// Copyright 2016 The Chromium Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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#ifndef BASE_TASK_SCHEDULER_SEQUENCE_H_
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#define BASE_TASK_SCHEDULER_SEQUENCE_H_
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#include <stddef.h>
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#include "base/base_export.h"
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#include "base/containers/queue.h"
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#include "base/macros.h"
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#include "base/memory/ref_counted.h"
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#include "base/optional.h"
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#include "base/sequence_token.h"
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#include "base/task_scheduler/scheduler_lock.h"
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#include "base/task_scheduler/sequence_sort_key.h"
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#include "base/task_scheduler/task.h"
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#include "base/threading/sequence_local_storage_map.h"
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namespace base {
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namespace internal {
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// A Sequence holds slots each containing up to a single Task that must be
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// executed in posting order.
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//
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// In comments below, an "empty Sequence" is a Sequence with no slot.
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//
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// Note: there is a known refcounted-ownership cycle in the Scheduler
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// architecture: Sequence -> Task -> TaskRunner -> Sequence -> ...
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// This is okay so long as the other owners of Sequence (PriorityQueue and
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// SchedulerWorker in alternation and
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// SchedulerWorkerPoolImpl::SchedulerWorkerDelegateImpl::GetWork()
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// temporarily) keep running it (and taking Tasks from it as a result). A
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// dangling reference cycle would only occur should they release their reference
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// to it while it's not empty. In other words, it is only correct for them to
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// release it after PopTask() returns false to indicate it was made empty by
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// that call (in which case the next PushTask() will return true to indicate to
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// the caller that the Sequence should be re-enqueued for execution).
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//
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// This class is thread-safe.
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class BASE_EXPORT Sequence : public RefCountedThreadSafe<Sequence> {
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public:
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Sequence();
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// Adds |task| in a new slot at the end of the Sequence. Returns true if the
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// Sequence was empty before this operation.
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bool PushTask(Task task);
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// Transfers ownership of the Task in the front slot of the Sequence to the
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// caller. The front slot of the Sequence will be nullptr and remain until
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// Pop(). Cannot be called on an empty Sequence or a Sequence whose front slot
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// is already nullptr.
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//
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// Because this method cannot be called on an empty Sequence, the returned
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// Optional<Task> is never nullptr. An Optional is used in preparation for the
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// merge between TaskScheduler and TaskQueueManager (in Blink).
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// https://crbug.com/783309
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Optional<Task> TakeTask();
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// Removes the front slot of the Sequence. The front slot must have been
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// emptied by TakeTask() before this is called. Cannot be called on an empty
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// Sequence. Returns true if the Sequence is empty after this operation.
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bool Pop();
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// Returns a SequenceSortKey representing the priority of the Sequence. Cannot
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// be called on an empty Sequence.
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SequenceSortKey GetSortKey() const;
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// Returns a token that uniquely identifies this Sequence.
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const SequenceToken& token() const { return token_; }
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SequenceLocalStorageMap* sequence_local_storage() {
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return &sequence_local_storage_;
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}
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private:
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friend class RefCountedThreadSafe<Sequence>;
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~Sequence();
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const SequenceToken token_ = SequenceToken::Create();
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// Synchronizes access to all members.
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mutable SchedulerLock lock_;
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// Queue of tasks to execute.
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base::queue<Task> queue_;
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// Number of tasks contained in the Sequence for each priority.
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size_t num_tasks_per_priority_[static_cast<int>(TaskPriority::HIGHEST) + 1] =
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{};
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// Holds data stored through the SequenceLocalStorageSlot API.
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SequenceLocalStorageMap sequence_local_storage_;
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DISALLOW_COPY_AND_ASSIGN(Sequence);
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};
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} // namespace internal
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} // namespace base
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#endif // BASE_TASK_SCHEDULER_SEQUENCE_H_
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