[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->SetCompletionCallback(

[dougarnett, 2016/09/29 15:40:05]

ah, so this is never meant to be used by the Task (it needs to define its own
callback and call it as necessary).
Perhaps the comment in the .h should specify that this is called by TaskQueue
when it is added (didn't quite get that).

[fgorski, 2016/09/29 17:37:37]

Done.

+      base::ThreadTaskRunnerHandle::Get(),
+      base::Bind(&TaskQueue::TaskCompleted, weak_ptr_factory_.GetWeakPtr()));
+  tasks_.push(std::move(task));
+  MaybeStartTask();
+}
+
+bool TaskQueue::HasTasks() const {
+  return !tasks_.empty() || CurrentlyRunning();
+}
+
+bool TaskQueue::CurrentlyRunning() const {
+  return current_task_.get() != nullptr;
+}
+
+void TaskQueue::MaybeStartTask() {
+  if (CurrentlyRunning())
+    return;
+  if (!HasTasks())
+    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