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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 <memory>
#include <queue>
#include <vector>
#include "base/base_export.h"
#include "base/macros.h"
#include "base/memory/ref_counted.h"
#include "base/task/task_scheduler/scheduler_lock.h"
#include "base/task/task_scheduler/sequence.h"
#include "base/task/task_scheduler/sequence_sort_key.h"
namespace base {
namespace internal {
// A PriorityQueue holds Sequences of Tasks. This class is thread-safe.
class BASE_EXPORT PriorityQueue {
// A Transaction can perform multiple operations atomically on a
// PriorityQueue. While a Transaction is alive, it is guaranteed that nothing
// else will access the PriorityQueue.
// A Worker needs to be able to Peek sequences from both its PriorityQueues
// (single-threaded and shared) and then Pop the sequence with the highest
// priority. If the Peek and the Pop are done through the same Transaction, it
// is guaranteed that the PriorityQueue hasn't changed between the 2
// operations.
class BASE_EXPORT Transaction {
// Inserts |sequence| in the PriorityQueue with |sequence_sort_key|.
// Note: |sequence_sort_key| is required as a parameter instead of being
// extracted from |sequence| in Push() to avoid this Transaction having a
// lock interdependency with |sequence|.
void Push(scoped_refptr<Sequence> sequence,
const SequenceSortKey& sequence_sort_key);
// Returns a reference to the SequenceSortKey representing the priority of
// the highest pending task in this PriorityQueue. The reference becomes
// invalid the next time that this PriorityQueue is modified.
// Cannot be called on an empty PriorityQueue.
const SequenceSortKey& PeekSortKey() const;
// Removes and returns the highest priority Sequence in this PriorityQueue.
// Cannot be called on an empty PriorityQueue.
scoped_refptr<Sequence> PopSequence();
// Returns true if the PriorityQueue is empty.
bool IsEmpty() const;
// Returns the number of Sequences in the PriorityQueue.
size_t Size() const;
friend class PriorityQueue;
explicit Transaction(PriorityQueue* outer_queue);
// Holds the lock of |outer_queue_| for the lifetime of this Transaction.
AutoSchedulerLock auto_lock_;
PriorityQueue* const outer_queue_;
// Begins a Transaction. This method cannot be called on a thread which has an
// active Transaction unless the last Transaction created on the thread was
// for the allowed predecessor specified in the constructor of this
// PriorityQueue.
std::unique_ptr<Transaction> BeginTransaction();
const SchedulerLock* container_lock() const { return &container_lock_; }
// A class combining a Sequence and the SequenceSortKey that determines its
// position in a PriorityQueue.
class SequenceAndSortKey;
using ContainerType = std::priority_queue<SequenceAndSortKey>;
// Synchronizes access to |container_|.
SchedulerLock container_lock_;
ContainerType container_;
} // namespace internal
} // namespace base