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https://issues.apache.org/jira/browse/KAFKA-14942?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Steven Schlansker resolved KAFKA-14942.
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Resolution: Invalid
> CopyOnWriteMap implements ConcurrentMap but does not implement required
> default methods
> ---------------------------------------------------------------------------------------
>
> Key: KAFKA-14942
> URL: https://issues.apache.org/jira/browse/KAFKA-14942
> Project: Kafka
> Issue Type: Bug
> Components: clients
> Affects Versions: 3.4.0
> Reporter: Steven Schlansker
> Priority: Minor
>
> Hi Kafka team,
> I was reading through the kafka-clients CopyOnWriteMap while investigating a
> problem in a different library, and I think it is declaring that it is a
> ConcurrentMap but does not completely implement that interface.
> In particular, it inherits e.g. computeIfAbsent as a default method from Map,
> which is noted to be a non-atomic implementation, and is not synchronized in
> any way. I think this can lead to a reader experiencing a map whose contents
> are not consistent with any serial execution of write ops.
>
> Consider a thread T1 which calls computeIfAbsent("a", _ -> "1")
> T1 computeIfAbsent calls get("a") and observes null, and is then pre-empted
> T2 calls put("a", "2"), which copies the (empty) backing map and stores
> \{"a": "2"}
> T1 computeIfAbsent then wakes up, still thinking the value is null, and calls
> put("a", "1").
>
> This leads to the map finishing with the contents \{"a":"1"}, while any
> serial execution of these two operations should always finish with \{"a":"2"}.
>
> I think CopyOnWriteMap should either re-implement all mutating default
> methods at least as synchronized. If this is a special internal map and we
> know those will never be called, perhaps they should throw
> UnsupportedOperationException or at least document the class as not a
> complete and proper implementation.
>
> Thank you for your consideration.
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