Error in zero length client id when using 3.1.1

[karlw00t]

We ran into a issue over at home-assistant/core#8315 where users were getting errors after upgrading the mqtt client. I did a bit of digging on the subject and I think there is a bug in hbmqtt's implementation of version 3.1.1 of the MQTT protocol. It appears that 3.1.1 allows for clients to send a zero length client id to the server, and according to the spec:

A Server MAY allow a Client to supply a ClientId that has a length of zero bytes, however if it does so the Server MUST treat this as a special case and assign a unique ClientId to that Client. It MUST then process the CONNECT packet as if the Client had provided that unique ClientId

It appears when getting the handler in the broker [1] it checks if client_id is True, but it's a zero length string, so the function returns None.

I think there are two options here, refuse zero length client ids, or generate a random_id and store that for later use.

[njouanin]

I think we should generate an unique ID and proceed with the connection. Could you try to fix this and provide a PR ?

[karlw00t]

Are there design doc anywhere? I'm trying to wrap my head around what's going on, but there a few places ([1] [2]) where variable names are not clear. My current question is what does "parent" mean on an instance of a Session class? It appears that if we can't find a cached session [3], then we should do the same client_id gen that's done here [4].

[1] https://github.com/karlw00t/hbmqtt/blob/master/hbmqtt/session.py#L84
[2] https://github.com/karlw00t/hbmqtt/blob/master/hbmqtt/broker.py#L371
[3] https://github.com/karlw00t/hbmqtt/blob/master/hbmqtt/broker.py#L374
[4] https://github.com/karlw00t/hbmqtt/blob/master/hbmqtt/broker.py#L367