Some users have reported seeing their consumer group become stuck in the CompletingRebalance phase when using the cooperative-sticky assignor. Based on the request metadata we were able to deduce that multiple consumers were reporting the same partition(s) in their "ownedPartitions" field of the consumer protocol. Since this is an invalid state, the input causes the cooperative-sticky assignor to detect that something is wrong and throw an IllegalStateException. If the consumer application is set up to simply retry, this will cause the group to appear to hang in the rebalance state.
The "ownedPartitions" field is encoded based on the ConsumerCoordinator's SubscriptionState, which was assumed to always be up to date. However there may be cases where the consumer has dropped out of the group but fails to clear the SubscriptionState, allowing it to report some partitions as owned that have since been reassigned to another member.
We should (a) fix the sticky assignment algorithm to resolve cases of improper input conditions by invalidating the "ownedPartitions" in cases of double ownership, and (b) shore up the ConsumerCoordinator logic to better handle rejoining the group and keeping its internal state consistent. See
KAFKA-12983 for more details on (b)