KAFKA-12759: Rebalance when a static member's subscription changes (classic protocol)

[danoSF]

Under the classic rebalance protocol with static membership, a static member that rejoins during the Stable state with the same selected protocol but a different set of subscribed topics did not trigger a rebalance. The newly subscribed topics (for example those discovered by a topics.regex member) were therefore never assigned to the group until the whole group was bounced for longer than session.timeout.ms.

This is a consistency gap rather than a new capability. Every other classic-group join path that can change a member's subscription already rebalances: dynamic and known-member rejoins go through classicGroupJoinExistingMember, which rebalances whenever !member.matches(request.protocols()) (a byte equality of the embedded protocol metadata, so any subscription change trips it), and the classic-to-consumer upgrade bridge already bumps the group epoch via hasMemberSubscriptionChanged. The static-member replace path (updateStaticMemberThenRebalanceOrCompleteJoin) was the sole exception: to avoid needless rebalances when a static instance simply reconnects with a new member id, it short-circuited on the selected protocol name alone and never performed the metadata comparison the other paths rely on.

This patch closes the gap by also comparing the rejoining member's own previous and new subscribed topics. The subscription is parsed with the version-safe V0 prefix already used by ClassicGroup#computeSubscribedTopics (the coordinator has parsed the embedded classic subscription this way for years, e.g. for offset expiration), so no new parsing capability is introduced. Only the subscribed topics are compared; owned partitions and user data do not require a new assignment and must not trigger a rebalance. When the consumer protocol cannot be parsed the coordinator falls back to its protocol-based behaviour, mirroring computeSubscribedTopics. The previous subscription is captured before the member's protocols are replaced.

Tests: GroupMetadataManagerTest covers a subscription that grows and one that shrinks (rebalance), an owned-partitions-only change and an unchanged subscription across protocol versions (no rebalance), and a multi-member static group asserting the decision is based on the rejoining member's own subscription rather than the group-wide union. JoinGroupRequestTest adds an end-to-end test reproducing the reported scenario against a broker: a static member that rejoins with an additional topic triggers a rebalance.