class Rack::Session::Cookie

def write_session(req, session_id, session, options)
  session = session.merge("session_id" => session_id)
  session_data = coder.encode(session)
  if @secrets.first
    session_data << "--#{generate_hmac(session_data, @secrets.first)}"
  end
  if session_data.size > (4096 - @key.size)
    req.get_header(RACK_ERRORS).puts("Warning! Rack::Session::Cookie data size exceeds 4K.")
    nil
  else
    SessionId.new(session_id, session_data)
  end
end