webrtc/modules/video_coding/receiver_unittest.cc

/*  Copyright (c) 2013 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 <string.h>

#include <list>
#include <queue>

#include "testing/gtest/include/gtest/gtest.h"
#include "webrtc/base/checks.h"
#include "webrtc/modules/video_coding/encoded_frame.h"
#include "webrtc/modules/video_coding/packet.h"
#include "webrtc/modules/video_coding/receiver.h"
#include "webrtc/modules/video_coding/test/stream_generator.h"
#include "webrtc/modules/video_coding/timing.h"
#include "webrtc/modules/video_coding/test/test_util.h"
#include "webrtc/system_wrappers/include/clock.h"
#include "webrtc/system_wrappers/include/critical_section_wrapper.h"

namespace webrtc {

class TestVCMReceiver : public ::testing::Test {
 protected:
  enum { kWidth = 640 };
  enum { kHeight = 480 };

  TestVCMReceiver()
      : clock_(new SimulatedClock(0)),
        timing_(clock_.get()),
        receiver_(&timing_, clock_.get(), &event_factory_) {

    stream_generator_.reset(new
        StreamGenerator(0, clock_->TimeInMilliseconds()));
  }

  virtual void SetUp() {
    receiver_.Reset();
  }

  int32_t InsertPacket(int index) {
    VCMPacket packet;
    bool packet_available = stream_generator_->GetPacket(&packet, index);
    EXPECT_TRUE(packet_available);
    if (!packet_available)
      return kGeneralError;  // Return here to avoid crashes below.
    return receiver_.InsertPacket(packet, kWidth, kHeight);
  }

  int32_t InsertPacketAndPop(int index) {
    VCMPacket packet;
    bool packet_available = stream_generator_->PopPacket(&packet, index);
    EXPECT_TRUE(packet_available);
    if (!packet_available)
      return kGeneralError;  // Return here to avoid crashes below.
    return receiver_.InsertPacket(packet, kWidth, kHeight);
  }

  int32_t InsertFrame(FrameType frame_type, bool complete) {
    int num_of_packets = complete ? 1 : 2;
    stream_generator_->GenerateFrame(
        frame_type, (frame_type != kEmptyFrame) ? num_of_packets : 0,
        (frame_type == kEmptyFrame) ? 1 : 0, clock_->TimeInMilliseconds());
    int32_t ret = InsertPacketAndPop(0);
    if (!complete) {
      // Drop the second packet.
      VCMPacket packet;
      stream_generator_->PopPacket(&packet, 0);
    }
    clock_->AdvanceTimeMilliseconds(kDefaultFramePeriodMs);
    return ret;
  }

  bool DecodeNextFrame() {
    int64_t render_time_ms = 0;
    VCMEncodedFrame* frame =
        receiver_.FrameForDecoding(0, render_time_ms, false);
    if (!frame)
      return false;
    receiver_.ReleaseFrame(frame);
    return true;
  }

  rtc::scoped_ptr<SimulatedClock> clock_;
  VCMTiming timing_;
  NullEventFactory event_factory_;
  VCMReceiver receiver_;
  rtc::scoped_ptr<StreamGenerator> stream_generator_;
};

TEST_F(TestVCMReceiver, RenderBufferSize_AllComplete) {
  EXPECT_EQ(0, receiver_.RenderBufferSizeMs());
  EXPECT_GE(InsertFrame(kVideoFrameKey, true), kNoError);
  int num_of_frames = 10;
  for (int i = 0; i < num_of_frames; ++i) {
    EXPECT_GE(InsertFrame(kVideoFrameDelta, true), kNoError);
  }
  EXPECT_EQ(num_of_frames * kDefaultFramePeriodMs,
            receiver_.RenderBufferSizeMs());
}

TEST_F(TestVCMReceiver, RenderBufferSize_SkipToKeyFrame) {
  EXPECT_EQ(0, receiver_.RenderBufferSizeMs());
  const int kNumOfNonDecodableFrames = 2;
  for (int i = 0; i < kNumOfNonDecodableFrames; ++i) {
    EXPECT_GE(InsertFrame(kVideoFrameDelta, true), kNoError);
  }
  const int kNumOfFrames = 10;
  EXPECT_GE(InsertFrame(kVideoFrameKey, true), kNoError);
  for (int i = 0; i < kNumOfFrames - 1; ++i) {
    EXPECT_GE(InsertFrame(kVideoFrameDelta, true), kNoError);
  }
  EXPECT_EQ((kNumOfFrames - 1) * kDefaultFramePeriodMs,
      receiver_.RenderBufferSizeMs());
}

