base/task/thread_pool/priority_queue.cc

// Copyright 2016 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "base/task/thread_pool/priority_queue.h"

#include <utility>

#include "base/check_op.h"
#include "base/compiler_specific.h"
#include "base/memory/ptr_util.h"

namespace base::internal {

// A class combining a TaskSource and the TaskSourceSortKey that determines its
// position in a PriorityQueue. Instances are only mutable via
// take_task_source() which can only be called once and renders its instance
// invalid after the call.
class PriorityQueue::TaskSourceAndSortKey {
 public:
  TaskSourceAndSortKey() = default;
  TaskSourceAndSortKey(RegisteredTaskSource task_source,
                       const TaskSourceSortKey& sort_key)
      : task_source_(std::move(task_source)), sort_key_(sort_key) {
    DCHECK(task_source_);
  }
  TaskSourceAndSortKey(const TaskSourceAndSortKey&) = delete;
  TaskSourceAndSortKey& operator=(const TaskSourceAndSortKey&) = delete;

  // Note: while |task_source_| should always be non-null post-move (i.e. we
  // shouldn't be moving an invalid TaskSourceAndSortKey around), there can't be
  // a DCHECK(task_source_) on moves as IntrusiveHeap moves elements on pop
  // instead of overwriting them: resulting in the move of a
  // TaskSourceAndSortKey with a null |task_source_| in Transaction::Pop()'s
  // implementation.
  TaskSourceAndSortKey(TaskSourceAndSortKey&& other) = default;
  TaskSourceAndSortKey& operator=(TaskSourceAndSortKey&& other) = default;

  // Extracts |task_source_| from this object. This object is invalid after this
  // call.
  RegisteredTaskSource take_task_source() {
    DCHECK(task_source_);
    task_source_->ClearImmediateHeapHandle();
    return std::move(task_source_);
  }

  // Compares this TaskSourceAndSortKey to |other| based on their respective
  // |sort_key_|. Used for a max-heap.
  bool operator<(const TaskSourceAndSortKey& other) const {
    return sort_key_ < other.sort_key_;
  }

  // Required by IntrusiveHeap.
  void SetHeapHandle(const HeapHandle& handle) {
    DCHECK(task_source_);
    task_source_->SetImmediateHeapHandle(handle);
  }

  // Required by IntrusiveHeap.
  void ClearHeapHandle() {
    // Ensure |task_source_| is not nullptr, which may be the case if
    // take_task_source() was called before this.
    if (task_source_) {
      task_source_->ClearImmediateHeapHandle();
    }
  }

  // Required by IntrusiveHeap.
  HeapHandle GetHeapHandle() const {
    if (task_source_) {
      return task_source_->GetImmediateHeapHandle();
    }
    return HeapHandle::Invalid();
  }

  const RegisteredTaskSource& task_source() const LIFETIME_BOUND {
    return task_source_;
  }
  RegisteredTaskSource& task_source() LIFETIME_BOUND { return task_source_; }

  const TaskSourceSortKey& sort_key() const LIFETIME_BOUND { return sort_key_; }

 private:
  RegisteredTaskSource task_source_;
  TaskSourceSortKey sort_key_;
};

PriorityQueue::PriorityQueue() = default;

PriorityQueue::~PriorityQueue() {
  if (!is_flush_task_sources_on_destroy_enabled_) {
    return;
  }

  while (!container_.empty()) {
    auto task_source = PopTaskSource();
    auto task = task_source.Clear();
    if (task) {
      std::move(task->task).Run();
    }
  }
}

PriorityQueue& PriorityQueue::operator=(PriorityQueue&& other) = default;

void PriorityQueue::Push(RegisteredTaskSource task_source,
                         TaskSourceSortKey task_source_sort_key) {
  container_.insert(
      TaskSourceAndSortKey(std::move(task_source), task_source_sort_key));
  IncrementNumTaskSourcesForThreadType(task_source_sort_key.thread_type());
}

const TaskSourceSortKey& PriorityQueue::PeekSortKey() const {
  DCHECK(!IsEmpty());
  return container_.top().sort_key();
}

