[mattrpav]

When multiple consumers share a durable subscription and ack messages out of order, a blind overwrite of LastAck can move the ack position backward. On restart, the recovery cursor replays already-acked messages. Check the update with a forward-only check so LastAck only advances.

Test demonstrates recovery cost when lastAckedSequence regresses due to blind overwrite during out-of-order acks. Two durable subscriptions share a topic; the fast subscription acks 499 of 500 messages in reverse order. Without the forward-only guard in MessageDatabase, lastAck regresses to sequence 1 and recovery scans all 499 entries.

Note: this impacts non-shared subscriptions as well

[cshannon]

LGTM, this is very unlikely to be an issue now with normal durables but with future shared subs i can see this being more of a problem so might as well protect against it

[cshannon]

Something I just thought of is we could do some modernization to the KahaDB index interfaces (like BTreeNode) and add some new methods that exist in Map that are useful, such as computeIfAbsent, computeIfPresent, merge, etc.

In particular, a merge method would be useful in this case. We could encapsulate the new check into a function and conditionally return the new/old value.

I don't think we need to do it now and would be a separate PR but there's probably other places in the code that could be a bit simpler with new methods like that.