TEST_F(TestVCMReceiver, RenderBufferSize_NotAllComplete) {
  EXPECT_EQ(0, receiver_.RenderBufferSizeMs());
  EXPECT_GE(InsertFrame(kVideoFrameKey, true), kNoError);
  int num_of_frames = 10;
  for (int i = 0; i < num_of_frames; ++i) {
    EXPECT_GE(InsertFrame(kVideoFrameDelta, true), kNoError);
  }
  num_of_frames++;
  EXPECT_GE(InsertFrame(kVideoFrameDelta, false), kNoError);
  for (int i = 0; i < num_of_frames; ++i) {
    EXPECT_GE(InsertFrame(kVideoFrameDelta, true), kNoError);
  }
  EXPECT_EQ((num_of_frames - 1) * kDefaultFramePeriodMs,
      receiver_.RenderBufferSizeMs());
}

TEST_F(TestVCMReceiver, RenderBufferSize_NoKeyFrame) {
  EXPECT_EQ(0, receiver_.RenderBufferSizeMs());
  int num_of_frames = 10;
  for (int i = 0; i < num_of_frames; ++i) {
    EXPECT_GE(InsertFrame(kVideoFrameDelta, true), kNoError);
  }
  int64_t next_render_time_ms = 0;
  VCMEncodedFrame* frame =
      receiver_.FrameForDecoding(10, next_render_time_ms, false);
  EXPECT_TRUE(frame == NULL);
  receiver_.ReleaseFrame(frame);
  EXPECT_GE(InsertFrame(kVideoFrameDelta, false), kNoError);
  for (int i = 0; i < num_of_frames; ++i) {
    EXPECT_GE(InsertFrame(kVideoFrameDelta, true), kNoError);
  }
  EXPECT_EQ(0, receiver_.RenderBufferSizeMs());
}

TEST_F(TestVCMReceiver, NonDecodableDuration_Empty) {
  // Enable NACK and with no RTT thresholds for disabling retransmission delay.
  receiver_.SetNackMode(kNack, -1, -1);
  const size_t kMaxNackListSize = 1000;
  const int kMaxPacketAgeToNack = 1000;
  const int kMaxNonDecodableDuration = 500;
  const int kMinDelayMs = 500;
  receiver_.SetNackSettings(kMaxNackListSize, kMaxPacketAgeToNack,
      kMaxNonDecodableDuration);
  EXPECT_GE(InsertFrame(kVideoFrameKey, true), kNoError);
  // Advance time until it's time to decode the key frame.
  clock_->AdvanceTimeMilliseconds(kMinDelayMs);
  EXPECT_TRUE(DecodeNextFrame());
  bool request_key_frame = false;
  std::vector<uint16_t> nack_list = receiver_.NackList(&request_key_frame);
  EXPECT_FALSE(request_key_frame);
}

TEST_F(TestVCMReceiver, NonDecodableDuration_NoKeyFrame) {
  // Enable NACK and with no RTT thresholds for disabling retransmission delay.
  receiver_.SetNackMode(kNack, -1, -1);
  const size_t kMaxNackListSize = 1000;
  const int kMaxPacketAgeToNack = 1000;
  const int kMaxNonDecodableDuration = 500;
  receiver_.SetNackSettings(kMaxNackListSize, kMaxPacketAgeToNack,
      kMaxNonDecodableDuration);
  const int kNumFrames = kDefaultFrameRate * kMaxNonDecodableDuration / 1000;
  for (int i = 0; i < kNumFrames; ++i) {
    EXPECT_GE(InsertFrame(kVideoFrameDelta, true), kNoError);
  }
  bool request_key_frame = false;
  std::vector<uint16_t> nack_list = receiver_.NackList(&request_key_frame);
  EXPECT_TRUE(request_key_frame);
}

