TaskScheduler [4/9] Priority Queue

base/task_scheduler/priority_queue.h

Index: base/task_scheduler/priority_queue.h
diff --git a/base/task_scheduler/priority_queue.h b/base/task_scheduler/priority_queue.h
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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.
+
+#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:
+  // A Sequence and the sort key that determines its position in the
+  // PriorityQueue.
+  struct SequenceAndSortKey {
+    SequenceAndSortKey(scoped_refptr<Sequence> sequence,
+                       SequenceSortKey sort_key);
+    ~SequenceAndSortKey();
+
+    // Returns a null SequenceAndSortKey.
+    static SequenceAndSortKey Null();

[gab, 2016/02/23 20:58:42]

I meant to have a default constructor instead, just like Time().

[robliao, 2016/02/23 21:00:35]

Could this instead be the default constructor?

[fdoray, 2016/02/24 15:03:53]

Done.

+
+    // Returns true if this is a null SequenceAndSortKey.
+    bool is_null() const;
+
+    scoped_refptr<Sequence> sequence;
+    SequenceSortKey sort_key;

[gab, 2016/02/23 20:58:42]

Should these be const? (and this class be marked "immutable" in its meta
comment?)

[fdoray, 2016/02/24 15:03:53]

Done.

+  };
+
+  // |sequence_inserted_callback| is a callback invoked when a sequence is added
+  // to the PriorityQueue.
+  explicit PriorityQueue(const Closure& sequence_inserted_callback);
+
+  // |sequence_inserted_callback| is a callback invoked when a sequence is added
+  // to the PriorityQueue. |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,
+                PriorityQueue* predecessor_priority_queue);
+
+  ~PriorityQueue();
+
+  class 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.
+    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;
+
+    Transaction(PriorityQueue* outer);
+
+    PriorityQueue* const outer_;
+
+    // Holds the lock of |outer_| for most of the lifetime of this Transaction.
+    // Using a scoped_ptr allows the destructor to release the lock before
+    // performing internal operations which don't require synchronization.

[gab, 2016/02/23 20:58:42]

s/don't require synchronization/have to be done outside of its scope/

(i.e. it's not just adding this complexity to free ASAP when synchronized work
is done but rather because it is required for the cleanup work to be
unsynchronized)

[fdoray, 2016/02/24 15:03:53]

Done.

+    scoped_ptr<AutoSchedulerLock> auto_lock_;
+
+    // Number of times that Push() has been called on this Transaction.
+    size_t num_pushed_sequences_;
+
+    DISALLOW_COPY_AND_ASSIGN(Transaction);
+  };
+
+  // Begins a Transaction which allows multiple operations to be performed
+  // atomically on the PriorityQueue. It is guaranteed that nothing else will
+  // modify the PriorityQueue while the returned Transaction is alive. 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;
+  };
+  using ContainerType =
+      std::priority_queue<scoped_ptr<SequenceAndSortKey>,
+                          std::vector<scoped_ptr<SequenceAndSortKey>>,
+                          SequenceAndSortKeyComparator>;
+
+  // Synchronizes access to |container_|.
+  SchedulerLock lock_;
+
+  ContainerType container_;
+
+  const Closure sequence_inserted_callback_;
+
+  DISALLOW_COPY_AND_ASSIGN(PriorityQueue);
+};
+
+}  // namespace internal
+}  // namespace base
+
+#endif  // BASE_TASK_SCHEDULER_PRIORITY_QUEUE_H_