[infinispan-issues] [JBoss JIRA] (ISPN-4286) Two concurrent putIfAbsent operations can both return null during rebalance

[Tristan Tarrant (JIRA)]

     [ https://issues.jboss.org/browse/ISPN-4286?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Tristan Tarrant updated ISPN-4286:
----------------------------------
    Fix Version/s: 9.0.0.Alpha3
                       (was: 9.0.0.Alpha2)


> 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.Alpha3
>
>
> 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}



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