TEST_F(TestVCMReceiver, NonDecodableDuration_OneIncomplete) {
  // Enable NACK and with no RTT thresholds for disabling retransmission delay.
  receiver_.SetNackMode(kNack, -1, -1);
  const size_t kMaxNackListSize = 1000;
  const int kMaxPacketAgeToNack = 1000;
  const int kMaxNonDecodableDuration = 500;
  const int kMaxNonDecodableDurationFrames = (kDefaultFrameRate *
      kMaxNonDecodableDuration + 500) / 1000;
  const int kMinDelayMs = 500;
  receiver_.SetNackSettings(kMaxNackListSize, kMaxPacketAgeToNack,
      kMaxNonDecodableDuration);
  receiver_.SetMinReceiverDelay(kMinDelayMs);
  int64_t key_frame_inserted = clock_->TimeInMilliseconds();
  EXPECT_GE(InsertFrame(kVideoFrameKey, true), kNoError);
  // Insert an incomplete frame.
  EXPECT_GE(InsertFrame(kVideoFrameDelta, false), kNoError);
  // Insert enough frames to have too long non-decodable sequence.
  for (int i = 0; i < kMaxNonDecodableDurationFrames;
       ++i) {
    EXPECT_GE(InsertFrame(kVideoFrameDelta, true), kNoError);
  }
  // Advance time until it's time to decode the key frame.
  clock_->AdvanceTimeMilliseconds(kMinDelayMs - clock_->TimeInMilliseconds() -
      key_frame_inserted);
  EXPECT_TRUE(DecodeNextFrame());
  // Make sure we get a key frame request.
  bool request_key_frame = false;
  std::vector<uint16_t> nack_list = receiver_.NackList(&request_key_frame);
  EXPECT_TRUE(request_key_frame);
}

TEST_F(TestVCMReceiver, NonDecodableDuration_NoTrigger) {
  // Enable NACK and with no RTT thresholds for disabling retransmission delay.
  receiver_.SetNackMode(kNack, -1, -1);
  const size_t kMaxNackListSize = 1000;
  const int kMaxPacketAgeToNack = 1000;
  const int kMaxNonDecodableDuration = 500;
  const int kMaxNonDecodableDurationFrames = (kDefaultFrameRate *
      kMaxNonDecodableDuration + 500) / 1000;
  const int kMinDelayMs = 500;
  receiver_.SetNackSettings(kMaxNackListSize, kMaxPacketAgeToNack,
      kMaxNonDecodableDuration);
  receiver_.SetMinReceiverDelay(kMinDelayMs);
  int64_t key_frame_inserted = clock_->TimeInMilliseconds();
  EXPECT_GE(InsertFrame(kVideoFrameKey, true), kNoError);
  // Insert an incomplete frame.
  EXPECT_GE(InsertFrame(kVideoFrameDelta, false), kNoError);
  // Insert all but one frame to not trigger a key frame request due to
  // too long duration of non-decodable frames.
  for (int i = 0; i < kMaxNonDecodableDurationFrames - 1;
       ++i) {
    EXPECT_GE(InsertFrame(kVideoFrameDelta, true), kNoError);
  }
  // Advance time until it's time to decode the key frame.
  clock_->AdvanceTimeMilliseconds(kMinDelayMs - clock_->TimeInMilliseconds() -
      key_frame_inserted);
  EXPECT_TRUE(DecodeNextFrame());
  // Make sure we don't get a key frame request since we haven't generated
  // enough frames.
  bool request_key_frame = false;
  std::vector<uint16_t> nack_list = receiver_.NackList(&request_key_frame);
  EXPECT_FALSE(request_key_frame);
}

TEST_F(TestVCMReceiver, NonDecodableDuration_NoTrigger2) {
  // Enable NACK and with no RTT thresholds for disabling retransmission delay.
  receiver_.SetNackMode(kNack, -1, -1);
  const size_t kMaxNackListSize = 1000;
  const int kMaxPacketAgeToNack = 1000;
  const int kMaxNonDecodableDuration = 500;
  const int kMaxNonDecodableDurationFrames = (kDefaultFrameRate *
      kMaxNonDecodableDuration + 500) / 1000;
  const int kMinDelayMs = 500;
  receiver_.SetNackSettings(kMaxNackListSize, kMaxPacketAgeToNack,
      kMaxNonDecodableDuration);
  receiver_.SetMinReceiverDelay(kMinDelayMs);
  int64_t key_frame_inserted = clock_->TimeInMilliseconds();
  EXPECT_GE(InsertFrame(kVideoFrameKey, true), kNoError);
  // Insert enough frames to have too long non-decodable sequence, except that
  // we don't have any losses.
  for (int i = 0; i < kMaxNonDecodableDurationFrames;
       ++i) {
    EXPECT_GE(InsertFrame(kVideoFrameDelta, true), kNoError);
  }
  // Insert an incomplete frame.
  EXPECT_GE(InsertFrame(kVideoFrameDelta, false), kNoError);
  // Advance time until it's time to decode the key frame.
  clock_->AdvanceTimeMilliseconds(kMinDelayMs - clock_->TimeInMilliseconds() -
      key_frame_inserted);
  EXPECT_TRUE(DecodeNextFrame());
  // Make sure we don't get a key frame request since the non-decodable duration
  // is only one frame.
  bool request_key_frame = false;
  std::vector<uint16_t> nack_list = receiver_.NackList(&request_key_frame);
  EXPECT_FALSE(request_key_frame);
}

