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This means the socket can be blocking, which makes administration easier. The drawback is that there is now a queue, including signalling, between the main thread and write thread. This consumes a bit more CPU; but in return the main thread is never blocked.
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send() no longer is blocking, and all sendNNN calls now return false if the call couldn't be executed. Additionally, the library now recovers much better from issues, like unexpected broker disconnects.
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It wasn't all that clear that this was the case.
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Protocol specification of MQTT 3.1.1 doesn't allow unsubscribes to fail.
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The specifications say that the broker MAY deduplicate messages on overlapping subscriptions. In result, some do, and some don't. It is now, by documentation, left to the user of this library to handle overlapping subscriptions properly, and depending on their broker, they may receive one or more times the same message when the subscriptions overlap.
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By using Happy Eyeballs, we stagger connections of a host resolves into multiple IPs. This is useful for IPv6 / IPv4 hosts, where one of the two can stutter. Sadly, creating a connection is rather complex, with many odd things that can happen along the way. For example, a writeable socket doesn't mean it is actually connected; it can also mean the socket is in an error state. This implementation is inspired by my own work on OpenTTD's variant of this.
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All administration should been taken care of now, and the only thing remaining is creating connect/disconnect and implementing the sendXXX functions.
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This contains no actual code yet, just the scaffolding to get started.