def checkin net_http_args
    stack = Thread.current[@key][net_http_args]

    raise ConnectionPool::Error, 'no connections are checked out' if
      stack.empty?

    conn = stack.pop

    if stack.empty?
      @available.push conn, connection_args: net_http_args
    end

    nil
  end
def checkin net_http_args
    stack = Thread.current[@key][net_http_args]

    raise ConnectionPool::Error, 'no connections are checked out' if
      stack.empty?

    conn = stack.pop

    if stack.empty?
      @available.push conn, connection_args: net_http_args
    end

    nil
  end
:nodoc: