mcollina / mosca / 725

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Pull #308

travis-ci

Handling of empty clientId for 3.1.1 connections

According to
http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/csprd02/mqtt-v3.1.1-csprd02.html#_Toc385349764

> 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 [MQTT-3.1.3-6].

> If the Client supplies a zero-byte ClientId, the Client MUST also set
> CleanSession to 1 [MQTT-3.1.3-7].
>
> If the Client supplies a zero-byte ClientId with CleanSession set to 0,
> the Server MUST respond to the CONNECT Packet with a CONNACK return code
> 0x02 (Identifier rejected) and then close the Network Connection
> [MQTT-3.1.3-8].
>
> If the Server rejects the ClientId it MUST respond to the CONNECT Packet
> with a CONNACK return code 0x02 (Identifier rejected) and then close the
> Network Connection [MQTT-3.1.3-9].

I was a bit unsure of how to implement tests for this behavior. The test
for the generation of a random ID when legitimately providing an empty
one is done by observing the publishNewClient message.

I cannot easily provide a test for the rejection-case, because the
mqtt-packet module would not allow me to generate invalid connect
packages (namely `clientId: null` + `cleanSession: 0 || protocolVersion:
3`

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