Asking for extra permissions dynamically with OmniAuth in Sinatra

extrapermomni.rb

  # This is called before the external site, so we can add dynamic Facebook permissions
  # for example. See http://www.mikepackdev.com/blog_posts/2-dynamically-requesting-facebook-permissions-with-omniauth
  get '/auth/:name/setup' do
    if session[:extra_permissions]
      request.env['omniauth.strategy'].options[:scope] = STANDARD_FACEBOOK_PERMISSIONS+','+session[:extra_permissions]
      session.delete(:extra_permissions)
    end
    # This looks odd, but is the expected way to indicate success to Omniauth!
    halt 404, 'Setup complete'
  end