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https://issues.apache.org/jira/browse/LUCENE-4190?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13407033#comment-13407033
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Michael McCandless commented on LUCENE-4190:
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I agree there is a real danger here if users accidentally point
IndexWriter at the wrong directory. This was found/fixed way in the
past already: LUCENE-385.
But I also don't want to go back to the hairy files(), extensions() we
used to require of all codec components.
Yet I think there's a good middle ground: only allow a codec to write
to _<seg>.* or _<seg>_*.* files (ie the ones created by
IndexFileNames). All of our codecs are (should be!) using IndexFileName.*
to compute a file name to write to.
In reality a codec already isn't free to just write to any file,
because then it may conflict with another codec doing the same thing.
So de-facto codecs already have a "private" namespace, prefixed by
_<seg> and further refined by _N (ie when there are multiple postings
formats in a single codec).
Since a general codec must already obey its private namespace (to not
step on other codecs) I think it's fine to enforce it?
> IndexWriter deletes non-Lucene files
> ------------------------------------
>
> Key: LUCENE-4190
> URL: https://issues.apache.org/jira/browse/LUCENE-4190
> Project: Lucene - Java
> Issue Type: Bug
> Reporter: Michael McCandless
> Assignee: Robert Muir
> Fix For: 4.0, 5.0
>
> Attachments: LUCENE-4190.patch, LUCENE-4190.patch
>
>
> Carl Austin raised a good issue in a comment on my Lucene 4.0.0 alpha blog
> post:
> http://blog.mikemccandless.com/2012/07/lucene-400-alpha-at-long-last.html
> IndexWriter will now (as of 4.0) delete all foreign files from the index
> directory. We made this change because Codecs are free to write to any files
> now, so the space of filenames is hard to "bound".
> But if the user accidentally uses the wrong directory (eg c:/) then we will
> in fact delete important stuff.
> I think we can at least use some simple criteria (must start with _, maybe
> must fit certain pattern eg _<base36>(_X).Y), so we are much less likely to
> delete a non-Lucene file....
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