- Electron version: 1.1.0
- Operating system: Windows
- Use case: cleanup and reporting tasks executed on quit (headless), which may take time and should not prevent from opening a new instance.
- Proposed solutions:
  - new _app.releaseSingleInstance()_ API which free the singleInstance lock
  - replace _app.releaseSingleInstance()_ API by an event listener (which may be listened/unlistened)
  - [app.makeSingleInstance](https://github.com/electron/electron/blob/master/docs/api/app.md#appmakesingleinstancecallback) returns the result of the callback function. So we can choose to quit or not in a more accurate way.
  - Provide a communication channel between the new instance and the main one (in callback arguments)

atom/browser/api/atom_api_app.cc
else if (name == "documents")
    return chrome::DIR_USER_DOCUMENTS;
  else if (name == "downloads")
    return chrome::DIR_DEFAULT_DOWNLOADS;
  else if (name == "music")
    return chrome::DIR_USER_MUSIC;
  else if (name == "pictures")
    return chrome::DIR_USER_PICTURES;
  else if (name == "videos")
    return chrome::DIR_USER_VIDEOS;
  else
    return -1;
}
bool NotificationCallbackWrapper(
    const ProcessSingleton::NotificationCallback& callback,
    const base::CommandLine::StringVector& cmd,
    const base::FilePath& cwd) {
  
  if (Browser::Get()->is_ready()) {
    callback.Run(cmd, cwd);
  } else {
    scoped_refptr<base::SingleThreadTaskRunner> task_runner(
        base::ThreadTaskRunnerHandle::Get());
    task_runner->PostTask(
        FROM_HERE, base::Bind(base::IgnoreResult(callback), cmd, cwd));
  }
  
  return !Browser::Get()->is_shutting_down();
}
void OnClientCertificateSelected(