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https://issues.apache.org/jira/browse/KAFKA-9668?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17052555#comment-17052555
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ASF GitHub Bot commented on KAFKA-9668:
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big-andy-coates commented on pull request #8233: KAFKA-9668: Iterating over
KafkaStreams.getAllMetadata() results in ConcurrentModificationException
URL: https://github.com/apache/kafka/pull/8233
Fixes [KAFKA-9668](https://issues.apache.org/jira/browse/KAFKA-9668)
`KafkaStreams.getAllMetadata()` returns
`StreamsMetadataState.getAllMetadata()`. All the latter methods is
`synchronized` it returns a reference to internal mutable state. Not only does
this break encapsulation, but it means any thread iterating over the returned
collection when the metadata gets rebuilt will encounter a
`ConcurrentModificationException`.
This change:
* switches from clearing and rebuild `allMetadata` when `onChange` is
called to building a new list and swapping this in. This is thread safe and has
the benefit that the returned list is not empty during a rebuild: you either
get the old or the new list.
* removes synchronisation from `getAllMetadata` and `getLocalMetadata`.
These are returning member variables. Synchronisation adds nothing.
* changes `getAllMetadata` to wrap its return value in an unmodifiable
wrapper to avoid breaking encapsulation.
* changes the getters in `StreamsMetadata` to wrap their return values in
unmodifiable wrapper to avoid breaking encapsulation.
Unit tests have been added to cover both changes classes to ensure
encapsulation and thread-safety are maintained.
### Committer Checklist (excluded from commit message)
- [ ] Verify design and implementation
- [ ] Verify test coverage and CI build status
- [ ] Verify documentation (including upgrade notes)
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> Iterating over KafkaStreams.getAllMetadata() results in
> ConcurrentModificationException
> ---------------------------------------------------------------------------------------
>
> Key: KAFKA-9668
> URL: https://issues.apache.org/jira/browse/KAFKA-9668
> Project: Kafka
> Issue Type: Bug
> Components: streams
> Affects Versions: 0.10.1.0
> Reporter: Andy Coates
> Assignee: Andy Coates
> Priority: Major
>
> `KafkaStreams.getAllMetadata()` returns
> `StreamsMetadataState.getAllMetadata()`. All the latter methods is
> `synchronized` it returns a reference to internal mutable state. Not only
> does this break encapsulation, but it means any thread iterating over the
> returned collection when the metadata gets rebuilt will encounter a
> `ConcurrentModificationException`.
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