lib/omniauth/strategy.rb
def setup_phase
      if options[:setup].respond_to?(:call)
        log :info, 'Setup endpoint detected, running now.'
        options[:setup].call(env)
      elsif options[:setup]
        log :info, 'Calling through to underlying application for setup.'
        setup_env = env.merge('PATH_INFO' => setup_path, 'REQUEST_METHOD' => 'GET')
        call_app!(setup_env)
      end
    end
def setup_phase
      if options[:setup].respond_to?(:call)
        log :info, 'Setup endpoint detected, running now.'
        options[:setup].call(env)
      elsif options[:setup]
        log :info, 'Calling through to underlying application for setup.'
        setup_env = env.merge('PATH_INFO' => setup_path, 'REQUEST_METHOD' => 'GET')
        call_app!(setup_env)
      end
    end
The setup phase looks for the `:setup` option to exist and,
 if it is, will call either the Rack endpoint supplied to the
 `:setup` option or it will call out to the setup path of the
 underlying application. This will default to `/auth/:provider/setup`.