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// Copyright 2021 The Chromium Authors
// 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_TASK_ORDER_H_
#define BASE_TASK_SEQUENCE_MANAGER_TASK_ORDER_H_
#include "base/base_export.h"
#include "base/task/sequence_manager/enqueue_order.h"
#include "base/time/time.h"
namespace base::sequence_manager {
struct Task;
namespace internal {
class Fence;
} // namespace internal
// `TaskOrder` represents the order of a `Task` relative to other `Task`s. The <
// operator on the set of all `TaskOrder`s is a strict total ordering [1].
// `TaskOrder` consists of the following:
// - `enqueue_order_`: The order the task was enqueued. It is assigned at
// posting time for immediate tasks and enqueue time for delayed tasks, which
// is the time at which a pending delayed task is moved to its `WorkQueue`
// (after its delay has expired, during a wake-up). This is the primary
// ordering for tasks. Delayed tasks that are enqueued during the same
// wake-up have the same `enqueue_order_` and their order is decided by
// `delayed_run_time_` and `sequence_num_`.
//
// - `delayed_run_time_`: The latest time at which a delayed task should run;
// only non-zero for delayed tasks. Before they become ripe, delayed tasks
// are maintained in a heap ordered by `latest_delayed_run_time`.
//
// - `sequence_num_`: a strictly increasing number assigned at posting time for
// all tasks. This is used to order delayed tasks if their `enqueue_order_`
// and `delayed_run_time_`s match.
//
// While `TaskOrder` can be used to order a set `Task`s, it is not necessarily
// the order that the associated tasks will run: tasks are executed in order of
// highest to lowest priority, tasks from disabled queues and queues blocked by
// fences are prevented from running, and sequence manager may choose immediate
// over delayed tasks to prevent starvation.
//
// [1] sequence_num is an int rollovers are possible, however it is extremely
// unlikely that two delayed tasks would have the same posting order and delayed
// run time.
class BASE_EXPORT TaskOrder {
public:
TaskOrder(const TaskOrder& other);
TaskOrder& operator=(const TaskOrder& other);
~TaskOrder();
EnqueueOrder enqueue_order() const { return enqueue_order_; }
int sequence_num() const { return sequence_num_; }
// TODO(crbug.com/40158967): Rename to latest_delayed_run_time() for clarity.
TimeTicks delayed_run_time() const { return delayed_run_time_; }
static TaskOrder CreateForTesting(EnqueueOrder enqueue_order,
TimeTicks delayed_run_time,
int sequence_num);
static TaskOrder CreateForTesting(EnqueueOrder enqueue_order);
friend bool operator==(const TaskOrder&, const TaskOrder&) = default;
bool operator>(const TaskOrder& other) const;
bool operator<(const TaskOrder& other) const;
bool operator<=(const TaskOrder& other) const;
bool operator>=(const TaskOrder& other) const;
protected:
TaskOrder(EnqueueOrder enqueue_order,
TimeTicks delayed_run_time,
int sequence_num);
private:
friend class internal::Fence;
friend struct Task;
EnqueueOrder enqueue_order_;
TimeTicks delayed_run_time_;
int sequence_num_;
};
} // namespace base::sequence_manager
#endif // BASE_TASK_SEQUENCE_MANAGER_TASK_ORDER_H_
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