Feature: make LWT Retain flag optional, Azure event grid compatibility

[fenax]

Hello,

We would like to use chirpstack-mqtt-forwarder with Azure Event Grid as our MQTT broker.

Azure Event Grid does not support the retain flag (yet) but chirpstack-mqtt-forwarder uses it on the LWT message.

Would it be possible to make the use of the retain flag configurable ?

If I were to submit a pull request for it, would it be accepted ?

Thanks.

[brocaar]

Are you sure the LWT is supported at all at the moment?

MQTT broker is adding more MQTT v5 features in the future to align more with the MQTT specifications. The following items detail the current differences between features supported by MQTT broker and the MQTT v5 specifications: Will message, Retain flag, Message ordering, and QoS 2 aren't supported.

https://learn.microsoft.com/en-us/azure/event-grid/mqtt-overview

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In general, I would like to avoid adding code to support MQTT brokers that do not fully implement the MQTT(v5) spec. Have you tried what happened when you connect the MQTT Forwarder? Maybe it is just ignoring the retained flag?

[fenax]

Sadly no, the broker closes the connection when it receives the CONNECT message with the will retain enabled as described in the MQTT standard [MQTT-3.2.2-13]

LWT is supported https://learn.microsoft.com/en-us/azure/event-grid/mqtt-support