Fix for an infinite loop in cooperative sticky assignor

[emasab]

when potential topic partitions are less than number of members.

Closes #4783

[mensfeld]

I can confirm this works :) I applied it via a patch in rdkafka-ruby and was not able to trigger it anymore. Thank you!

[emasab]

Thank you @mensfeld ! Couldn't push it earlier because we're reviewing company flows after the incident that happened to public repositories earlier in the month.

[mensfeld]

No worries. You motivated me to finally craft ability to patch librdkafka for rdkafka-ruby so now I can also expose the needed fork-protect APIs for MacOS. Thank you for all your hard work :pray:

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[mensfeld]

@pranavrth I've been running a similar patch for this for weeks, I have specs in rdkafka-ruby and karafka. This solves it as I was able to reproduce it on my side in 100%.

[pranavrth]

@mensfeld - I was saying that we don't need a test for this kind of trivial bug.