Members can miss the rebalance cancellation on coordinator change

The new coordinator sends first a CH_UPDATE command to cancel the existing rebalance, and then a REBALANCE_START command to start a new rebalance. But the CH_UPDATE command is sent asynchronously, so it's possible for some members to receive it after the REBALANCE_START command.

If that happens, that node will assume that it will receive the segments it requested for the previous rebalance. But with the ISPN-4484 fix, the provider node cancels the outbound transfer tasks when receiving a CH_UPDATE without a pendingCH, so the state requestor will never receive its segments.

Even without the ISPN-4484 fix this is a problem, although less obvious. Between the provider node receiving the CH_UPDATE and the REBALANCE_START commands, it won't have the requestor in its write CH, so the requestor can miss transactions.