server.rb

    def start(&block)
      raise ServerError, "already started." if @status != :Stop
      server_type = @config[:ServerType] || SimpleServer

      server_type.start{
        @logger.info \
          "#{self.class}#start: pid=#{$$} port=#{@listeners[0].addr[1]}"
        call_callback(:StartCallback)

        thgroup = ThreadGroup.new
        @status = :Running
        while @status == :Running
          begin
            if svrs = IO.select(@listeners, nil, nil, 2.0)
              svrs[0].each{|svr|
                @tokens.pop          # blocks while no token is there.
                if sock = accept_client(svr)
                  sock.do_not_reverse_lookup = config[:DoNotReverseLookup]
                  th = start_thread(sock, &block)
                  th[:WEBrickThread] = true
                  thgroup.add(th)
                else
                  @tokens.push(nil)
                end
              }
            end
          rescue Errno::EBADF, IOError => ex
            # if the listening socket was closed in GenericServer#shutdown,
            # IO::select raise it.
          rescue Exception => ex
            msg = "#{ex.class}: #{ex.message}\n\t#{ex.backtrace[0]}"
            @logger.error msg
          end
        end

        @logger.info "going to shutdown ..."
        thgroup.list.each{|th| th.join if th[:WEBrickThread] }
        call_callback(:StopCallback)
        @logger.info "#{self.class}#start done."
        @status = :Stop
      }
    end

utils.rb

    def create_listeners(address, port, logger=nil)
      unless port
        raise ArgumentError, "must specify port"
      end
      res = Socket::getaddrinfo(address, port,
                                Socket::AF_UNSPEC,   # address family
                                Socket::SOCK_STREAM, # socket type
                                0,                   # protocol
                                Socket::AI_PASSIVE)  # flag
      last_error = nil
      sockets = []
      res.each{|ai|
        begin
          while @socket.nil?
            begin
            @socket = TCPServer.new(ai[3], port)
            port = @socket.addr[1] if port == 0
            Utils::set_close_on_exec(@socket)
            rescue Errno::EADDRINUSE
              logger.warn("WEBRICK: port #{port} is already in use, trying #{port+1}")
              port = 1 + port
            end
          end
          sockets << @socket
        rescue => ex
          logger.warn("TCPServer Error: #{ex}") if logger
          last_error  = ex
        end
      }
      raise last_error if sockets.empty?
      return sockets
    end