modules/video_coding/frame_buffer2.cc

/*
 *  Copyright (c) 2016 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 "modules/video_coding/frame_buffer2.h"

#include <algorithm>
#include <cstdlib>
#include <iterator>
#include <queue>
#include <utility>
#include <vector>

#include "api/video/encoded_image.h"
#include "api/video/video_timing.h"
#include "common_types.h"  // NOLINT(build/include)
#include "modules/video_coding/include/video_coding_defines.h"
#include "modules/video_coding/jitter_estimator.h"
#include "modules/video_coding/timing.h"
#include "rtc_base/checks.h"
#include "rtc_base/experiments/rtt_mult_experiment.h"
#include "rtc_base/logging.h"
#include "rtc_base/numerics/sequence_number_util.h"
#include "rtc_base/trace_event.h"
#include "system_wrappers/include/clock.h"
#include "system_wrappers/include/field_trial.h"

namespace webrtc {
namespace video_coding {

namespace {
// Max number of frames the buffer will hold.
constexpr int kMaxFramesBuffered = 600;

// Max number of decoded frame info that will be saved.
constexpr int kMaxFramesHistory = 50;

// The time it's allowed for a frame to be late to its rendering prediction and
// still be rendered.
constexpr int kMaxAllowedFrameDelayMs = 5;

constexpr int64_t kLogNonDecodedIntervalMs = 5000;
}  // namespace

FrameBuffer::FrameBuffer(Clock* clock,
                         VCMJitterEstimator* jitter_estimator,
                         VCMTiming* timing,
                         VCMReceiveStatisticsCallback* stats_callback)
    : clock_(clock),
      jitter_estimator_(jitter_estimator),
      timing_(timing),
      inter_frame_delay_(clock_->TimeInMilliseconds()),
      last_decoded_frame_it_(frames_.end()),
      last_continuous_frame_it_(frames_.end()),
      num_frames_history_(0),
      num_frames_buffered_(0),
      stopped_(false),
      protection_mode_(kProtectionNack),
      stats_callback_(stats_callback),
      last_log_non_decoded_ms_(-kLogNonDecodedIntervalMs) {}

FrameBuffer::~FrameBuffer() {}

FrameBuffer::ReturnReason FrameBuffer::NextFrame(
    int64_t max_wait_time_ms,
    std::unique_ptr<EncodedFrame>* frame_out,
    bool keyframe_required) {
  TRACE_EVENT0("webrtc", "FrameBuffer::NextFrame");
  int64_t latest_return_time_ms =
      clock_->TimeInMilliseconds() + max_wait_time_ms;
  int64_t wait_ms = max_wait_time_ms;
  int64_t now_ms = 0;

  do {
    now_ms = clock_->TimeInMilliseconds();
    {
      rtc::CritScope lock(&crit_);
      new_continuous_frame_event_.Reset();
      if (stopped_)
        return kStopped;

      wait_ms = max_wait_time_ms;

      // Need to hold |crit_| in order to access frames_to_decode_. therefore we
      // set it here in the loop instead of outside the loop in order to not
      // acquire the lock unnecessarily.
      frames_to_decode_.clear();

      // |frame_it| points to the first frame after the
      // |last_decoded_frame_it_|.
      auto frame_it = frames_.end();
      if (last_decoded_frame_it_ == frames_.end()) {
        frame_it = frames_.begin();
      } else {
        frame_it = last_decoded_frame_it_;
        ++frame_it;
      }

      // |continuous_end_it| points to the first frame after the
      // |last_continuous_frame_it_|.
      auto continuous_end_it = last_continuous_frame_it_;
      if (continuous_end_it != frames_.end())
        ++continuous_end_it;

      for (; frame_it != continuous_end_it && frame_it != frames_.end();
           ++frame_it) {
        if (!frame_it->second.continuous ||
            frame_it->second.num_missing_decodable > 0) {
          continue;
        }

        EncodedFrame* frame = frame_it->second.frame.get();

        if (keyframe_required && !frame->is_keyframe())
          continue;

        // TODO(https://bugs.webrtc.org/9974): consider removing this check
        // as it may make a stream undecodable after a very long delay between
        // frames.
        if (last_decoded_frame_timestamp_ &&
            AheadOf(*last_decoded_frame_timestamp_, frame->Timestamp())) {
          continue;
        }

        // Only ever return all parts of a superframe. Therefore skip this
        // frame if it's not a beginning of a superframe.
        if (frame->inter_layer_predicted) {
          continue;
        }

