使用pbc时为什么会有内存泄漏？

[Oooocean]

    local req_name = "ZoneProtocol.GetSceneItemListReq"
    local S1 = snapshot()
    pb.encode(req_name, {sceneid=0})
    local S2 = snapshot()
    for k,v in pairs(S2) do
      if S1[k] == nil then
        print(k,v)
      end
    end

输出：
userdata: 0x7f0b21083cc0 table
0x7f0b1fc8b708 : S1 : ...iao/rp_server/pub/test_robot/bin/service/robot_agent.lua:146

userdata: 0x7f0b2101c040 table
0x7f0b1fca2dc0 : ZoneProtocol.GetSceneItemListReq