TEST_F(TestVCMReceiver, NonDecodableDuration_KeyFrameAfterIncompleteFrames) {
  // Enable NACK and with no RTT thresholds for disabling retransmission delay.
  receiver_.SetNackMode(kNack, -1, -1);
  const size_t kMaxNackListSize = 1000;
  const int kMaxPacketAgeToNack = 1000;
  const int kMaxNonDecodableDuration = 500;
  const int kMaxNonDecodableDurationFrames = (kDefaultFrameRate *
      kMaxNonDecodableDuration + 500) / 1000;
  const int kMinDelayMs = 500;
  receiver_.SetNackSettings(kMaxNackListSize, kMaxPacketAgeToNack,
      kMaxNonDecodableDuration);
  receiver_.SetMinReceiverDelay(kMinDelayMs);
  int64_t key_frame_inserted = clock_->TimeInMilliseconds();
  EXPECT_GE(InsertFrame(kVideoFrameKey, true), kNoError);
  // Insert an incomplete frame.
  EXPECT_GE(InsertFrame(kVideoFrameDelta, false), kNoError);
  // Insert enough frames to have too long non-decodable sequence.
  for (int i = 0; i < kMaxNonDecodableDurationFrames;
       ++i) {
    EXPECT_GE(InsertFrame(kVideoFrameDelta, true), kNoError);
  }
  EXPECT_GE(InsertFrame(kVideoFrameKey, true), kNoError);
  // Advance time until it's time to decode the key frame.
  clock_->AdvanceTimeMilliseconds(kMinDelayMs - clock_->TimeInMilliseconds() -
      key_frame_inserted);
  EXPECT_TRUE(DecodeNextFrame());
  // Make sure we don't get a key frame request since we have a key frame
  // in the list.
  bool request_key_frame = false;
  std::vector<uint16_t> nack_list = receiver_.NackList(&request_key_frame);
  EXPECT_FALSE(request_key_frame);
}

// A simulated clock, when time elapses, will insert frames into the jitter
// buffer, based on initial settings.
class SimulatedClockWithFrames : public SimulatedClock {
 public:
  SimulatedClockWithFrames(StreamGenerator* stream_generator,
                           VCMReceiver* receiver)
      : SimulatedClock(0),
        stream_generator_(stream_generator),
        receiver_(receiver) {}
  virtual ~SimulatedClockWithFrames() {}

  // If |stop_on_frame| is true and next frame arrives between now and
  // now+|milliseconds|, the clock will be advanced to the arrival time of next
  // frame.
  // Otherwise, the clock will be advanced by |milliseconds|.
  //
  // For both cases, a frame will be inserted into the jitter buffer at the
  // instant when the clock time is timestamps_.front().arrive_time.
  //
  // Return true if some frame arrives between now and now+|milliseconds|.
  bool AdvanceTimeMilliseconds(int64_t milliseconds, bool stop_on_frame) {
    return AdvanceTimeMicroseconds(milliseconds * 1000, stop_on_frame);
  };

  bool AdvanceTimeMicroseconds(int64_t microseconds, bool stop_on_frame) {
    int64_t start_time = TimeInMicroseconds();
    int64_t end_time = start_time + microseconds;
    bool frame_injected = false;
    while (!timestamps_.empty() &&
           timestamps_.front().arrive_time <= end_time) {
      RTC_DCHECK(timestamps_.front().arrive_time >= start_time);

