def app
      @app || @app_build_lock.synchronize {
        @app ||= begin
          stack = default_middleware_stack
          config.middleware = build_middleware.merge_into(stack)
          config.middleware.build(endpoint)
        end
      }
    end
def app
      @app || @app_build_lock.synchronize {
        @app ||= begin
          stack = default_middleware_stack
          config.middleware = build_middleware.merge_into(stack)
          config.middleware.build(endpoint)
        end
      }
    end
Returns the underlying Rack application for this engine.