&
                     ext.direction != webrtc::RtpTransceiverDirection::kStopped;
            }) != header_extensions_to_negotiate_.end();
    if (!uses_gfd) {
      for (RtpHeaderExtensionCapability& ext :
           header_extensions_to_negotiate_) {
        if (ext.uri == RtpExtension::kVideoLayersAllocationUri ||
            ext.uri == RtpExtension::kDependencyDescriptorUri) {
          ext.direction = RtpTransceiverDirection::kSendRecv;
        }
      }
    }
  }
}

RtpTransceiver::~RtpTransceiver() {
  // TODO(tommi): On Android, when running PeerConnectionClientTest (e.g.
  // PeerConnectionClientTest#testCameraSwitch), the instance doesn't get
  // deleted on `thread_`. See if we can fix that.
  if (!stopped_) {
    RTC_DCHECK_RUN_ON(thread_);
    StopInternal();
  }

  RTC_CHECK(!channel_) << "Missing call to ClearChannel?";
}

RTCError RtpTransceiver::CreateChannel(
    absl::string_view mid,
    Call* call_ptr,
    const cricket::MediaConfig& media_config,
    bool srtp_required,
    CryptoOptions crypto_options,
    const cricket::AudioOptions& audio_options,
    const cricket::VideoOptions& video_options,
    VideoBitrateAllocatorFactory* video_bitrate_allocator_factory,
    std::function<RtpTransportInternal*(absl::string_view)> transport_lookup) {
  RTC_DCHECK_RUN_ON(thread_);
  if (!media_engine()) {
    // TODO(hta): Must be a better way
    return RTCError(RTCErrorType::INTERNAL_ERROR,
                    "No media engine for mid=" + std::string(mid));
  }
  std::unique_ptr<cricket::ChannelInterface> new_channel;
  if (media_type() == cricket::MEDIA_TYPE_AUDIO) {
    // TODO(bugs.webrtc.org/11992): CreateVideoChannel internally switches to
    // the worker thread. We shouldn't be using the `call_ptr_` hack here but
    // simply be on the worker thread and use `call_` (update upstream code).
    RTC_DCHECK(call_ptr);
    RTC_DCHECK(media_engine());
    // TODO(bugs.webrtc.org/11992): Remove this workaround after updates in
    // PeerConnection and add the expectation that we're already on the right
    // thread.
    context()->worker_thread()->BlockingCall([&] {
      RTC_DCHECK_RUN_ON(context()->worker_thread());

      AudioCodecPairId codec_pair_id = AudioCodecPairId::Create();

      std::unique_ptr<cricket::VoiceMediaSendChannelInterface>
          media_send_channel = media_engine()->voice().CreateSendChannel(
              call_ptr, media_config, audio_options, crypto_options,
              codec_pair_id);
      if (!media_send_channel) {
        // TODO(bugs.webrtc.org/14912): Consider CHECK or reporting failure
        return;
      }
      std::unique_ptr<cricket::VoiceMediaReceiveChannelInterface>
          media_receive_channel = media_engine()->voice().CreateReceiveChannel(
              call_ptr, media_config, audio_options, crypto_options,
              codec_pair_id);
      if (!media_receive_channel) {
        return;
      }
      // Note that this is safe because both sending and
      // receiving channels will be deleted at the same time.
      media_send_channel->SetSsrcListChangedCallback(
          [receive_channel =
               media_receive_channel.get()](const std::set<uint32_t>& choices) {
            receive_channel->ChooseReceiverReportSsrc(choices);
          });

      new_channel = std::make_unique<cricket::VoiceChannel>(
          context()->worker_thread(), context()->network_thread(),
          context()->signaling_thread(), std::move(media_send_channel),
          std::move(media_receive_channel), mid, srtp_required, crypto_options,
          context()->ssrc_generator());
    });
  } else {
    RTC_DCHECK_EQ(cricket::MEDIA_TYPE_VIDEO, media_type());

