chromium/base/task/thread_pool/sequence.h

// 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.

#ifndef BASE_TASK_THREAD_POOL_SEQUENCE_H_
#define BASE_TASK_THREAD_POOL_SEQUENCE_H_

#include <stddef.h>

#include "base/base_export.h"
#include "base/containers/intrusive_heap.h"
#include "base/containers/queue.h"
#include "base/sequence_token.h"
#include "base/task/task_traits.h"
#include "base/task/thread_pool/pooled_parallel_task_runner.h"
#include "base/task/thread_pool/task.h"
#include "base/task/thread_pool/task_source.h"
#include "base/task/thread_pool/task_source_sort_key.h"
#include "base/thread_annotations.h"
#include "base/threading/sequence_local_storage_map.h"

namespace base {
namespace internal {

// A Sequence is intended to hold delayed tasks and immediate tasks.
// Delayed tasks are held in a prority_queue until they are ripe and
// immediate tasks in a simple fifo queue.
// When Sequence::TakeTask is called, we select the next appropriate task
// from both queues and return it.
// Each queue holds slots each containing up to a single Task that must be
// executed in posting/runtime order.
//
// In comments below, an "empty Sequence" is a Sequence with no slot.
//
// Note: there is a known refcounted-ownership cycle in the Scheduler
// architecture: Sequence -> Task -> TaskRunner -> Sequence -> ...
// This is okay so long as the other owners of Sequence (PriorityQueue and
// WorkerThread in alternation and
// ThreadGroup::WorkerThreadDelegateImpl::GetWork()
// temporarily) keep running it (and taking Tasks from it as a result). A
// dangling reference cycle would only occur should they release their reference
// to it while it's not empty. In other words, it is only correct for them to
// release it after PopTask() returns false to indicate it was made empty by
// that call (in which case the next PushImmediateTask() will return true to
// indicate to the caller that the Sequence should be re-enqueued for
// execution). This class is thread-safe.
class BASE_EXPORT Sequence : public TaskSource { … };

}  // namespace internal
}  // namespace base

#endif  // BASE_TASK_THREAD_POOL_SEQUENCE_H_