src/base/task/sequence_manager/tasks.h

// Copyright 2018 The Chromium 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 BASE_TASK_SEQUENCE_MANAGER_TASKS_H_
#define BASE_TASK_SEQUENCE_MANAGER_TASKS_H_

#include "base/pending_task.h"
#include "base/task/sequence_manager/enqueue_order.h"

namespace base {
namespace sequence_manager {

constexpr int kTaskTypeNone = 0;

namespace internal {

// Wrapper around PostTask method arguments and the assigned task type.
// Eventually it becomes a PendingTask once accepted by a TaskQueueImpl.
struct BASE_EXPORT PostedTask {
  explicit PostedTask(OnceClosure callback = OnceClosure(),
                      Location location = Location(),
                      TimeDelta delay = TimeDelta(),
                      Nestable nestable = Nestable::kNestable,
                      int task_type = kTaskTypeNone);
  PostedTask(PostedTask&& move_from) noexcept;
  ~PostedTask();

  OnceClosure callback;
  Location location;
  TimeDelta delay;
  Nestable nestable;
  int task_type;

  DISALLOW_COPY_AND_ASSIGN(PostedTask);
};

// Represents a time at which a task wants to run. Tasks scheduled for the
// same point in time will be ordered by their sequence numbers.
struct DelayedWakeUp {
  TimeTicks time;
  int sequence_num;

  bool operator!=(const DelayedWakeUp& other) const {
    return time != other.time || other.sequence_num != sequence_num;
  }

  bool operator==(const DelayedWakeUp& other) const {
    return !(*this != other);
  }

  bool operator<=(const DelayedWakeUp& other) const {
    if (time == other.time) {
      // Debug gcc builds can compare an element against itself.
      DCHECK(sequence_num != other.sequence_num || this == &other);
      // |sequence_num| is int and might wrap around to a negative number when
      // casted from EnqueueOrder. This way of comparison handles that properly.
      return (sequence_num - other.sequence_num) <= 0;
    }
    return time < other.time;
  }
};

}  // namespace internal

// PendingTask with extra metadata for SequenceManager.
struct BASE_EXPORT Task : public PendingTask {
  Task(internal::PostedTask posted_task,
       TimeTicks desired_run_time,
       internal::EnqueueOrder sequence_order,
       internal::EnqueueOrder enqueue_order = internal::EnqueueOrder());

  internal::DelayedWakeUp delayed_wake_up() const {
    return internal::DelayedWakeUp{delayed_run_time, sequence_num};
  }

  // SequenceManager is particularly sensitive to enqueue order,
  // so we have accessors for safety.
  internal::EnqueueOrder enqueue_order() const {
    DCHECK(enqueue_order_);
    return enqueue_order_;
  }

  void set_enqueue_order(internal::EnqueueOrder enqueue_order) {
    DCHECK(!enqueue_order_);
    enqueue_order_ = enqueue_order;
  }

  bool enqueue_order_set() const { return enqueue_order_; }

  int task_type;

 private:
  // Similar to |sequence_num|, but ultimately the |enqueue_order| is what
  // the scheduler uses for task ordering. For immediate tasks |enqueue_order|
  // is set when posted, but for delayed tasks it's not defined until they are
  // enqueued. This is because otherwise delayed tasks could run before
  // an immediate task posted after the delayed task.
  internal::EnqueueOrder enqueue_order_;
};

}  // namespace sequence_manager
}  // namespace base

#endif  // BASE_TASK_SEQUENCE_MANAGER_TASKS_H_