KAFKA-19926: Apply rebalance delay also for later epochs if the group is empty

[RaidenE1]

In #20755 we introduce a initial
rebalance delay for streams group, now we would like to expand it to all
rebalance when group is empty to avoid frequent rebalance in a small
time period

Reviewers: Lucas Brutschy lbrutschy@confluent.io

[Copilot]

Pull request overview

This PR extends the rebalance delay mechanism from only applying to initial group creation (epoch 0) to applying whenever a group becomes empty, regardless of the group's epoch. This helps avoid frequent rebalances when members repeatedly leave and rejoin an empty group in a short time period.

Key Changes

• The rebalance delay now triggers for any empty group (not just epoch 0)
• Assignment epoch handling during delay periods supports groups with non-zero epochs
• Test coverage ensures the existing test doesn't interfere with the new delay behavior

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/GroupMetadataManager.java	Changed rebalance delay condition from groupEpoch() == 0 to isEmpty(), removed assignment epoch check during delay, and updated target assignment epoch calculation to use Math.max(1, group.assignmentEpoch()) to support groups that became empty after having assignments
group-coordinator/src/test/java/org/apache/kafka/coordinator/group/GroupMetadataManagerTest.java	Added test for rebalance delay when joining empty group with non-zero epoch, and disabled delay for existing test that wasn't designed to handle it

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