[jira] [Created] (KAFKA-14942) CopyOnWriteMap implements ConcurrentMap but does not implement required default methods

["Steven Schlansker (Jira)" <ji...@apache.org>]

Steven Schlansker created KAFKA-14942:
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             Summary: 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


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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