    // TODO(bugs.webrtc.org/11992): CreateVideoChannel internally switches to
    // the worker thread. We shouldn't be using the `call_ptr_` hack here but
    // simply be on the worker thread and use `call_` (update upstream code).
    context()->worker_thread()->BlockingCall([&] {
      RTC_DCHECK_RUN_ON(context()->worker_thread());

      std::unique_ptr<cricket::VideoMediaSendChannelInterface>
          media_send_channel = media_engine()->video().CreateSendChannel(
              call_ptr, media_config, video_options, crypto_options,
              video_bitrate_allocator_factory);
      if (!media_send_channel) {
        return;
      }

      std::unique_ptr<cricket::VideoMediaReceiveChannelInterface>
          media_receive_channel = media_engine()->video().CreateReceiveChannel(
              call_ptr, media_config, video_options, crypto_options);
      if (!media_receive_channel) {
        return;
      }
      // Note that this is safe because both sending and
      // receiving channels will be deleted at the same time.
      media_send_channel->SetSsrcListChangedCallback(
          [receive_channel =
               media_receive_channel.get()](const std::set<uint32_t>& choices) {
            receive_channel->ChooseReceiverReportSsrc(choices);
          });

      new_channel = std::make_unique<cricket::VideoChannel>(
          context()->worker_thread(), context()->network_thread(),
          context()->signaling_thread(), std::move(media_send_channel),
          std::move(media_receive_channel), mid, srtp_required, crypto_options,
          context()->ssrc_generator());
    });
  }
  if (!new_channel) {
    // TODO(hta): Must be a better way
    return RTCError(RTCErrorType::INTERNAL_ERROR,
                    "Failed to create channel for mid=" + std::string(mid));
  }
  SetChannel(std::move(new_channel), transport_lookup);
  return RTCError::OK();
}

void RtpTransceiver::SetChannel(
    std::unique_ptr<cricket::ChannelInterface> channel,
    std::function<RtpTransportInternal*(const std::string&)> transport_lookup) {
  RTC_DCHECK_RUN_ON(thread_);
  RTC_DCHECK(channel);
  RTC_DCHECK(transport_lookup);
  RTC_DCHECK(!channel_);
  // Cannot set a channel on a stopped transceiver.
  if (stopped_) {
    return;
  }

  RTC_LOG_THREAD_BLOCK_COUNT();

  RTC_DCHECK_EQ(media_type(), channel->media_type());
  signaling_thread_safety_ = PendingTaskSafetyFlag::Create();

  std::unique_ptr<cricket::ChannelInterface> channel_to_delete;

  // An alternative to this, could be to require SetChannel to be called
  // on the network thread. The channel object operates for the most part
  // on the network thread, as part of its initialization being on the network
  // thread is required, so setting a channel object as part of the construction
  // (without thread hopping) might be the more efficient thing to do than
  // how SetChannel works today.
  // Similarly, if the channel() accessor is limited to the network thread, that
  // helps with keeping the channel implementation requirements being met and
  // avoids synchronization for accessing the pointer or network related state.
  context()->network_thread()->BlockingCall([&]() {
    if (channel_) {
      channel_->SetFirstPacketReceivedCallback(nullptr);
      channel_->SetRtpTransport(nullptr);
      channel_to_delete = std::move(channel_);
    }

    channel_ = std::move(channel);

    channel_->SetRtpTransport(transport_lookup(channel_->mid()));
    channel_->SetFirstPacketReceivedCallback(
        [thread = thread_, flag = signaling_thread_safety_, this]() mutable {
          thread->PostTask(
              SafeTask(std::move(flag), [this]() { OnFirstPacketReceived(); }));
        });
  });
  PushNewMediaChannelAndDeleteChannel(nullptr);

