Github user pnowojski commented on the issue:
https://github.com/apache/flink/pull/4321
Good catch with with this spinning, I missed that.
Checking per each iteration for assigned partitions is unfortunately
costly, because there is no cheap `isEmpty()` method. The one that I have found
`consumer.assignment()` is pretty costly (creates quite a lot of objects and
takes some locks). I wouldn't want to call it very often.
I could move this variable to local scope of `run()` function, but it would
be a little bit more error prone (in case some refactoring and for example
calling `reassignPartitions()` from somewhere else outside of the `run()`
method).
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