private CoordinatorResult<Void, CoordinatorRecord> completeClassicGroupJoin(String groupId) {
        ClassicGroup group;
        try {
            group = getOrMaybeCreateClassicGroup(groupId, false);
        } catch (UnknownMemberIdException | GroupIdNotFoundException exception) {
            log.debug("Cannot find the group, skipping rebalance stage.", exception);
            return EMPTY_RESULT;
        }
        return completeClassicGroupJoin(group);
    }
An overload of {@link GroupMetadataManager#completeClassicGroupJoin(ClassicGroup)} used as
timeout operation. It additionally looks up the group by the id and checks the group type.
completeClassicGroupJoin will only be called if the group is CLASSIC.
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/GroupMetadataManager.java