TaskScheduler [4/9] Priority Queue

base/task_scheduler/priority_queue.h

+// 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.
+
+#ifndef BASE_TASK_SCHEDULER_PRIORITY_QUEUE_H_
+#define BASE_TASK_SCHEDULER_PRIORITY_QUEUE_H_
+
+#include <queue>
+#include <vector>
+
+#include "base/base_export.h"
+#include "base/callback.h"
+#include "base/macros.h"
+#include "base/memory/ref_counted.h"
+#include "base/memory/scoped_ptr.h"
+#include "base/task_scheduler/scheduler_lock.h"
+#include "base/task_scheduler/sequence.h"
+#include "base/task_scheduler/sequence_sort_key.h"
+#include "base/threading/non_thread_safe.h"
+
+namespace base {
+namespace internal {
+
+// A PriorityQueue holds Sequences of Tasks. This class is thread-safe.
+class BASE_EXPORT PriorityQueue {
+ public:
+  // An immutable struct combining a Sequence and the sort key that determines
+  // its position in a PriorityQueue.
+  struct BASE_EXPORT SequenceAndSortKey {
+    // Constructs a null SequenceAndSortKey.
+    SequenceAndSortKey();
+
+    // Constructs a SequenceAndSortKey with the given |sequence| and |sort_key|.
+    SequenceAndSortKey(scoped_refptr<Sequence> sequence,
+                       const SequenceSortKey& sort_key);
+
+    ~SequenceAndSortKey();
+
+    // Returns true if this is a null SequenceAndSortKey.
+    bool is_null() const { return !sequence; }
+
+    const scoped_refptr<Sequence> sequence;
+    const SequenceSortKey sort_key;
+  };
+
+  // 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 WorkerThread 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 : public NonThreadSafe {
+   public:
+    ~Transaction();
+
+    // Inserts |sequence_and_sort_key| in the PriorityQueue.
+    void Push(scoped_ptr<SequenceAndSortKey> sequence_and_sort_key);
+
+    // Returns the SequenceAndSortKey with the highest priority or a null
+    // SequenceAndSortKey if the PriorityQueue is empty. The reference becomes
+    // invalid the next time that a Sequence is popped from the PriorityQueue.
+    const SequenceAndSortKey& Peek() const;
+
+    // Removes the SequenceAndSortKey with the highest priority from the
+    // PriorityQueue. Cannot be called on an empty PriorityQueue.
+    void Pop();
+
+   private:
+    friend class PriorityQueue;
+
+    explicit Transaction(PriorityQueue* outer_queue);
+
+    // Holds the lock of |outer_queue_| for most of the lifetime of this
+    // Transaction. Using a scoped_ptr allows the destructor to release the lock
+    // before performing internal operations which have to be done outside of
+    // its scope.
+    scoped_ptr<AutoSchedulerLock> auto_lock_;
+
+    PriorityQueue* const outer_queue_;
+
+    // Number of times that Push() has been called on this Transaction.
+    size_t num_pushed_sequences_ = 0;
+
+    DISALLOW_COPY_AND_ASSIGN(Transaction);
+  };
+
+  // |sequence_inserted_callback| is a non-null callback invoked when the
+  // Transaction is done for each Push that was performed with the Transaction.
+  explicit PriorityQueue(const Closure& sequence_inserted_callback);
+
+  // |sequence_inserted_callback| is a non-null callback invoked when the
+  // Transaction is done for each Push that was performed with the Transaction.
+  // |predecessor_priority_queue| is a PriorityQueue for which a thread is
+  // allowed to have an active Transaction when it creates a Transaction for
+  // this PriorityQueue.
+  PriorityQueue(const Closure& sequence_inserted_callback,
+                const PriorityQueue* predecessor_priority_queue);
+
+  ~PriorityQueue();
+
+  // 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.
+  scoped_ptr<Transaction> BeginTransaction();
+
+ private:
+  struct SequenceAndSortKeyComparator {
+    bool operator()(const scoped_ptr<SequenceAndSortKey>& left,
+                    const scoped_ptr<SequenceAndSortKey>& right) const {
+      return left->sort_key < right->sort_key;
+    }
+  };
+  using ContainerType =
+      std::priority_queue<scoped_ptr<SequenceAndSortKey>,
+                          std::vector<scoped_ptr<SequenceAndSortKey>>,
+                          SequenceAndSortKeyComparator>;
+
+  // Synchronizes access to |container_|.
+  SchedulerLock container_lock_;
+
+  ContainerType container_;
+
+  const Closure sequence_inserted_callback_;
+
+  // A null SequenceAndSortKey returned by Peek() when the PriorityQueue is
+  // empty.
+  const SequenceAndSortKey empty_sequence_and_sort_key_;
+
+  DISALLOW_COPY_AND_ASSIGN(PriorityQueue);
+};
+
+}  // namespace internal
+}  // namespace base
+
+#endif  // BASE_TASK_SCHEDULER_PRIORITY_QUEUE_H_