Move blink scheduler implementation into a component

content/child/scheduler/prioritizing_task_queue_selector.h

-// Copyright 2014 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 CONTENT_CHILD_SCHEDULER_PRIORITIZING_TASK_QUEUE_SELECTOR_H_
-#define CONTENT_CHILD_SCHEDULER_PRIORITIZING_TASK_QUEUE_SELECTOR_H_
-
-#include <set>
-
-#include "base/compiler_specific.h"
-#include "base/threading/thread_checker.h"
-#include "content/child/scheduler/task_queue_selector.h"
-#include "content/common/content_export.h"
-
-namespace content {
-
-// A PrioritizingTaskQueueSelector is a TaskQueueSelector which is used by the
-// SchedulerHelper to enable prioritization of particular task queues.
-class CONTENT_EXPORT PrioritizingTaskQueueSelector
-    : NON_EXPORTED_BASE(public TaskQueueSelector) {
- public:
-  enum QueuePriority {
-    // Queues with control priority will run before any other queue, and will
-    // explicitly starve other queues. Typically this should only be used for
-    // private queues which perform control operations.
-    CONTROL_PRIORITY,
-    // Queues with high priority will be selected preferentially over normal or
-    // best effort queues. The selector will ensure that high priority queues
-    // cannot completely starve normal priority queues.
-    HIGH_PRIORITY,
-    // Queues with normal priority are the default.
-    NORMAL_PRIORITY,
-    // Queues with best effort priority will only be run if all other queues are
-    // empty. They can be starved by the other queues.
-    BEST_EFFORT_PRIORITY,
-    // Must be the last entry.
-    QUEUE_PRIORITY_COUNT,
-    FIRST_QUEUE_PRIORITY = CONTROL_PRIORITY,
-  };
-
-  PrioritizingTaskQueueSelector();
-  ~PrioritizingTaskQueueSelector() override;
-
-  // Set the priority of |queue_index| to |priority|.
-  void SetQueuePriority(size_t queue_index, QueuePriority priority);
-
-  // Enable the |queue_index| with a priority of |priority|. By default all
-  // queues are enabled with normal priority.
-  void EnableQueue(size_t queue_index, QueuePriority priority);
-
-  // Disable the |queue_index|.
-  void DisableQueue(size_t queue_index);
-
-  // Whether |queue_index| is enabled.
-  bool IsQueueEnabled(size_t queue_index) const;
-
-  // TaskQueueSelector implementation:
-  void RegisterWorkQueues(
-      const std::vector<const base::TaskQueue*>& work_queues) override;
-  bool SelectWorkQueueToService(size_t* out_queue_index) override;
-  void AsValueInto(base::trace_event::TracedValue* state) const override;
-  void SetTaskQueueSelectorObserver(Observer* observer) override;
-
- protected:
-  // Disable the |queue_index|. Returns true if the queue was previously enabled
-  // otherwise returns false.
-  bool DisableQueueInternal(size_t queue_index);
-
- private:
-  // Returns true if queueA contains an older task than queueB.
-  static bool IsOlder(const base::TaskQueue* queueA,
-                      const base::TaskQueue* queueB);
-
-  // Returns the priority which is next after |priority|.
-  static QueuePriority NextPriority(QueuePriority priority);
-
-  static const char* PriorityToString(QueuePriority priority);
-
-  // Return true if |out_queue_index| indicates the index of the queue with
-  // the oldest pending task from the set of queues of |priority|, or
-  // false if all queues of that priority are empty.
-  bool ChooseOldestWithPriority(QueuePriority priority,
-                                size_t* out_queue_index) const;
-
-  // Returns true if |queue_index| is enabled with the given |priority|.
-  bool QueueEnabledWithPriority(size_t queue_index,
-                                QueuePriority priority) const;
-
-  // Called whenever the selector chooses a task queue for execution with the
-  // priority |priority|.
-  void DidSelectQueueWithPriority(QueuePriority priority);
-
-  // Number of high priority tasks which can be run before a normal priority
-  // task should be selected to prevent starvation.
-  // TODO(rmcilroy): Check if this is a good value.
-  static const size_t kMaxStarvationTasks = 5;
-
-  base::ThreadChecker main_thread_checker_;
-  std::vector<const base::TaskQueue*> work_queues_;
-  std::set<size_t> queue_priorities_[QUEUE_PRIORITY_COUNT];
-  size_t starvation_count_;
-  Observer* task_queue_selector_observer_;  // NOT OWNED
-  DISALLOW_COPY_AND_ASSIGN(PrioritizingTaskQueueSelector);
-};
-
-}  // namespace content
-
-#endif  // CONTENT_CHILD_SCHEDULER_PRIORITIZING_TASK_QUEUE_SELECTOR_H_