feat: preUnsubscribe handler

[chfritz]

Invoked then a client unsubscribes from topics. Just like authorizeSubscribe, this function allows modifying the topics before executing the unsubscribe. This is needed in cases where topics are modified in authorizeSubscribe. For example:

aedes.authorizeSubscribe = (client, subscription, callback) => {
  // overwrite subscription: force client to its namespace
  subscription.topic = `/${client.id}/${subscription.topic}`;
  callback(null, subscription);
}

aedes.preUnsubscribe = (client, packet, callback) => {
  // overwrite unsubscriptions: force client to its namespace
  for (let i in packet.unsubscriptions) {
    packet.unsubscriptions[i] = `/${client.id}/${packet.unsubscriptions[i]}`;
  }
  callback(client, packet)
}

[coveralls]

Pull Request Test Coverage Report for Build 709159642

• 5 of 5 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.002%) to 99.555%

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Totals
Change from base Build 689232499:	0.002%
Covered Lines:	804
Relevant Lines:	805

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💛 - Coveralls

[gnought]

The main question is what's the use case of modifying subscription topic in authorizeSubscribe ?

[robertsLando]

@gnought You mean preSubscribe ?

[chfritz]

@gnought the example I'm providing in the description is the use-case I need it for: I need to force clients into their own namespace, without the clients needing to know about their namespaces. Clients may subscribe to #, i.e., get everything, but will actually be reduced to /${client.id}/#, i.e., everything in their namespace only.

[chfritz]

@robertsLando I've made preUnsubscribe async as you suggested.

[gnought]

I believe you also hook authorizePublish to modify the PUBLISH topic, am I right?

[chfritz]

yes, exactly, @gnought .

[gnought]

How about if we add the support of client isolation? Will it fit in your case? @chfritz

[chfritz]

@mcollina : will do. @gnought : maybe. What do you mean by support for client isolation?

[gnought]

Just like your use case, we prepend some client-specific string in client topics.

[chfritz]

I see. Yes, I think that would work for me.