pipeline.h

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// Copyright 2013 The Flutter Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
 
#ifndef FLUTTER_SHELL_COMMON_PIPELINE_H_
#define FLUTTER_SHELL_COMMON_PIPELINE_H_
 
#include <deque>
#include <memory>
#include <mutex>
 
#include "flutter/flow/frame_timings.h"
#include "flutter/flow/layers/layer_tree.h"
#include "flutter/fml/macros.h"
#include "flutter/fml/memory/ref_counted.h"
#include "flutter/fml/synchronization/semaphore.h"
#include "flutter/fml/trace_event.h"
 
namespace flutter {
 
struct PipelineProduceResult {
  // Whether the item was successfully pushed into the pipeline.
  bool success = false;
  // Whether it is the first item of the pipeline. Only valid when 'success' is
  // 'true'.
  bool is_first_item = false;
};
 
enum class PipelineConsumeResult {
  // NOLINTBEGIN(readability-identifier-naming)
  NoneAvailable,
  Done,
  MoreAvailable,
  // NOLINTEND(readability-identifier-naming)
};
 
size_t GetNextPipelineTraceID();
 
/// A thread-safe queue of resources for a single consumer and a single
/// producer, with a maximum queue depth.
///
/// Pipelines support two key operations: produce and consume.
///
/// The consumer calls |Consume| to wait for a resource to be produced and
/// consume it when ready.
///
/// The producer calls |Produce| to generate a `ProducerContinuation` which
/// provides a means to enqueue a resource in the pipeline, if the pipeline is
/// below its maximum depth. When the resource has been prepared, the producer
/// calls `Complete` on the continuation, which enqueues the resource and
/// signals the waiting consumer.
///
/// Pipelines generate the following tracing information:
/// * PipelineItem: async flow tracking time taken from the time a producer
///   calls |Produce| to the time a consumer consumes calls |Consume|.
/// * PipelineProduce: async flow tracking time taken from the time a producer
///   calls |Produce| to the time they complete the `ProducerContinuation` with
///   a resource.
/// * Pipeline Depth: counter of inflight resource producers.
///
/// The primary use of this class is as the frame pipeline used in Flutter's
/// animator/rasterizer.
template <class R>
class Pipeline {
 public:
  using Resource = R;
  using ResourcePtr = std::unique_ptr<Resource>;
 
  /// Denotes a spot in the pipeline reserved for the producer to finish
  /// preparing a completed pipeline resource.
  class ProducerContinuation {
   public:
    ProducerContinuation() : trace_id_(0) {}
 
    ProducerContinuation(ProducerContinuation&& other)
        : continuation_(other.continuation_), trace_id_(other.trace_id_) {
      other.continuation_ = nullptr;
      other.trace_id_ = 0;
    }
 
    ProducerContinuation& operator=(ProducerContinuation&& other) {
      std::swap(continuation_, other.continuation_);
      std::swap(trace_id_, other.trace_id_);
      return *this;
    }
 
    ~ProducerContinuation() {
      if (continuation_) {
        continuation_(nullptr, trace_id_);
        TRACE_EVENT_ASYNC_END0("flutter", "PipelineProduce", trace_id_);
        // The continuation is being dropped on the floor. End the flow.
        TRACE_FLOW_END("flutter", "PipelineItem", trace_id_);
        TRACE_EVENT_ASYNC_END0("flutter", "PipelineItem", trace_id_);
      }
    }
 
    /// Completes the continuation with the specified resource.
    [[nodiscard]] PipelineProduceResult Complete(ResourcePtr resource) {
      PipelineProduceResult result;
      if (continuation_) {
        result = continuation_(std::move(resource), trace_id_);
        continuation_ = nullptr;
        TRACE_EVENT_ASYNC_END0("flutter", "PipelineProduce", trace_id_);
        TRACE_FLOW_STEP("flutter", "PipelineItem", trace_id_);
      }
      return result;
    }
 
    explicit operator bool() const { return continuation_ != nullptr; }
 
   private:
    friend class Pipeline;
    using Continuation =
        std::function<PipelineProduceResult(ResourcePtr, size_t)>;
 
    Continuation continuation_;
    uint64_t trace_id_;
 
    ProducerContinuation(const Continuation& continuation, uint64_t trace_id)
        : continuation_(continuation), trace_id_(trace_id) {
      TRACE_EVENT_ASYNC_BEGIN0_WITH_FLOW_IDS("flutter", "PipelineItem",
                                             trace_id_, /*flow_id_count=*/1,
                                             /*flow_ids=*/&trace_id);
      TRACE_FLOW_BEGIN("flutter", "PipelineItem", trace_id_);
      TRACE_EVENT_ASYNC_BEGIN0("flutter", "PipelineProduce", trace_id_);
    }
 
