Enable shared subscription for MQTTv5

[mbench757]

I've tested this on an ESP32 C3 with mosquitto mqtt broker. This allows shared subscription topic to find the correct callback for processing.

[EdJoPaTo]

I havnt read about shared subscriptions yet, only looked at the code.

Why is it not directly the second if statement? Why first the indexOf? According to the second if the $ is the first character anyway so startsWith should have better performance then too. Am I missing something here?

Suggested change

	else if ((i = topic1.indexOf('$')) >= 0)
	{
	// shared subscription for MQTTv5
	if (topic1.startsWith("$share"))
	{
	return topic1.endsWith(topic2);
	}
	else if (topic1.startsWith("$share")) // shared subscription for MQTTv5
	{
	return topic1.endsWith(topic2);

[mbench757]

Take a look a the man page here: https://mosquitto.org/man/mosquitto-8.html. Under "Broker Status", it is possible to have a topic starting with "$SYS".

"Clients can find information about the broker by subscribing to topics in the $SYS hierarchy as follows."

So I only implemented "$share" at this point, as I selfishly only need the round-robin shared subscription for two esp32 to take turns in solving a task.

[EdJoPaTo]

$SYS is irrelevant here as its unrelated to shared topics?

As far as I understand the shared topics, the topic is always like this:
$share/groupid/topic (example: $share/my-shared-subscriber-group/myhome/groundfloor/+/temperature)

So shared topics always start with "$share". The first if is irrelevant as the second if is both more precise and better performance wise?

Also something I noticed: shared topic subscriptions also support wildcards (+ and #) which this implementation currently isn't. For a first version not a problem but should be noted as a known issue I guess.

Shared topics look interesting, I should look into them more.