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// Copyright 2016 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.
#include <stddef.h>
#include <memory>
#include "base/base_export.h"
#include "base/containers/queue.h"
#include "base/macros.h"
#include "base/memory/ref_counted.h"
#include "base/sequence_token.h"
#include "base/task_scheduler/scheduler_lock.h"
#include "base/task_scheduler/sequence_sort_key.h"
#include "base/task_scheduler/task.h"
#include "base/task_scheduler/task_traits.h"
#include "base/threading/sequence_local_storage_map.h"
namespace base {
namespace internal {
// A Sequence holds slots each containing up to a single Task that must be
// executed in posting 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
// SchedulerWorker in alternation and
// SchedulerWorkerPoolImpl::SchedulerWorkerDelegateImpl::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 PushTask() 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 RefCountedThreadSafe<Sequence> {
// Adds |task| in a new slot at the end of the Sequence. Returns true if the
// Sequence was empty before this operation.
bool PushTask(std::unique_ptr<Task> task);
// Transfers ownership of the Task in the front slot of the Sequence to the
// caller. The front slot of the Sequence will be nullptr and remain until
// Pop(). Cannot be called on an empty Sequence or a Sequence whose front slot
// is already nullptr.
std::unique_ptr<Task> TakeTask();
// Returns the TaskTraits of the Task in front of the Sequence. Cannot be
// called on an empty Sequence or on a Sequence whose front slot is empty.
TaskTraits PeekTaskTraits() const;
// Removes the front slot of the Sequence. The front slot must have been
// emptied by TakeTask() before this is called. Cannot be called on an empty
// Sequence. Returns true if the Sequence is empty after this operation.
bool Pop();
// Returns a SequenceSortKey representing the priority of the Sequence. Cannot
// be called on an empty Sequence.
SequenceSortKey GetSortKey() const;
// Returns a token that uniquely identifies this Sequence.
const SequenceToken& token() const { return token_; }
SequenceLocalStorageMap* sequence_local_storage() {
return &sequence_local_storage_;
friend class RefCountedThreadSafe<Sequence>;
const SequenceToken token_ = SequenceToken::Create();
// Synchronizes access to all members.
mutable SchedulerLock lock_;
// Queue of tasks to execute.
base::queue<std::unique_ptr<Task>> queue_;
// Number of tasks contained in the Sequence for each priority.
size_t num_tasks_per_priority_[static_cast<int>(TaskPriority::HIGHEST) + 1] =
// Holds data stored through the SequenceLocalStorageSlot API.
SequenceLocalStorageMap sequence_local_storage_;
} // namespace internal
} // namespace base