_call (Rack::File)

    # File lib/rack/file.rb, line 25
        def _call(env)
          @path_info = Utils.unescape(env["PATH_INFO"])
          return forbidden  if @path_info.include? ".."

          @path = F.join(@root, @path_info)

          begin
            if F.file?(@path) && F.readable?(@path)
              serving
            else
              raise Errno::EPERM
            end
          rescue SystemCallError
            not_found
          end
        end