  RTC_DCHECK_BLOCK_COUNT_NO_MORE_THAN(2);
}

void RtpTransceiver::ClearChannel() {
  RTC_DCHECK_RUN_ON(thread_);

  if (!channel_) {
    return;
  }

  RTC_LOG_THREAD_BLOCK_COUNT();

  if (channel_) {
    signaling_thread_safety_->SetNotAlive();
    signaling_thread_safety_ = nullptr;
  }
  std::unique_ptr<cricket::ChannelInterface> channel_to_delete;

  context()->network_thread()->BlockingCall([&]() {
    if (channel_) {
      channel_->SetFirstPacketReceivedCallback(nullptr);
      channel_->SetRtpTransport(nullptr);
      channel_to_delete = std::move(channel_);
    }
  });

  RTC_DCHECK_BLOCK_COUNT_NO_MORE_THAN(1);
  PushNewMediaChannelAndDeleteChannel(std::move(channel_to_delete));

  RTC_DCHECK_BLOCK_COUNT_NO_MORE_THAN(2);
}

void RtpTransceiver::PushNewMediaChannelAndDeleteChannel(
    std::unique_ptr<cricket::ChannelInterface> channel_to_delete) {
  // The clumsy combination of pushing down media channel and deleting
  // the channel is due to the desire to do both things in one Invoke().
  if (!channel_to_delete && senders_.empty() && receivers_.empty()) {
    return;
  }
  context()->worker_thread()->BlockingCall([&]() {
    // Push down the new media_channel, if any, otherwise clear it.
    auto* media_send_channel =
        channel_ ? channel_->media_send_channel() : nullptr;
    for (const auto& sender : senders_) {
      sender->internal()->SetMediaChannel(media_send_channel);
    }

    auto* media_receive_channel =
        channel_ ? channel_->media_receive_channel() : nullptr;
    for (const auto& receiver : receivers_) {
      receiver->internal()->SetMediaChannel(media_receive_channel);
    }

    // Destroy the channel, if we had one, now _after_ updating the receivers
    // who might have had references to the previous channel.
    if (channel_to_delete) {
      channel_to_delete.reset(nullptr);
    }
  });
}

void RtpTransceiver::AddSender(
    rtc::scoped_refptr<RtpSenderProxyWithInternal<RtpSenderInternal>> sender) {
  RTC_DCHECK_RUN_ON(thread_);
  RTC_DCHECK(!stopped_);
  RTC_DCHECK(!unified_plan_);
  RTC_DCHECK(sender);
  RTC_DCHECK_EQ(media_type(), sender->media_type());
  RTC_DCHECK(!absl::c_linear_search(senders_, sender));

  std::vector<cricket::Codec> send_codecs =
      media_type() == cricket::MEDIA_TYPE_VIDEO
          ? media_engine()->video().send_codecs(false)
          : media_engine()->voice().send_codecs();
  sender->internal()->SetSendCodecs(send_codecs);
  senders_.push_back(sender);
}

bool RtpTransceiver::RemoveSender(RtpSenderInterface* sender) {
  RTC_DCHECK(!unified_plan_);
  if (sender) {
    RTC_DCHECK_EQ(media_type(), sender->media_type());
  }
  auto it = absl::c_find(senders_, sender);
  if (it == senders_.end()) {
    return false;
  }
  (*it)->internal()->Stop();
  senders_.erase(it);
  return true;
}

void RtpTransceiver::AddReceiver(
    rtc::scoped_refptr<RtpReceiverProxyWithInternal<RtpReceiverInternal>>
        receiver) {
  RTC_DCHECK_RUN_ON(thread_);
  RTC_DCHECK(!stopped_);
  RTC_DCHECK(!unified_plan_);
  RTC_DCHECK(receiver);
  RTC_DCHECK_EQ(media_type(), receiver->media_type());
  RTC_DCHECK(!absl::c_linear_search(receivers_, receiver));
  receivers_.push_back(receiver);
}

bool RtpTransceiver::RemoveReceiver(RtpReceiverInterface* receiver) {
  RTC_DCHECK_RUN_ON(thread_);
  RTC_DCHECK(!unified_plan_);
  if (receiver) {
    RTC_DCHECK_EQ(media_type(), receiver->media_type());
  }
  auto it = absl::c_find(receivers_, receiver);
  if (it == receivers_.end()) {
    return false;
  }

