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https://issues.apache.org/jira/browse/LUCENE-4876?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13720526#comment-13720526
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Adrien Grand commented on LUCENE-4876:
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bq. This is currently impossible because of SetOnce.
The merge schedulers don't have a SetOnce<IndexWriter> so if a user replaces
the MergePolicy and all objects that have a SetOnce in its IndexWriterConfig
and forgets the merge scheduler, the problem remains.
I don't really like this SetOnce<?> trick. If a variable should only be set
once, it should be final and set in the constructor?
bq. how cruel it is to expose clone semantics on end-users
I fully agree. In this issue I tried to make clone consistently used across
stateful objects held by an IndexWriterConfig object but ideally
IndexWriterConfig should only carry stateless objects (in particular none of
them should have an IndexWriter as a member) so that we never need to clone it
or any of its members when reusing it.
> IndexWriterConfig.clone should clone the MergeScheduler
> -------------------------------------------------------
>
> Key: LUCENE-4876
> URL: https://issues.apache.org/jira/browse/LUCENE-4876
> Project: Lucene - Core
> Issue Type: Bug
> Reporter: Adrien Grand
> Assignee: Adrien Grand
> Fix For: 4.3
>
> Attachments: LUCENE-4876.patch, LUCENE-4876.patch, LUCENE-4876.patch
>
>
> ConcurrentMergeScheduler has a List<MergeThread> member to track the running
> merging threads, so IndexWriterConfig.clone should clone the merge scheduler
> so that both IndexWriterConfig instances are independant.
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