scheduler: Maintain a constant enqueue order for every task

third_party/WebKit/Source/platform/scheduler/base/enqueue_order.h

 // Copyright 2015 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 THIRD_PARTY_WEBKIT_SOURCE_PLATFORM_SCHEDULER_BASE_ENQUEUE_ORDER_H_
 #define THIRD_PARTY_WEBKIT_SOURCE_PLATFORM_SCHEDULER_BASE_ENQUEUE_ORDER_H_
 
 #include <stdint.h>
 
 #include "base/synchronization/lock.h"
+#include "public/platform/WebCommon.h"
 
 namespace blink {
 namespace scheduler {
 namespace internal {
 
-using EnqueueOrder = uint64_t;
+// The enqueue order is used to uniquely order tasks. The primary ordering key
+// is the desired run time of a task, followed by the sequence number. Note the
+// scheduler assumes sequence numbers will not overflow.
+struct BLINK_PLATFORM_EXPORT EnqueueOrder {
+  base::TimeTicks delayed_run_time;
+  uint64_t sequence_num;

[alex clarke (OOO till 29th), 2017/04/13 07:46:00]

I wonder if we should have

using SequenceNum = uint64_t;

[Sami, 2017/04/18 10:47:49]

Good idea, done.

 
-// TODO(scheduler-dev): Remove explicit casts when c++17 comes.
-enum class EnqueueOrderValues : EnqueueOrder {
-  // Invalid EnqueueOrder.
+  bool is_null() const { return delayed_run_time.is_null() && !sequence_num; }
+
+  bool operator==(const EnqueueOrder& other) const;
+  bool operator<=(const EnqueueOrder& other) const;
+  bool operator<(const EnqueueOrder& other) const;
+};
+
+std::ostream& operator<<(std::ostream&, const EnqueueOrder&);
+
+enum class EnqueueOrderSequenceNumberValues {
+  // Invalid sequence number.
   NONE = 0,
 
-  // Earliest possible EnqueueOrder, to be used for fence blocking.
+  // Earliest possible sequence number, to be used for fence blocking.
   BLOCKING_FENCE = 1,
   FIRST = 2,
 };
 
-// A 64bit integer used to provide ordering of tasks. NOTE The scheduler assumes

[alex clarke (OOO till 29th), 2017/04/13 07:46:00]

We should keep the comment about not overflowing.

[Sami, 2017/04/18 10:47:49]

Yep, it's now further up the file where the enqueue order is defined.

-// these values will not overflow.
 class EnqueueOrderGenerator {
  public:
   EnqueueOrderGenerator();
   ~EnqueueOrderGenerator();
 
-  // Returns a monotonically increasing integer, starting from one. Can be
+  // Returns a monotonically increasing enqueue order, starting from one. Can be
   // called from any thread.
-  EnqueueOrder GenerateNext();
-
-  static bool IsValidEnqueueOrder(EnqueueOrder enqueue_order) {
-    return enqueue_order != 0ull;
-  }
+  EnqueueOrder GenerateNext(base::TimeTicks delayed_run_time);
 
  private:
   base::Lock lock_;
-  EnqueueOrder enqueue_order_;
+  uint64_t sequence_num_;
 };
 
 }  // namespace internal
 }  // namespace scheduler
 }  // namespace blink
 
 #endif  // THIRD_PARTY_WEBKIT_SOURCE_PLATFORM_SCHEDULER_BASE_ENQUEUE_ORDER_H_