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

components/offline_pages/core/task_queue.cc

+// 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.
+
+#include "components/offline_pages/core/task_queue.h"
+
+#include "base/bind.h"
+#include "base/threading/thread_task_runner_handle.h"
+
+namespace offline_pages {
+
+TaskQueue::TaskQueue() : weak_ptr_factory_(this) {}
+
+TaskQueue::~TaskQueue() {}
+
+void TaskQueue::AddTask(std::unique_ptr<Task> task) {
+  task->SetTaskCompletionCallback(
+      base::ThreadTaskRunnerHandle::Get(),
+      base::Bind(&TaskQueue::TaskCompleted, weak_ptr_factory_.GetWeakPtr()));
+  tasks_.push(std::move(task));
+  MaybeStartTask();
+}
+
+bool TaskQueue::HasPendingTasks() const {
+  return !tasks_.empty() || HasRunningTask();
+}
+
+bool TaskQueue::HasRunningTask() const {
+  return current_task_.get() != nullptr;
+}
+
+void TaskQueue::MaybeStartTask() {
+  if (HasRunningTask() || !HasPendingTasks())
+    return;
+
+  current_task_ = std::move(tasks_.front());
+  tasks_.pop();
+  current_task_->Run();
+}
+
+void TaskQueue::TaskCompleted(Task* task) {
+  DCHECK_EQ(task, current_task_.get());
+  if (task == current_task_.get()) {
+    current_task_.reset(nullptr);
+    MaybeStartTask();
+  }
+}
+
+}  // namespace offline_pages