Fix message loss when forwarding messages from MQTTv5 to MQTTv3 in async mode

[inad9300]

Description

So far, a test case showing the message loss has been added. The actual fix needs to be developed (I won't venture myself into those waters).

Please, note that I haven't nor plan to sign the CLA. This pull request is just a convenient way for me to share my findings. As far as I am concerned, you are free to use of the code I provide in this pull request in whichever way you see fit. Please, copy it, modify it and complete it as you wish.

Related Issue

https://github.com/hivemq/hivemq-mqtt-client/issues/618

Type of Change

• [ ] 📚 Examples / docs / tutorials / dependencies update
• [x] 🔧 Bug fix (non-breaking change which fixes an issue)
• [ ] 🥂 Improvement (non-breaking change which improves an existing feature)
• [ ] 🚀 New feature (non-breaking change which adds functionality)
• [ ] 💥 Breaking change (fix or feature that would cause existing functionality to change)
• [ ] 🔐 Security fix

Checklist

• [x] I've written tests (if applicable) for all new methods and classes that I created.
• [ ] I've added documentation as necessary so users can easily use and understand this feature/fix.

[cla-bot[bot]]

Thank you for your pull request and welcome to our community. We require contributors to sign our Contributor License Agreement, and we don't seem to have the users @inad9300 on file. In order for us to review and merge your code, please sign our Contributor License Agreement to get yourself added. You'll find the CLA and more information here: https://github.com/hivemq/hivemq-community/blob/master/CONTRIBUTING.adoc#contributor-license-agreement

[pglombardo]

Hi @inad9300 - thanks for the helpful bug report and tests! I'll take a closer look and post back next week.

[SgtSilvio]

Closing this as the issue you see is caused by a threading problem in the test code and is not an issue of the library. Feel free to comment and ask for clarifications.