[michaelklishin]

This PR has two parts.

Restriction of Multi-Segment Matchers

Having multiple # segments is usually not necessary and results
in higher resource use for no real benefit.

MQTTv5 restricts patterns to one such matcher.
This PR caps the count at two to accommodate for
certain exotic but still realistic use cases.

Trie Matching Refactoring

This PR also introduces a memoization (caching) in
a trie matching function, further reducing the computational cost of using
up to two multi-segment matchers #.

Other

Per discussion with @ansd @dcorbacho.

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[splatteredbits]

This is a breaking change and should have been queued to a major version of RabbitMQ. We have 10 routing keys that use three or more # characters. How does this impact existing routing keys? Is there a way to turn off this behavior?

[michaelklishin]

@splatteredbits that's a security fix, so you are welcome. There is no justification for using three or four multi-segment # patterns.

[michaelklishin]

Is there a way to turn off this behavior?

There is no. Multiple # segments are unnecessary (they match multiple segments and are used for the "tail part" of a pattern). We support two for what I mention as "exotic use cases" above.

There is no limit on how many single segment matchers, *, there can be.