False warning log for not committed offsets on CooperativeStickyAssignor strategy

[hrabur]

spring-kafka:3.0.12 (code of main and 3.0.x branches show the same problem)

Recently we switched to partition.assignment.strategy: org.apache.kafka.clients.consumer.CooperativeStickyAssignor, as a consequence, we started to observe false positive warning messages during the restart of our service

These offsets could not be committed; partition(s) lost during rebalance: {}

The message led us to the following code in spring-kafka

spring-kafka/spring-kafka/src/main/java/org/springframework/kafka/listener/KafkaMessageListenerContainer.java

Line 1714 in 659b066

this.logger.warn(() -> "These offsets could not be committed; partition(s) lost during rebalance: "

We believe the the warning is false positive, because as seen from the log message the uncommitted map is empty (... rebalance: {}), i.e. all offsets are properly committed.

As it is seen in the code the warning log message is printed always, without taking into consideration if uncommitted is empty or not

[artembilan]

I think you are right and we can check for empty collection before printing that warning.

Please, consider to contribute the fix: https://github.com/spring-projects/spring-kafka/blob/main/CONTRIBUTING.adoc