Trejkaz created LUCENE-6658:
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Summary: 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: 5.2.1, 4.10.4
Reporter: Trejkaz
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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