mqtt client receives msg with null topic in some cases

[zhengguoxin]

In most cases, mqtt client can receive the subscribed topic, but in some cases, mqtt client receives messages with null topic. I can reproduce this situation by trying to hibernate the server and then resume it.

But I don't know how to solve this problem when the received topic is null

[doc-han]

You will need to first figure out whether the null value is caused by the broker or the client library.

[zhengguoxin]

This is not caused by the broker, because when receive null topic, other clients can receive it normally. But when I restart nodejs, this mqtt client can receive correct topic value

[tennessine]

你用的什么broker，怎么能跑出来null topic

[zhengguoxin]

你用的什么broker，怎么能跑出来null topic

和broker无关，用的是emqx的。我找到规律了，当mqtt连接成功后，如果后续的代码有一个长时间的cpu密集运算，比如一个耗时很长的for循环，那么在循环中，实际上mqtt客户端没有和broke连接了（因为cpu被分配给循环）。等循环结束后，mqtt会自动重连成功，但是这个时候虽然可以接收信息，但是topic居然是null。

既然能收到消息，说明topic寻址是正常，而回调中topic为空，应该是这个mqtt的bug导致。

这个问题在部分场景下可以利用 v5版本的 properties特性来携带topic，但是这个场景不能解决所有的问题，

希望有大侠能解决这个bug，谢谢

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[robertsLando]

MQTT 5.0.0 BETA is now available! Try it out and give us feedback: npm i mqtt@beta. It may fix your issues