decode_synchronizer.h

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/*

 *  Copyright (c) 2022 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.

*/

#ifndef VIDEO_DECODE_SYNCHRONIZER_H_

#define VIDEO_DECODE_SYNCHRONIZER_H_

#include <stdint.h>

#include <functional>

#include <memory>

#include <set>

#include <utility>

#include "absl/types/optional.h"

#include "api/metronome/metronome.h"

#include "api/sequence_checker.h"

#include "api/task_queue/task_queue_base.h"

#include "api/units/timestamp.h"

#include "rtc_base/checks.h"

#include "rtc_base/thread_annotations.h"

#include "video/frame_decode_scheduler.h"

#include "video/frame_decode_timing.h"

namespace webrtc {

// DecodeSynchronizer synchronizes the frame scheduling by coalescing decoding

// on the metronome.

//

// A video receive stream can use the DecodeSynchronizer by receiving a

// FrameDecodeScheduler instance with `CreateSynchronizedFrameScheduler()`.

// This instance implements FrameDecodeScheduler and can be used as a normal

// scheduler. This instance is owned by the receive stream, and is borrowed by

// the DecodeSynchronizer. The DecodeSynchronizer will stop borrowing the

// instance when `FrameDecodeScheduler::Stop()` is called, after which the

// scheduler may be destroyed by the receive stream.

//

// When a frame is scheduled for decode by a receive stream using the

// DecodeSynchronizer, it will instead be executed on the metronome during the

// tick interval where `max_decode_time` occurs. For example, if a frame is

// scheduled for decode in 50ms and the tick interval is 20ms, then the frame

// will be released for decoding in 2 ticks. See below for illustration,

//

// In the case where the decode time is in the past, or must occur before the

// next metronome tick then the frame will be released right away, allowing a

// delayed stream to catch up quickly.

//

// DecodeSynchronizer is single threaded - all method calls must run on the

// `worker_queue_`.

class DecodeSynchronizer {

 public:

  DecodeSynchronizer(Clock* clock,

                     Metronome* metronome,

                     TaskQueueBase* worker_queue);

  ~DecodeSynchronizer();

  DecodeSynchronizer(const DecodeSynchronizer&) = delete;

  DecodeSynchronizer& operator=(const DecodeSynchronizer&) = delete;

  std::unique_ptr<FrameDecodeScheduler> CreateSynchronizedFrameScheduler();

 private:

  class ScheduledFrame {

   public:

    ScheduledFrame(uint32_t rtp_timestamp,

                   FrameDecodeTiming::FrameSchedule schedule,

                   FrameDecodeScheduler::FrameReleaseCallback callback);

    // Disallow copy since `callback` should only be moved.

    ScheduledFrame(const ScheduledFrame&) = delete;

    ScheduledFrame& operator=(const ScheduledFrame&) = delete;

    ScheduledFrame(ScheduledFrame&&) = default;

    ScheduledFrame& operator=(ScheduledFrame&&) = default;

    // Executes `callback_`.

    void RunFrameReleaseCallback() &&;

    uint32_t rtp_timestamp() const { return rtp_timestamp_; }

    Timestamp LatestDecodeTime() const;

   private:

    uint32_t rtp_timestamp_;

    FrameDecodeTiming::FrameSchedule schedule_;

    FrameDecodeScheduler::FrameReleaseCallback callback_;

};

  class SynchronizedFrameDecodeScheduler : public FrameDecodeScheduler {

   public:

    explicit SynchronizedFrameDecodeScheduler(DecodeSynchronizer* sync);

    ~SynchronizedFrameDecodeScheduler() override;

    // Releases the outstanding frame for decoding. This invalidates

    // `next_frame_`. There must be a frame scheduled.

    ScheduledFrame ReleaseNextFrame();

    // Returns `next_frame_.schedule.max_decode_time`. There must be a frame

    // scheduled when this is called.

    Timestamp LatestDecodeTime();

    // FrameDecodeScheduler implementation.

    absl::optional<uint32_t> ScheduledRtpTimestamp() override;

    void ScheduleFrame(uint32_t rtp,

                       FrameDecodeTiming::FrameSchedule schedule,

                       FrameReleaseCallback cb) override;

    void CancelOutstanding() override;

    void Stop() override;

   private:

    DecodeSynchronizer* sync_;

    absl::optional<ScheduledFrame> next_frame_;

    bool stopped_ = false;

};

  void OnFrameScheduled(SynchronizedFrameDecodeScheduler* scheduler);

  void RemoveFrameScheduler(SynchronizedFrameDecodeScheduler* scheduler);

  void ScheduleNextTick();

  void OnTick();

  Clock* const clock_;

  TaskQueueBase* const worker_queue_;

  Metronome* const metronome_;

  Timestamp expected_next_tick_ = Timestamp::PlusInfinity();

  std::set<SynchronizedFrameDecodeScheduler*> schedulers_

      RTC_GUARDED_BY(worker_queue_);

  bool tick_scheduled_ = false;

  ScopedTaskSafetyDetached safety_;

};

}  // namespace webrtc

#endif  // VIDEO_DECODE_SYNCHRONIZER_H_