Two concurrent putIfAbsent operations can both return null during rebalance --------------------------------------------------------------------------- Key: ISPN-4286 URL: https://issues.jboss.org/browse/ISPN-4286 Project: Infinispan Issue Type: Bug Components: Core, State Transfer Affects Versions: 6.0.2.Final Reporter: Dan Berindei Assignee: Dan Berindei Priority: Critical Fix For: 9.0.0.Alpha2 If the cache topology changes while executing a putIfAbsent operation, the old primary owner will throw an OutdatedTopologyException, and the originator will retry on the new owner. When retrying the PutKeyValueCommand on the new primary owner, we compare the current value with the command's new value. If they are equal, we assume that the initial command wrote the old value, and we return {{null}}. However, the value might have been written by another putIfAbsent operation. So we could have two {{putIfAbsent(k, v)}} operations, both returning {{null}}. {code} A is the originator, B is the primary owner, k = null A -> B: putIfAbsent(k, v1) B dies before writing v, C is now primary owner D -> C: putIfAbsent(k, v1) // another put operation from D, with the same value C -> D: null // correct A -> C: retry_putIfAbsent(k, v1) C -> A: null // C assumes A is overwriting its own value, so it's also returning null {code}