        // Gather all remaining frames for the same superframe.
        std::vector<FrameMap::iterator> current_superframe;
        current_superframe.push_back(frame_it);
        bool last_layer_completed =
            frame_it->second.frame->is_last_spatial_layer;
        FrameMap::iterator next_frame_it = frame_it;
        while (true) {
          ++next_frame_it;
          if (next_frame_it == frames_.end() ||
              next_frame_it->first.picture_id != frame->id.picture_id ||
              !next_frame_it->second.continuous) {
            break;
          }
          // Check if the next frame has some undecoded references other than
          // the previous frame in the same superframe.
          size_t num_allowed_undecoded_refs =
              (next_frame_it->second.frame->inter_layer_predicted) ? 1 : 0;
          if (next_frame_it->second.num_missing_decodable >
              num_allowed_undecoded_refs) {
            break;
          }
          // All frames in the superframe should have the same timestamp.
          if (frame->Timestamp() != next_frame_it->second.frame->Timestamp()) {
            RTC_LOG(LS_WARNING)
                << "Frames in a single superframe have different"
                   " timestamps. Skipping undecodable superframe.";
            break;
          }
          current_superframe.push_back(next_frame_it);
          last_layer_completed =
              next_frame_it->second.frame->is_last_spatial_layer;
        }
        // Check if the current superframe is complete.
        // TODO(bugs.webrtc.org/10064): consider returning all available to
        // decode frames even if the superframe is not complete yet.
        if (!last_layer_completed) {
          continue;
        }

        frames_to_decode_ = std::move(current_superframe);

        if (frame->RenderTime() == -1) {
          frame->SetRenderTime(
              timing_->RenderTimeMs(frame->Timestamp(), now_ms));
        }
        wait_ms = timing_->MaxWaitingTime(frame->RenderTime(), now_ms);

        // This will cause the frame buffer to prefer high framerate rather
        // than high resolution in the case of the decoder not decoding fast
        // enough and the stream has multiple spatial and temporal layers.
        // For multiple temporal layers it may cause non-base layer frames to be
        // skipped if they are late.
        if (wait_ms < -kMaxAllowedFrameDelayMs)
          continue;

        break;
      }
    }  // rtc::Critscope lock(&crit_);

    wait_ms = std::min<int64_t>(wait_ms, latest_return_time_ms - now_ms);
    wait_ms = std::max<int64_t>(wait_ms, 0);
  } while (new_continuous_frame_event_.Wait(wait_ms));

  {
    rtc::CritScope lock(&crit_);
    now_ms = clock_->TimeInMilliseconds();
    std::vector<EncodedFrame*> frames_out;
    for (const FrameMap::iterator& frame_it : frames_to_decode_) {
      RTC_DCHECK(frame_it != frames_.end());
      EncodedFrame* frame = frame_it->second.frame.release();

      if (!frame->delayed_by_retransmission()) {
        int64_t frame_delay;

        if (inter_frame_delay_.CalculateDelay(frame->Timestamp(), &frame_delay,
                                              frame->ReceivedTime())) {
          jitter_estimator_->UpdateEstimate(frame_delay, frame->size());
        }

        float rtt_mult = protection_mode_ == kProtectionNackFEC ? 0.0 : 1.0;
        if (RttMultExperiment::RttMultEnabled()) {
          rtt_mult = RttMultExperiment::GetRttMultValue();
        }
        timing_->SetJitterDelay(jitter_estimator_->GetJitterEstimate(rtt_mult));
        timing_->UpdateCurrentDelay(frame->RenderTime(), now_ms);
      } else {
        if (RttMultExperiment::RttMultEnabled() ||
            webrtc::field_trial::IsEnabled("WebRTC-AddRttToPlayoutDelay"))
          jitter_estimator_->FrameNacked();
      }

      // Gracefully handle bad RTP timestamps and render time issues.
      if (HasBadRenderTiming(*frame, now_ms)) {
        jitter_estimator_->Reset();
        timing_->Reset();
        frame->SetRenderTime(timing_->RenderTimeMs(frame->Timestamp(), now_ms));
      }

      UpdateJitterDelay();
      UpdateTimingFrameInfo();
      PropagateDecodability(frame_it->second);