  (*it)->internal()->Stop();
  context()->worker_thread()->BlockingCall([&]() {
    // `Stop()` will clear the receiver's pointer to the media channel.
    (*it)->internal()->SetMediaChannel(nullptr);
  });

  receivers_.erase(it);
  return true;
}

rtc::scoped_refptr<RtpSenderInternal> RtpTransceiver::sender_internal() const {
  RTC_DCHECK(unified_plan_);
  RTC_CHECK_EQ(1u, senders_.size());
  return rtc::scoped_refptr<RtpSenderInternal>(senders_[0]->internal());
}

rtc::scoped_refptr<RtpReceiverInternal> RtpTransceiver::receiver_internal()
    const {
  RTC_DCHECK(unified_plan_);
  RTC_CHECK_EQ(1u, receivers_.size());
  return rtc::scoped_refptr<RtpReceiverInternal>(receivers_[0]->internal());
}

cricket::MediaType RtpTransceiver::media_type() const {
  return media_type_;
}

absl::optional<std::string> RtpTransceiver::mid() const {
  return mid_;
}

void RtpTransceiver::OnFirstPacketReceived() {
  for (const auto& receiver : receivers_) {
    receiver->internal()->NotifyFirstPacketReceived();
  }
}

rtc::scoped_refptr<RtpSenderInterface> RtpTransceiver::sender() const {
  RTC_DCHECK(unified_plan_);
  RTC_CHECK_EQ(1u, senders_.size());
  return senders_[0];
}

rtc::scoped_refptr<RtpReceiverInterface> RtpTransceiver::receiver() const {
  RTC_DCHECK(unified_plan_);
  RTC_CHECK_EQ(1u, receivers_.size());
  return receivers_[0];
}

void RtpTransceiver::set_current_direction(RtpTransceiverDirection direction) {
  RTC_LOG(LS_INFO) << "Changing transceiver (MID=" << mid_.value_or("<not set>")
                   << ") current direction from "
                   << (current_direction_ ? RtpTransceiverDirectionToString(
                                                *current_direction_)
                                          : "<not set>")
                   << " to " << RtpTransceiverDirectionToString(direction)
                   << ".";
  current_direction_ = direction;
  if (RtpTransceiverDirectionHasSend(*current_direction_)) {
    has_ever_been_used_to_send_ = true;
  }
}

void RtpTransceiver::set_fired_direction(
    absl::optional<RtpTransceiverDirection> direction) {
  fired_direction_ = direction;
}

bool RtpTransceiver::stopped() const {
  RTC_DCHECK_RUN_ON(thread_);
  return stopped_;
}

bool RtpTransceiver::stopping() const {
  RTC_DCHECK_RUN_ON(thread_);
  return stopping_;
}

RtpTransceiverDirection RtpTransceiver::direction() const {
  if (unified_plan_ && stopping())
    return RtpTransceiverDirection::kStopped;

  return direction_;
}

RTCError RtpTransceiver::SetDirectionWithError(
    RtpTransceiverDirection new_direction) {
  if (unified_plan_ && stopping()) {
    LOG_AND_RETURN_ERROR(RTCErrorType::INVALID_STATE,
                         "Cannot set direction on a stopping transceiver.");
  }
  if (new_direction == direction_)
    return RTCError::OK();

  if (new_direction == RtpTransceiverDirection::kStopped) {
    LOG_AND_RETURN_ERROR(RTCErrorType::INVALID_PARAMETER,
                         "The set direction 'stopped' is invalid.");
  }

  direction_ = new_direction;
  on_negotiation_needed_();

  return RTCError::OK();
}

absl::optional<RtpTransceiverDirection> RtpTransceiver::current_direction()
    const {
  if (unified_plan_ && stopped())
    return RtpTransceiverDirection::kStopped;

  return current_direction_;
}

absl::optional<RtpTransceiverDirection> RtpTransceiver::fired_direction()
    const {
  return fired_direction_;
}

void RtpTransceiver::StopSendingAndReceiving() {
  // 1. Let sender be transceiver.[[Sender]].
  // 2. Let receiver be transceiver.[[Receiver]].
  //
  // 3. Stop sending media with sender.
  //
  RTC_DCHECK_RUN_ON(thread_);

