For older versions of the JoinGroup API, the coordinator implements a static timeout for new members of 5 minutes. This timeout is implemented using the heartbeat purgatory and we expect that the delayed operation will be force completed if the member successfully joins. This is implemented in GroupCoordinator with the following logic:
However, heartbeat completion depends on this check:
Since we invoke the join callback first, we will fall to the second branch. This will only return true when the latest heartbeat plus session timeout exceeds the deadline. The deadline in this case depends only on the statically configured new member timeout, which means the heartbeat cannot complete until about 5 minutes have passed. If the member falls out of the group before then, then the heartbeat ultimately expires, which may trigger a spurious rebalance.
Newer versions of the protocol are not affected by this bug because we return immediately the first time a member joins the group.