    FML_DISALLOW_COPY_AND_ASSIGN(ProducerContinuation);
  };
 
  explicit Pipeline(uint32_t depth)
      : empty_(depth), available_(0), inflight_(0) {}
 
  ~Pipeline() = default;
 
  bool IsValid() const { return empty_.IsValid() && available_.IsValid(); }
 
  /// Creates a `ProducerContinuation` that a producer can use to add a
  /// resource to the queue.
  ///
  /// If the queue is already at its maximum depth, the `ProducerContinuation`
  /// is returned with success = false.
  ProducerContinuation Produce() {
    if (!empty_.TryWait()) {
      return {};
    }
    ++inflight_;
    FML_TRACE_COUNTER("flutter", "Pipeline Depth",
                      reinterpret_cast<int64_t>(this),      //
                      "frames in flight", inflight_.load()  //
    );
 
    return ProducerContinuation{
        std::bind(&Pipeline::ProducerCommit, this, std::placeholders::_1,
                  std::placeholders::_2),  // continuation
        GetNextPipelineTraceID()};         // trace id
  }
 
  /// Creates a `ProducerContinuation` that will only push the task if the
  /// queue is empty.
  ///
  /// Prefer using |Produce|. ProducerContinuation returned by this method
  /// doesn't guarantee that the frame will be rendered.
  ProducerContinuation ProduceIfEmpty() {
    if (!empty_.TryWait()) {
      return {};
    }
    ++inflight_;
    FML_TRACE_COUNTER("flutter", "Pipeline Depth",
                      reinterpret_cast<int64_t>(this),      //
                      "frames in flight", inflight_.load()  //
    );
 
    return ProducerContinuation{
        std::bind(&Pipeline::ProducerCommitIfEmpty, this, std::placeholders::_1,
                  std::placeholders::_2),  // continuation
        GetNextPipelineTraceID()};         // trace id
  }
 
  using Consumer = std::function<void(ResourcePtr)>;
 
  /// @note Procedure doesn't copy all closures.
  [[nodiscard]] PipelineConsumeResult Consume(const Consumer& consumer) {
    if (consumer == nullptr) {
      return PipelineConsumeResult::NoneAvailable;
    }
 
    if (!available_.TryWait()) {
      return PipelineConsumeResult::NoneAvailable;
    }
 
    ResourcePtr resource;
    size_t trace_id = 0;
    size_t items_count = 0;
 
    {
      std::scoped_lock lock(queue_mutex_);
      std::tie(resource, trace_id) = std::move(queue_.front());
      queue_.pop_front();
      items_count = queue_.size();
    }
 
    consumer(std::move(resource));
 
    empty_.Signal();
    --inflight_;
 
    TRACE_FLOW_END("flutter", "PipelineItem", trace_id);
    TRACE_EVENT_ASYNC_END0("flutter", "PipelineItem", trace_id);
 
    return items_count > 0 ? PipelineConsumeResult::MoreAvailable
                           : PipelineConsumeResult::Done;
  }
 
 private:
  fml::Semaphore empty_;
  fml::Semaphore available_;
  std::atomic<int> inflight_;
  std::mutex queue_mutex_;
  std::deque<std::pair<ResourcePtr, size_t>> queue_;
 
  /// Commits a produced resource to the queue and signals the consumer that a
  /// resource is available.
  PipelineProduceResult ProducerCommit(ResourcePtr resource, size_t trace_id) {
    bool is_first_item = false;
    {
      std::scoped_lock lock(queue_mutex_);
      is_first_item = queue_.empty();
      queue_.emplace_back(std::move(resource), trace_id);
    }
 
    // Ensure the queue mutex is not held as that would be a pessimization.
    available_.Signal();
    return {.success = true, .is_first_item = is_first_item};
  }
 
  PipelineProduceResult ProducerCommitIfEmpty(ResourcePtr resource,
                                              size_t trace_id) {
    {
      std::scoped_lock lock(queue_mutex_);
      if (!queue_.empty()) {
        // Bail if the queue is not empty, opens up spaces to produce other
        // frames.
        empty_.Signal();
        return {.success = false, .is_first_item = false};
      }
      queue_.emplace_back(std::move(resource), trace_id);
    }
 
    // Ensure the queue mutex is not held as that would be a pessimization.
    available_.Signal();
    return {.success = true, .is_first_item = true};
  }
 
  FML_DISALLOW_COPY_AND_ASSIGN(Pipeline);
};
 
}  // namespace flutter
 
#endif  // FLUTTER_SHELL_COMMON_PIPELINE_H_