      SimulatedClock::AdvanceTimeMicroseconds(timestamps_.front().arrive_time -
                                              TimeInMicroseconds());
      GenerateAndInsertFrame((timestamps_.front().render_time + 500) / 1000);
      timestamps_.pop();
      frame_injected = true;

      if (stop_on_frame)
        return frame_injected;
    }

    if (TimeInMicroseconds() < end_time) {
      SimulatedClock::AdvanceTimeMicroseconds(end_time - TimeInMicroseconds());
    }
    return frame_injected;
  };

  // Input timestamps are in unit Milliseconds.
  // And |arrive_timestamps| must be positive and in increasing order.
  // |arrive_timestamps| determine when we are going to insert frames into the
  // jitter buffer.
  // |render_timestamps| are the timestamps on the frame.
  void SetFrames(const int64_t* arrive_timestamps,
                 const int64_t* render_timestamps,
                 size_t size) {
    int64_t previous_arrive_timestamp = 0;
    for (size_t i = 0; i < size; i++) {
      RTC_CHECK(arrive_timestamps[i] >= previous_arrive_timestamp);
      timestamps_.push(TimestampPair(arrive_timestamps[i] * 1000,
                                     render_timestamps[i] * 1000));
      previous_arrive_timestamp = arrive_timestamps[i];
    }
  }

 private:
  struct TimestampPair {
    TimestampPair(int64_t arrive_timestamp, int64_t render_timestamp)
        : arrive_time(arrive_timestamp), render_time(render_timestamp) {}

    int64_t arrive_time;
    int64_t render_time;
  };

  void GenerateAndInsertFrame(int64_t render_timestamp_ms) {
    VCMPacket packet;
    stream_generator_->GenerateFrame(FrameType::kVideoFrameKey,
                                     1,  // media packets
                                     0,  // empty packets
                                     render_timestamp_ms);

    bool packet_available = stream_generator_->PopPacket(&packet, 0);
    EXPECT_TRUE(packet_available);
    if (!packet_available)
      return;  // Return here to avoid crashes below.
    receiver_->InsertPacket(packet, 640, 480);
  }

  std::queue<TimestampPair> timestamps_;
  StreamGenerator* stream_generator_;
  VCMReceiver* receiver_;
};

// Use a SimulatedClockWithFrames
// Wait call will do either of these:
// 1. If |stop_on_frame| is true, the clock will be turned to the exact instant
// that the first frame comes and the frame will be inserted into the jitter
// buffer, or the clock will be turned to now + |max_time| if no frame comes in
// the window.
// 2. If |stop_on_frame| is false, the clock will be turn to now + |max_time|,
// and all the frames arriving between now and now + |max_time| will be
// inserted into the jitter buffer.
//
// This is used to simulate the JitterBuffer getting packets from internet as
// time elapses.

class FrameInjectEvent : public EventWrapper {
 public:
  FrameInjectEvent(SimulatedClockWithFrames* clock, bool stop_on_frame)
      : clock_(clock), stop_on_frame_(stop_on_frame) {}

  bool Set() override { return true; }

  EventTypeWrapper Wait(unsigned long max_time) override {
    if (clock_->AdvanceTimeMilliseconds(max_time, stop_on_frame_) &&
        stop_on_frame_) {
      return EventTypeWrapper::kEventSignaled;
    } else {
      return EventTypeWrapper::kEventTimeout;
    }
  }

 private:
  SimulatedClockWithFrames* clock_;
  bool stop_on_frame_;
};

class VCMReceiverTimingTest : public ::testing::Test {
 protected:

  VCMReceiverTimingTest()

      : clock_(&stream_generator_, &receiver_),
        stream_generator_(0, clock_.TimeInMilliseconds()),
        timing_(&clock_),
        receiver_(
            &timing_,
            &clock_,
            rtc::scoped_ptr<EventWrapper>(new FrameInjectEvent(&clock_, false)),
            rtc::scoped_ptr<EventWrapper>(
                new FrameInjectEvent(&clock_, true))) {}


  virtual void SetUp() { receiver_.Reset(); }

  SimulatedClockWithFrames clock_;
  StreamGenerator stream_generator_;
  VCMTiming timing_;
  VCMReceiver receiver_;
};

// Test whether VCMReceiver::FrameForDecoding handles parameter
// |max_wait_time_ms| correctly:
// 1. The function execution should never take more than |max_wait_time_ms|.
// 2. If the function exit before now + |max_wait_time_ms|, a frame must be
//    returned.
TEST_F(VCMReceiverTimingTest, FrameForDecoding) {
  const size_t kNumFrames = 100;
  const int kFramePeriod = 40;
  int64_t arrive_timestamps[kNumFrames];
  int64_t render_timestamps[kNumFrames];
  int64_t next_render_time;