      AdvanceLastDecodedFrame(frame_it);
      last_decoded_frame_timestamp_ = frame->Timestamp();
      frames_out.push_back(frame);
    }

    if (!frames_out.empty()) {
      if (frames_out.size() == 1) {
        frame_out->reset(frames_out[0]);
      } else {
        frame_out->reset(CombineAndDeleteFrames(frames_out));
      }
      return kFrameFound;
    }
  }  // rtc::Critscope lock(&crit_)

  if (latest_return_time_ms - now_ms > 0) {
    // If |next_frame_it_ == frames_.end()| and there is still time left, it
    // means that the frame buffer was cleared as the thread in this function
    // was waiting to acquire |crit_| in order to return. Wait for the
    // remaining time and then return.
    return NextFrame(latest_return_time_ms - now_ms, frame_out);
  }
  return kTimeout;
}

bool FrameBuffer::HasBadRenderTiming(const EncodedFrame& frame,
                                     int64_t now_ms) {
  // Assume that render timing errors are due to changes in the video stream.
  int64_t render_time_ms = frame.RenderTimeMs();
  // Zero render time means render immediately.
  if (render_time_ms == 0) {
    return false;
  }
  if (render_time_ms < 0) {
    return true;
  }
  const int64_t kMaxVideoDelayMs = 10000;
  if (std::abs(render_time_ms - now_ms) > kMaxVideoDelayMs) {
    int frame_delay = static_cast<int>(std::abs(render_time_ms - now_ms));
    RTC_LOG(LS_WARNING)
        << "A frame about to be decoded is out of the configured "
        << "delay bounds (" << frame_delay << " > " << kMaxVideoDelayMs
        << "). Resetting the video jitter buffer.";
    return true;
  }
  if (static_cast<int>(timing_->TargetVideoDelay()) > kMaxVideoDelayMs) {
    RTC_LOG(LS_WARNING) << "The video target delay has grown larger than "
                        << kMaxVideoDelayMs << " ms.";
    return true;
  }
  return false;
}

void FrameBuffer::SetProtectionMode(VCMVideoProtection mode) {
  TRACE_EVENT0("webrtc", "FrameBuffer::SetProtectionMode");
  rtc::CritScope lock(&crit_);
  protection_mode_ = mode;
}

void FrameBuffer::Start() {
  TRACE_EVENT0("webrtc", "FrameBuffer::Start");
  rtc::CritScope lock(&crit_);
  stopped_ = false;
}

void FrameBuffer::Stop() {
  TRACE_EVENT0("webrtc", "FrameBuffer::Stop");
  rtc::CritScope lock(&crit_);
  stopped_ = true;
  new_continuous_frame_event_.Set();
}

void FrameBuffer::Clear() {
  rtc::CritScope lock(&crit_);
  ClearFramesAndHistory();
}

void FrameBuffer::UpdateRtt(int64_t rtt_ms) {
  rtc::CritScope lock(&crit_);
  jitter_estimator_->UpdateRtt(rtt_ms);
}

bool FrameBuffer::ValidReferences(const EncodedFrame& frame) const {
  if (frame.id.picture_id < 0)
    return false;

  for (size_t i = 0; i < frame.num_references; ++i) {
    if (frame.references[i] < 0 || frame.references[i] >= frame.id.picture_id)
      return false;

    for (size_t j = i + 1; j < frame.num_references; ++j) {
      if (frame.references[i] == frame.references[j])
        return false;
    }
  }

  if (frame.inter_layer_predicted && frame.id.spatial_layer == 0)
    return false;

  return true;
}

void FrameBuffer::UpdatePlayoutDelays(const EncodedFrame& frame) {
  TRACE_EVENT0("webrtc", "FrameBuffer::UpdatePlayoutDelays");
  PlayoutDelay playout_delay = frame.EncodedImage().playout_delay_;
  if (playout_delay.min_ms >= 0)
    timing_->set_min_playout_delay(playout_delay.min_ms);

  if (playout_delay.max_ms >= 0)
    timing_->set_max_playout_delay(playout_delay.max_ms);

  if (!frame.delayed_by_retransmission())
    timing_->IncomingTimestamp(frame.Timestamp(), frame.ReceivedTime());
}

int64_t FrameBuffer::InsertFrame(std::unique_ptr<EncodedFrame> frame) {
  TRACE_EVENT0("webrtc", "FrameBuffer::InsertFrame");
  RTC_DCHECK(frame);
  if (stats_callback_)
    stats_callback_->OnCompleteFrame(frame->is_keyframe(), frame->size(),
                                     frame->contentType());
  const VideoLayerFrameId& id = frame->id;

  rtc::CritScope lock(&crit_);

  int64_t last_continuous_picture_id =
      last_continuous_frame_it_ == frames_.end()
          ? -1
          : last_continuous_frame_it_->first.picture_id;

