Subscription aliases don't seem to work correctly

[dustin]

What did I do

Subscribed to topics with an alias limit set to request the broker to alias messages to the same topic.

This does require a broker that supports aliases, which I don't believe mosquitto does.

What happened

After the first message (establishing the aliases), subsequent messages don't properly report the topic.

Expected

If a message doesn't contain a topic, but contains an alias reference, then the alias should be dereferenced and the topic displayed.

Environment

• OS: OS version, e.g. macOS Ventura 13.1
• MQTTX version, e.g. v1.6.0

Details

Subbed to tmp/# on broker.emqx.io and sent two messages:

mosquitto_pub -d -h broker.emqx.io -t tmp/x -m hello
mosquitto_pub -d -h broker.emqx.io -t tmp/x -m hello

Compare to the output of my client subscribed at the same time:

tmp/x → hello
  TopicAlias 1
tmp/x → hello
  TopicAlias 1

[ysfscream]

Hi, sorry I'm late in replying. As I understand it, your mean it should eventually show the topic from which this message was received, right?

[dustin]

It should always show the topic. If it receives an empty topic with no known alias, there's a state mismatch between the client and the broker and he connection should be dropped. Otherwise, the client does know the topic and should display it.