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https://issues.apache.org/jira/browse/LUCENE-6658?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14614724#comment-14614724
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Uwe Schindler commented on LUCENE-6658:
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Just as unrelated recommandation: We should move the IndexUpgrader and its
merge policy to backward-codecs.jar. Makes no sense to have this in core.
Should I open new issue about that?
> IndexUpgrader doesn't upgrade an index if it has zero segments
> --------------------------------------------------------------
>
> Key: LUCENE-6658
> URL: https://issues.apache.org/jira/browse/LUCENE-6658
> Project: Lucene - Core
> Issue Type: Bug
> Components: core/index
> Affects Versions: 4.10.4, 5.2.1
> Reporter: Trejkaz
> Attachments: LUCENE-6658.patch
>
>
> IndexUpgrader uses merges to do its job. Therefore, if you use it to upgrade
> an index with no segments, it will do nothing - it won't even update the
> version numbers in the segments file, meaning that later versions of Lucene
> will fail to open the index, despite the fact that you "upgraded" it.
> The suggested workaround when this was raised on the mailing list in January
> seems to be to use filesystem magic to look at the files, figure out whether
> there are any segments, and write a new empty index if there are none.
> This sounds easy, but there are probably traps. For instance, there might be
> files in the directory which don't really belong to the index. Earlier
> versions of Lucene used to have a FilenameFilter which was usable to
> distinguish one from the other, but that seems to have disappeared, making it
> less obvious how to do this.
> This issue is presumed to exist in 3.x as well, I just haven't encountered it
> yet because the only empty indices I have hit have been later versions.
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