Github user tillrohrmann commented on the issue: https://github.com/apache/flink/pull/3278 Ah now I understand what the problem of `testAtLeastOnceProdcuerFailsIfFlushingDisabled` was. Can't we mock the `KafkaProducer` to control when the record's callbacks are triggered? That way we have full control over the execution and can write a proper test.
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