  // 4. Send an RTCP BYE for each RTP stream that was being sent by sender, as
  // specified in [RFC3550].
  for (const auto& sender : senders_)
    sender->internal()->Stop();

  // Signal to receiver sources that we're stopping.
  for (const auto& receiver : receivers_)
    receiver->internal()->Stop();

  context()->worker_thread()->BlockingCall([&]() {
    // 5 Stop receiving media with receiver.
    for (const auto& receiver : receivers_)
      receiver->internal()->SetMediaChannel(nullptr);
  });

  stopping_ = true;
  direction_ = RtpTransceiverDirection::kInactive;
}

RTCError RtpTransceiver::StopStandard() {
  RTC_DCHECK_RUN_ON(thread_);
  // If we're on Plan B, do what Stop() used to do there.
  if (!unified_plan_) {
    StopInternal();
    return RTCError::OK();
  }
  // 1. Let transceiver be the RTCRtpTransceiver object on which the method is
  // invoked.
  //
  // 2. Let connection be the RTCPeerConnection object associated with
  // transceiver.
  //
  // 3. If connection.[[IsClosed]] is true, throw an InvalidStateError.
  if (is_pc_closed_) {
    LOG_AND_RETURN_ERROR(RTCErrorType::INVALID_STATE,
                         "PeerConnection is closed.");
  }

  // 4. If transceiver.[[Stopping]] is true, abort these steps.
  if (stopping_)
    return RTCError::OK();

  // 5. Stop sending and receiving given transceiver, and update the
  // negotiation-needed flag for connection.
  StopSendingAndReceiving();
  on_negotiation_needed_();

  return RTCError::OK();
}

void RtpTransceiver::StopInternal() {
  RTC_DCHECK_RUN_ON(thread_);
  StopTransceiverProcedure();
}

void RtpTransceiver::StopTransceiverProcedure() {
  RTC_DCHECK_RUN_ON(thread_);
  // As specified in the "Stop the RTCRtpTransceiver" procedure
  // 1. If transceiver.[[Stopping]] is false, stop sending and receiving given
  // transceiver.
  if (!stopping_)
    StopSendingAndReceiving();

  // 2. Set transceiver.[[Stopped]] to true.
  stopped_ = true;

  // Signal the updated change to the senders.
  for (const auto& sender : senders_)
    sender->internal()->SetTransceiverAsStopped();

  // 3. Set transceiver.[[Receptive]] to false.
  // 4. Set transceiver.[[CurrentDirection]] to null.
  current_direction_ = absl::nullopt;
}

RTCError RtpTransceiver::SetCodecPreferences(
    rtc::ArrayView<RtpCodecCapability> codec_capabilities) {
  RTC_DCHECK(unified_plan_);
  // 3. If codecs is an empty list, set transceiver's [[PreferredCodecs]] slot
  // to codecs and abort these steps.
  if (codec_capabilities.empty()) {
    codec_preferences_.clear();
    return RTCError::OK();
  }

  // 4. Remove any duplicate values in codecs.
  std::vector<RtpCodecCapability> codecs;
  absl::c_remove_copy_if(codec_capabilities, std::back_inserter(codecs),
                         [&codecs](const RtpCodecCapability& codec) {
                           return absl::c_linear_search(codecs, codec);
                         });

