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

[Dan Berindei (JIRA)]

Dan Berindei created ISPN-4286:
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             Summary: 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
             Fix For: 7.0.0.Final


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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