  if (!ValidReferences(*frame)) {
    RTC_LOG(LS_WARNING) << "Frame with (picture_id:spatial_id) ("
                        << id.picture_id << ":"
                        << static_cast<int>(id.spatial_layer)
                        << ") has invalid frame references, dropping frame.";
    return last_continuous_picture_id;
  }

  if (num_frames_buffered_ >= kMaxFramesBuffered) {
    if (frame->is_keyframe()) {
      RTC_LOG(LS_WARNING) << "Inserting keyframe (picture_id:spatial_id) ("
                          << id.picture_id << ":"
                          << static_cast<int>(id.spatial_layer)
                          << ") but buffer is full, clearing"
                          << " buffer and inserting the frame.";
      ClearFramesAndHistory();
    } else {
      RTC_LOG(LS_WARNING) << "Frame with (picture_id:spatial_id) ("
                          << id.picture_id << ":"
                          << static_cast<int>(id.spatial_layer)
                          << ") could not be inserted due to the frame "
                          << "buffer being full, dropping frame.";
      return last_continuous_picture_id;
    }
  }

  if (last_decoded_frame_it_ != frames_.end() &&
      id <= last_decoded_frame_it_->first) {
    if (AheadOf(frame->Timestamp(), *last_decoded_frame_timestamp_) &&
        frame->is_keyframe()) {
      // If this frame has a newer timestamp but an earlier picture id then we
      // assume there has been a jump in the picture id due to some encoder
      // reconfiguration or some other reason. Even though this is not according
      // to spec we can still continue to decode from this frame if it is a
      // keyframe.
      RTC_LOG(LS_WARNING)
          << "A jump in picture id was detected, clearing buffer.";
      ClearFramesAndHistory();
      last_continuous_picture_id = -1;
    } else {
      RTC_LOG(LS_WARNING) << "Frame with (picture_id:spatial_id) ("
                          << id.picture_id << ":"
                          << static_cast<int>(id.spatial_layer)
                          << ") inserted after frame ("
                          << last_decoded_frame_it_->first.picture_id << ":"
                          << static_cast<int>(
                                 last_decoded_frame_it_->first.spatial_layer)
                          << ") was handed off for decoding, dropping frame.";
      return last_continuous_picture_id;
    }
  }

  // Test if inserting this frame would cause the order of the frames to become
  // ambiguous (covering more than half the interval of 2^16). This can happen
  // when the picture id make large jumps mid stream.
  if (!frames_.empty() && id < frames_.begin()->first &&
      frames_.rbegin()->first < id) {
    RTC_LOG(LS_WARNING)
        << "A jump in picture id was detected, clearing buffer.";
    ClearFramesAndHistory();
    last_continuous_picture_id = -1;
  }

  auto info = frames_.emplace(id, FrameInfo()).first;

  if (info->second.frame) {
    RTC_LOG(LS_WARNING) << "Frame with (picture_id:spatial_id) ("
                        << id.picture_id << ":"
                        << static_cast<int>(id.spatial_layer)
                        << ") already inserted, dropping frame.";
    return last_continuous_picture_id;
  }

  if (!UpdateFrameInfoWithIncomingFrame(*frame, info))
    return last_continuous_picture_id;
  UpdatePlayoutDelays(*frame);

  info->second.frame = std::move(frame);
  ++num_frames_buffered_;

  if (info->second.num_missing_continuous == 0) {
    info->second.continuous = true;
    PropagateContinuity(info);
    last_continuous_picture_id = last_continuous_frame_it_->first.picture_id;

    // Since we now have new continuous frames there might be a better frame
    // to return from NextFrame. Signal that thread so that it again can choose
    // which frame to return.
    new_continuous_frame_event_.Set();
  }

  return last_continuous_picture_id;
}

void FrameBuffer::PropagateContinuity(FrameMap::iterator start) {
  TRACE_EVENT0("webrtc", "FrameBuffer::PropagateContinuity");
  RTC_DCHECK(start->second.continuous);
  if (last_continuous_frame_it_ == frames_.end())
    last_continuous_frame_it_ = start;

  std::queue<FrameMap::iterator> continuous_frames;
  continuous_frames.push(start);

  // A simple BFS to traverse continuous frames.
  while (!continuous_frames.empty()) {
    auto frame = continuous_frames.front();
    continuous_frames.pop();

    if (last_continuous_frame_it_->first < frame->first)
      last_continuous_frame_it_ = frame;

