When static membership (KIP-345) and incremental rebalancing (KIP-429) are turned on at the same time, that upon failure it is possible some partitions are not assigned to anyone. The event sequence is the following:
1. An assignment (task1) from rebalance is sent to an existing static member with owned list (partition1, partition2), hence upon receiving the assignment the static member is supposed to revoke partition2 and then re-join the group to trigger another rebalance.
2. The member crashed before re-join the group, lost all of its assigned partitions. However since this member is static with long session timeout, it was not kicked out of the group yet at the coordinator side.
3. The member resumes and then re-join with a known instance.id. The coordinator would not trigger a rebalance in this case and just give it the previous assignment (partition1), and since the member has forgot about its previous owned partitions it would just take partition1 and not re-join.
4. As a result the partition2 is not owned by this member any more but not re-assigned to anyone; until the next rebalance it would not be fetched by any member of the group.
The key here is that today we are relying on the member's local memory to calculate the added / revoked diff based on (owned, assigned). But if the member crashed and lost all of its owned partition, AND if it is a static member whose re-join would not trigger a new rebalance, this will break.