RegisteredTaskSource& PriorityQueue::PeekTaskSource() const {
  DCHECK(!IsEmpty());

  // The const_cast on Min() is okay since modifying the TaskSource cannot alter
  // the sort order of TaskSourceAndSortKey.
  auto& task_source_and_sort_key =
      const_cast<PriorityQueue::TaskSourceAndSortKey&>(container_.top());
  return task_source_and_sort_key.task_source();
}

RegisteredTaskSource PriorityQueue::PopTaskSource() {
  DCHECK(!IsEmpty());

  // The const_cast on Min() is okay since the TaskSourceAndSortKey is
  // transactionally being popped from |container_| right after and taking its
  // TaskSource does not alter its sort order.
  auto& task_source_and_sort_key =
      const_cast<TaskSourceAndSortKey&>(container_.top());
  DecrementNumTaskSourcesForThreadType(
      task_source_and_sort_key.sort_key().thread_type());
  RegisteredTaskSource task_source =
      task_source_and_sort_key.take_task_source();
  container_.pop();
  return task_source;
}

RegisteredTaskSource PriorityQueue::RemoveTaskSource(
    const TaskSource& task_source) {
  if (IsEmpty()) {
    return nullptr;
  }

  const HeapHandle heap_handle = task_source.immediate_heap_handle();
  if (!heap_handle.IsValid()) {
    return nullptr;
  }

  TaskSourceAndSortKey& task_source_and_sort_key =
      const_cast<PriorityQueue::TaskSourceAndSortKey&>(
          container_.at(heap_handle));
  DCHECK_EQ(task_source_and_sort_key.task_source().get(), &task_source);
  RegisteredTaskSource registered_task_source =
      task_source_and_sort_key.take_task_source();

  DecrementNumTaskSourcesForThreadType(
      task_source_and_sort_key.sort_key().thread_type());
  container_.erase(heap_handle);
  return registered_task_source;
}

void PriorityQueue::UpdateSortKey(const TaskSource& task_source,
                                  TaskSourceSortKey sort_key) {
  if (IsEmpty()) {
    return;
  }

  const HeapHandle heap_handle = task_source.immediate_heap_handle();
  if (!heap_handle.IsValid()) {
    return;
  }

  auto old_sort_key = container_.at(heap_handle).sort_key();
  auto registered_task_source =
      const_cast<PriorityQueue::TaskSourceAndSortKey&>(
          container_.at(heap_handle))
          .take_task_source();

  DecrementNumTaskSourcesForThreadType(old_sort_key.thread_type());
  IncrementNumTaskSourcesForThreadType(sort_key.thread_type());

  container_.Replace(
      heap_handle,
      TaskSourceAndSortKey(std::move(registered_task_source), sort_key));
}

bool PriorityQueue::IsEmpty() const {
  return container_.empty();
}

size_t PriorityQueue::Size() const {
  return container_.size();
}

void PriorityQueue::EnableFlushTaskSourcesOnDestroyForTesting() {
  DCHECK(!is_flush_task_sources_on_destroy_enabled_);
  is_flush_task_sources_on_destroy_enabled_ = true;
}

void PriorityQueue::swap(PriorityQueue& other) {
  container_.swap(other.container_);
  std::swap(num_foreground_task_sources_, other.num_foreground_task_sources_);
  std::swap(num_background_task_sources_, other.num_background_task_sources_);
  std::swap(is_flush_task_sources_on_destroy_enabled_,
            other.is_flush_task_sources_on_destroy_enabled_);
}

void PriorityQueue::DecrementNumTaskSourcesForThreadType(
    ThreadType thread_type) {
  if (thread_type != ThreadType::kBackground) {
    DCHECK_GT(num_foreground_task_sources_, 0U);
    --num_foreground_task_sources_;
  } else {
    DCHECK_GT(num_background_task_sources_, 0U);
    --num_background_task_sources_;
  }
}

void PriorityQueue::IncrementNumTaskSourcesForThreadType(
    ThreadType thread_type) {
  if (thread_type != ThreadType::kBackground) {
    ++num_foreground_task_sources_;
  } else {
    ++num_background_task_sources_;
  }
}

}  // namespace base::internal