    // Loop through all dependent frames, and if that frame no longer has
    // any unfulfilled dependencies then that frame is continuous as well.
    for (size_t d = 0; d < frame->second.dependent_frames.size(); ++d) {
      auto frame_ref = frames_.find(frame->second.dependent_frames[d]);
      RTC_DCHECK(frame_ref != frames_.end());

      // TODO(philipel): Look into why we've seen this happen.
      if (frame_ref != frames_.end()) {
        --frame_ref->second.num_missing_continuous;
        if (frame_ref->second.num_missing_continuous == 0) {
          frame_ref->second.continuous = true;
          continuous_frames.push(frame_ref);
        }
      }
    }
  }
}

void FrameBuffer::PropagateDecodability(const FrameInfo& info) {
  TRACE_EVENT0("webrtc", "FrameBuffer::PropagateDecodability");
  for (size_t d = 0; d < info.dependent_frames.size(); ++d) {
    auto ref_info = frames_.find(info.dependent_frames[d]);
    RTC_DCHECK(ref_info != frames_.end());
    // TODO(philipel): Look into why we've seen this happen.
    if (ref_info != frames_.end()) {
      RTC_DCHECK_GT(ref_info->second.num_missing_decodable, 0U);
      --ref_info->second.num_missing_decodable;
    }
  }
}

void FrameBuffer::AdvanceLastDecodedFrame(FrameMap::iterator decoded) {
  TRACE_EVENT0("webrtc", "FrameBuffer::AdvanceLastDecodedFrame");
  if (last_decoded_frame_it_ == frames_.end()) {
    last_decoded_frame_it_ = frames_.begin();
  } else {
    RTC_DCHECK(last_decoded_frame_it_->first < decoded->first);
    ++last_decoded_frame_it_;
  }
  --num_frames_buffered_;
  ++num_frames_history_;

  // First, delete non-decoded frames from the history.
  while (last_decoded_frame_it_ != decoded) {
    if (last_decoded_frame_it_->second.frame)
      --num_frames_buffered_;
    last_decoded_frame_it_ = frames_.erase(last_decoded_frame_it_);
  }

  // Then remove old history if we have too much history saved.
  if (num_frames_history_ > kMaxFramesHistory) {
    frames_.erase(frames_.begin());
    --num_frames_history_;
  }
}

bool FrameBuffer::UpdateFrameInfoWithIncomingFrame(const EncodedFrame& frame,
                                                   FrameMap::iterator info) {
  TRACE_EVENT0("webrtc", "FrameBuffer::UpdateFrameInfoWithIncomingFrame");
  const VideoLayerFrameId& id = frame.id;

  RTC_DCHECK(last_decoded_frame_it_ == frames_.end() ||
             last_decoded_frame_it_->first < info->first);

  // In this function we determine how many missing dependencies this |frame|
  // has to become continuous/decodable. If a frame that this |frame| depend
  // on has already been decoded then we can ignore that dependency since it has
  // already been fulfilled.
  //
  // For all other frames we will register a backwards reference to this |frame|
  // so that |num_missing_continuous| and |num_missing_decodable| can be
  // decremented as frames become continuous/are decoded.
  struct Dependency {
    VideoLayerFrameId id;
    bool continuous;
  };
  std::vector<Dependency> not_yet_fulfilled_dependencies;

  // Find all dependencies that have not yet been fulfilled.
  for (size_t i = 0; i < frame.num_references; ++i) {
    VideoLayerFrameId ref_key(frame.references[i], frame.id.spatial_layer);
    auto ref_info = frames_.find(ref_key);

    // Does |frame| depend on a frame earlier than the last decoded one?
    if (last_decoded_frame_it_ != frames_.end() &&
        ref_key <= last_decoded_frame_it_->first) {
      // Was that frame decoded? If not, this |frame| will never become
      // decodable.
      if (ref_info == frames_.end()) {
        int64_t now_ms = clock_->TimeInMilliseconds();
        if (last_log_non_decoded_ms_ + kLogNonDecodedIntervalMs < now_ms) {
          RTC_LOG(LS_WARNING)
              << "Frame with (picture_id:spatial_id) (" << id.picture_id << ":"
              << static_cast<int>(id.spatial_layer)
              << ") depends on a non-decoded frame more previous than"
              << " the last decoded frame, dropping frame.";
          last_log_non_decoded_ms_ = now_ms;
        }
        return false;
      }
    } else {
      bool ref_continuous =
          ref_info != frames_.end() && ref_info->second.continuous;
      not_yet_fulfilled_dependencies.push_back({ref_key, ref_continuous});
    }
  }

