[Offline pages] Introduces TaskQueue to serialize tasks that asynchronously access SQLStore

components/offline_pages/core/task_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 COMPONENTS_OFFLINE_PAGES_CORE_TASK_QUEUE_H_
+#define COMPONENTS_OFFLINE_PAGES_CORE_TASK_QUEUE_H_
+
+#include <memory>
+#include <queue>
+
+#include "base/callback.h"
+#include "base/macros.h"
+#include "base/memory/ref_counted.h"
+#include "base/memory/weak_ptr.h"
+#include "components/offline_pages/core/task.h"
+
+namespace offline_pages {
+
+// Class for coordinating |Task|s in relation to access to a specific resource.
+// As a task, we understand a set of asynchronous operations (possibly switching
+// threads) that access a set of sensitive resource(s). Because the resource
+// state is modified and individual steps of a task are asynchronous, allowing
+// certain tasks to run in parallel may lead to incorrect results. This class
+// allows for ordering of tasks in a FIFO manner, to ensure two tasks modifying
+// a resources are not run at the same time.
+//
+// Consumers of this class should create an instance of TaskQueue and implement
+// tasks that need to be run sequentially. New task will only be started when
+// the previous one calls |Task::TaskComplete|.
+class TaskQueue {
+ public:
+  TaskQueue();
+  ~TaskQueue();
+
+  // Adds a task to the queue. Queue takes ownership of the task.
+  void AddTask(std::unique_ptr<Task> task);
+  // Whether the task queue has any pending (not-running) tasks.
+  bool HasPendingTasks() const;
+  // Whether there is a task currently running.
+  bool HasRunningTask() const;
+
+ private:
+  // Checks whether there are any tasks to run, as well as whether no task is
+  // currently running. When both are met, it will start the next task in the
+  // queue.
+  void MaybeStartTask();
+
+  // Callback for informing the queue that a task was completed.
+  void TaskCompleted(Task* task);
+
+  // Currently running tasks.
+  std::unique_ptr<Task> current_task_;
+
+  // A FIFO queue of tasks that will be run using this task queue.
+  std::queue<std::unique_ptr<Task>> tasks_;
+
+  base::WeakPtrFactory<TaskQueue> weak_ptr_factory_;
+
+  DISALLOW_COPY_AND_ASSIGN(TaskQueue);
+};
+
+}  // namespace offline_pages
+
+#endif  // COMPONENTS_OFFLINE_PAGES_CORE_TASK_QUEUE_H_