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https://issues.apache.org/jira/browse/LUCENE-6808?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14804589#comment-14804589
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Michael McCandless commented on LUCENE-6808:
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The intention of the {{FieldComparator.value}} API is that you would use it to
look up a value after all collection is complete, e.g. to fill the fields to
create the {{FieldDoc}} for this hit, and so the comparator would naturally not
be changing it anymore.
I don't think this API should have to make a deep clone of the value it
returns; this is just extra per-hit garbage created for searches sorting by
field ...
But I don't really understand what SOLR-6168 is trying to do ... maybe there is
another way to do it w/o abusing this API?
> Term(Ord)ValComparator.value return BytesRefs that are re-used internally
> -------------------------------------------------------------------------
>
> Key: LUCENE-6808
> URL: https://issues.apache.org/jira/browse/LUCENE-6808
> Project: Lucene - Core
> Issue Type: Bug
> Reporter: Hoss Man
> Attachments: LUCENE-6808.patch
>
>
> While working on SOLR-6168, which involves some non-trivial usage of
> {{FieldComparator}}, I discovered some weird bugs anytime I was using
> TermOrdValComparator. I ultimately tracked this down to the fact that the
> {{BytesRef}} instances returned by {{TermOrdValComparator.value(int slot)}}
> are backed by {{BytesRefBuilder}} instances that the Comparator hangs on to
> and re-uses -- so the values a caller gets back from
> {{FieldComparator.value(slot)}} might be changed out from under it before it
> has a chance to use that value in something like
> {{FieldComparator.compareValues(first,second)}}.
> The general approach when dealing with BytesRef instances (as i understand
> it) is that the caller is responsible for making a copy if it wants to hang
> on to it -- but in this case that would violate the generic API of
> FieldComparator -- callers would have to pay attention to when a
> {{FieldComparator}} is a
> {{FieldComparator<BytesRef>}} and do casting to copy the BytesRef.
> It seems like the right solution is for {{TermOrdValComparator.value(slot)}}
> (and {{TermValComparator.value(slot)}} which has a similar BytesRef usage) to
> return {{BytesRef.deepCopyOf(values\[slot\])}}
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