  // Construct test samples.
  // render_timestamps are the timestamps stored in the Frame;
  // arrive_timestamps controls when the Frame packet got received.
  for (size_t i = 0; i < kNumFrames; i++) {
    // Preset frame rate to 25Hz.
    // But we add a reasonable deviation to arrive_timestamps to mimic Internet
    // fluctuation.
    arrive_timestamps[i] =
        (i + 1) * kFramePeriod + (i % 10) * ((i % 2) ? 1 : -1);
    render_timestamps[i] = (i + 1) * kFramePeriod;
  }

  clock_.SetFrames(arrive_timestamps, render_timestamps, kNumFrames);

  // Record how many frames we finally get out of the receiver.
  size_t num_frames_return = 0;

  const int64_t kMaxWaitTime = 30;

  // Ideally, we should get all frames that we input in InitializeFrames.
  // In the case that FrameForDecoding kills frames by error, we rely on the
  // build bot to kill the test.
  while (num_frames_return < kNumFrames) {
    int64_t start_time = clock_.TimeInMilliseconds();
    VCMEncodedFrame* frame =
        receiver_.FrameForDecoding(kMaxWaitTime, next_render_time, false);
    int64_t end_time = clock_.TimeInMilliseconds();

    // In any case the FrameForDecoding should not wait longer than
    // max_wait_time.
    // In the case that we did not get a frame, it should have been waiting for
    // exactly max_wait_time. (By the testing samples we constructed above, we
    // are sure there is no timing error, so the only case it returns with NULL
    // is that it runs out of time.)
    if (frame) {
      receiver_.ReleaseFrame(frame);
      ++num_frames_return;
      EXPECT_GE(kMaxWaitTime, end_time - start_time);
    } else {
      EXPECT_EQ(kMaxWaitTime, end_time - start_time);
    }
  }
}

// Test whether VCMReceiver::FrameForDecoding handles parameter
// |prefer_late_decoding| and |max_wait_time_ms| correctly:
// 1. The function execution should never take more than |max_wait_time_ms|.
// 2. If the function exit before now + |max_wait_time_ms|, a frame must be
//    returned and the end time must be equal to the render timestamp - delay
//    for decoding and rendering.
TEST_F(VCMReceiverTimingTest, FrameForDecodingPreferLateDecoding) {
  const size_t kNumFrames = 100;
  const int kFramePeriod = 40;

  int64_t arrive_timestamps[kNumFrames];
  int64_t render_timestamps[kNumFrames];
  int64_t next_render_time;

  int render_delay_ms;
  int max_decode_ms;
  int dummy;
  timing_.GetTimings(&dummy, &max_decode_ms, &dummy, &dummy, &dummy, &dummy,
                     &render_delay_ms);

  // Construct test samples.
  // render_timestamps are the timestamps stored in the Frame;
  // arrive_timestamps controls when the Frame packet got received.
  for (size_t i = 0; i < kNumFrames; i++) {
    // Preset frame rate to 25Hz.
    // But we add a reasonable deviation to arrive_timestamps to mimic Internet
    // fluctuation.
    arrive_timestamps[i] =
        (i + 1) * kFramePeriod + (i % 10) * ((i % 2) ? 1 : -1);
    render_timestamps[i] = (i + 1) * kFramePeriod;
  }

  clock_.SetFrames(arrive_timestamps, render_timestamps, kNumFrames);

  // Record how many frames we finally get out of the receiver.
  size_t num_frames_return = 0;
  const int64_t kMaxWaitTime = 30;
  bool prefer_late_decoding = true;
  while (num_frames_return < kNumFrames) {
    int64_t start_time = clock_.TimeInMilliseconds();

    VCMEncodedFrame* frame =
        receiver_.FrameForDecoding(kMaxWaitTime, next_render_time,
                                   prefer_late_decoding);
    int64_t end_time = clock_.TimeInMilliseconds();
    if (frame) {
      EXPECT_EQ(frame->RenderTimeMs() - max_decode_ms - render_delay_ms,
                end_time);
      receiver_.ReleaseFrame(frame);
      ++num_frames_return;
    } else {
      EXPECT_EQ(kMaxWaitTime, end_time - start_time);
    }
  }
}

}  // namespace webrtc