  // Does |frame| depend on the lower spatial layer?
  if (frame.inter_layer_predicted) {
    VideoLayerFrameId ref_key(frame.id.picture_id, frame.id.spatial_layer - 1);
    auto ref_info = frames_.find(ref_key);

    bool lower_layer_continuous =
        ref_info != frames_.end() && ref_info->second.continuous;
    bool lower_layer_decoded = last_decoded_frame_it_ != frames_.end() &&
                               last_decoded_frame_it_->first == ref_key;

    if (!lower_layer_continuous || !lower_layer_decoded) {
      not_yet_fulfilled_dependencies.push_back(
          {ref_key, lower_layer_continuous});
    }
  }

  info->second.num_missing_continuous = not_yet_fulfilled_dependencies.size();
  info->second.num_missing_decodable = not_yet_fulfilled_dependencies.size();

  for (const Dependency& dep : not_yet_fulfilled_dependencies) {
    if (dep.continuous)
      --info->second.num_missing_continuous;

    frames_[dep.id].dependent_frames.push_back(id);
  }

  return true;
}

void FrameBuffer::UpdateJitterDelay() {
  TRACE_EVENT0("webrtc", "FrameBuffer::UpdateJitterDelay");
  if (!stats_callback_)
    return;

  int decode_ms;
  int max_decode_ms;
  int current_delay_ms;
  int target_delay_ms;
  int jitter_buffer_ms;
  int min_playout_delay_ms;
  int render_delay_ms;
  if (timing_->GetTimings(&decode_ms, &max_decode_ms, &current_delay_ms,
                          &target_delay_ms, &jitter_buffer_ms,
                          &min_playout_delay_ms, &render_delay_ms)) {
    stats_callback_->OnFrameBufferTimingsUpdated(
        decode_ms, max_decode_ms, current_delay_ms, target_delay_ms,
        jitter_buffer_ms, min_playout_delay_ms, render_delay_ms);
  }
}

void FrameBuffer::UpdateTimingFrameInfo() {
  TRACE_EVENT0("webrtc", "FrameBuffer::UpdateTimingFrameInfo");
  absl::optional<TimingFrameInfo> info = timing_->GetTimingFrameInfo();
  if (info && stats_callback_)
    stats_callback_->OnTimingFrameInfoUpdated(*info);
}

void FrameBuffer::ClearFramesAndHistory() {
  TRACE_EVENT0("webrtc", "FrameBuffer::ClearFramesAndHistory");
  frames_.clear();
  last_decoded_frame_it_ = frames_.end();
  last_continuous_frame_it_ = frames_.end();
  frames_to_decode_.clear();
  num_frames_history_ = 0;
  num_frames_buffered_ = 0;
}

EncodedFrame* FrameBuffer::CombineAndDeleteFrames(
    const std::vector<EncodedFrame*>& frames) const {
  RTC_DCHECK(!frames.empty());
  EncodedFrame* first_frame = frames[0];
  EncodedFrame* last_frame = frames.back();
  size_t total_length = 0;
  for (size_t i = 0; i < frames.size(); ++i) {
    total_length += frames[i]->size();
  }
  first_frame->VerifyAndAllocate(total_length);

  // Spatial index of combined frame is set equal to spatial index of its top
  // spatial layer.
  first_frame->SetSpatialIndex(last_frame->id.spatial_layer);
  first_frame->id.spatial_layer = last_frame->id.spatial_layer;

  first_frame->video_timing_mutable()->network2_timestamp_ms =
      last_frame->video_timing().network2_timestamp_ms;
  first_frame->video_timing_mutable()->receive_finish_ms =
      last_frame->video_timing().receive_finish_ms;

  // Append all remaining frames to the first one.
  uint8_t* buffer = first_frame->MutableBuffer() + first_frame->size();
  for (size_t i = 1; i < frames.size(); ++i) {
    EncodedFrame* next_frame = frames[i];
    memcpy(buffer, next_frame->Buffer(), next_frame->size());
    buffer += next_frame->size();
    delete next_frame;
  }
  first_frame->set_size(total_length);
  return first_frame;
}

FrameBuffer::FrameInfo::FrameInfo() = default;
FrameBuffer::FrameInfo::FrameInfo(FrameInfo&&) = default;
FrameBuffer::FrameInfo::~FrameInfo() = default;

}  // namespace video_coding
}  // namespace webrtc