Fix task concurrency in components/update_client

components/update_client/update_client_internal.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 COMPONENTS_UPDATE_CLIENT_UPDATE_CLIENT_INTERNAL_H_
 #define COMPONENTS_UPDATE_CLIENT_UPDATE_CLIENT_INTERNAL_H_
 
 #include "components/update_client/update_client.h"
 
 #include <queue>
@@ skipping 47 matching lines @@
   void OnTaskComplete(const CompletionCallback& completion_callback,
                       Task* task,
                       int error);
 
   void NotifyObservers(Observer::Events event, const std::string& id);
 
   base::ThreadChecker thread_checker_;
 
   scoped_refptr<Configurator> config_;
 
-  // Contains the tasks that are queued up.
+  // Contains the tasks that are pending. In the current implementation,
+  // only update tasks (background tasks) are queued up. These tasks are
+  // pending while they are in this queue. They are not being handled for
+  // the moment.
   std::queue<Task*> task_queue_;
 
-  // Contains all tasks in progress.
+  // Contains all tasks in progress. These are the tasks that the update engine
+  // is executing at one moment. Install tasks are run concurrently, update
+  // tasks are always serialized, and update tasks are queued up if install
+  // tasks are running. In addition, concurrent install tasks for the same id
+  // are not allowed.
   std::set<Task*> tasks_;
 
   // TODO(sorin): try to make the ping manager an observer of the service.
   scoped_ptr<PingManager> ping_manager_;
   scoped_ptr<UpdateEngine> update_engine_;
 
   base::ObserverList<Observer> observer_list_;
 
   // Used to post responses back to the main thread.
   scoped_refptr<base::SingleThreadTaskRunner> main_task_runner_;
 
   // Used to execute blocking tasks.
   scoped_refptr<base::SequencedTaskRunner> blocking_task_runner_;
 
   DISALLOW_COPY_AND_ASSIGN(UpdateClientImpl);
 };
 
 }  // namespace update_client
 
 #endif  // COMPONENTS_UPDATE_CLIENT_UPDATE_CLIENT_INTERNAL_H_