  // 6. to 8.
  RTCError result;
  std::vector<cricket::Codec> recv_codecs;
  if (media_type_ == cricket::MEDIA_TYPE_AUDIO) {
    recv_codecs = media_engine()->voice().recv_codecs();
  } else if (media_type_ == cricket::MEDIA_TYPE_VIDEO) {
    recv_codecs = media_engine()->video().recv_codecs(context()->use_rtx());
  }
  result = VerifyCodecPreferences(codecs, recv_codecs,
                                  context()->env().field_trials());

  if (result.ok()) {
    codec_preferences_ = codecs;
  }

  return result;
}

std::vector<RtpHeaderExtensionCapability>
RtpTransceiver::GetHeaderExtensionsToNegotiate() const {
  return header_extensions_to_negotiate_;
}

std::vector<RtpHeaderExtensionCapability>
RtpTransceiver::GetNegotiatedHeaderExtensions() const {
  RTC_DCHECK_RUN_ON(thread_);
  std::vector<RtpHeaderExtensionCapability> result;
  result.reserve(header_extensions_to_negotiate_.size());
  for (const auto& ext : header_extensions_to_negotiate_) {
    auto negotiated = absl::c_find_if(negotiated_header_extensions_,
                                      [&ext](const RtpExtension& negotiated) {
                                        return negotiated.uri == ext.uri;
                                      });
    RtpHeaderExtensionCapability capability(ext.uri);
    // TODO(bugs.webrtc.org/7477): extend when header extensions support
    // direction.
    capability.direction = negotiated != negotiated_header_extensions_.end()
                               ? RtpTransceiverDirection::kSendRecv
                               : RtpTransceiverDirection::kStopped;
    result.push_back(capability);
  }
  return result;
}

// Helper function to determine mandatory-to-negotiate extensions.
// See https://www.rfc-editor.org/rfc/rfc8834#name-header-extensions
// and https://w3c.github.io/webrtc-extensions/#rtcrtptransceiver-interface
// Since BUNDLE is offered by default, MID is mandatory and can not be turned
// off via this API.
bool IsMandatoryHeaderExtension(const std::string& uri) {
  return uri == RtpExtension::kMidUri;
}

RTCError RtpTransceiver::SetHeaderExtensionsToNegotiate(
    rtc::ArrayView<const RtpHeaderExtensionCapability> header_extensions) {
  // https://w3c.github.io/webrtc-extensions/#dom-rtcrtptransceiver-setheaderextensionstonegotiate
  if (header_extensions.size() != header_extensions_to_negotiate_.size()) {
    return RTCError(RTCErrorType::INVALID_MODIFICATION,
                    "Size of extensions to negotiate does not match.");
  }
  // For each index i of extensions, run the following steps: ...
  for (size_t i = 0; i < header_extensions.size(); i++) {
    const auto& extension = header_extensions[i];
    if (extension.uri != header_extensions_to_negotiate_[i].uri) {
      return RTCError(RTCErrorType::INVALID_MODIFICATION,
                      "Reordering extensions is not allowed.");
    }
    if (IsMandatoryHeaderExtension(extension.uri) &&
        extension.direction != RtpTransceiverDirection::kSendRecv) {
      return RTCError(RTCErrorType::INVALID_MODIFICATION,
                      "Attempted to stop a mandatory extension.");
    }

    // TODO(bugs.webrtc.org/7477): Currently there are no recvonly extensions so
    // this can not be checked: "When there exists header extension capabilities
    // that have directions other than kSendRecv, restrict extension.direction
    // as to not exceed that capability."
  }

  // Apply mutation after error checking.
  for (size_t i = 0; i < header_extensions.size(); i++) {
    header_extensions_to_negotiate_[i].direction =
        header_extensions[i].direction;
  }

  return RTCError::OK();
}

void RtpTransceiver::OnNegotiationUpdate(
    SdpType sdp_type,
    const cricket::MediaContentDescription* content) {
  RTC_DCHECK_RUN_ON(thread_);
  RTC_DCHECK(content);
  if (sdp_type == SdpType::kAnswer)
    negotiated_header_extensions_ = content->rtp_header_extensions();
}

void RtpTransceiver::SetPeerConnectionClosed() {
  is_pc_closed_ = true;
}

}  // namespace webrtc