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From "David Smiley (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (LUCENE-5152) Lucene FST is not immutable
Date Fri, 02 Aug 2013 18:29:50 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-5152?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13727927#comment-13727927
] 

David Smiley commented on LUCENE-5152:
--------------------------------------

I've been bitten by bugs in my code related to sharing BytesRefs.  It's not clear in the APIs
who "owns" a BytesRef received from a method call.  In other words, can a recipient know wether
it can safely cache it for use later or wether it's not safe because it could change suddenly
later.  It's not even just that since there are two levels of ownership, the BytesRef instance
and then the underlying char array.  This could be partially addressed with more documentation;
I did that in my own code but that didn't stop another bug.  

I have a rough idea in which a BytesRef has a boolean flag or two to indicate how share-able
it is, and then a conditional clone method that either returns the current instance or returns
shallow copy, or does a deep copy depending on those flags.  I dunno, just a half-baked idea.
                
> Lucene FST is not immutable
> ---------------------------
>
>                 Key: LUCENE-5152
>                 URL: https://issues.apache.org/jira/browse/LUCENE-5152
>             Project: Lucene - Core
>          Issue Type: Bug
>          Components: core/FSTs
>    Affects Versions: 4.4
>            Reporter: Simon Willnauer
>            Priority: Blocker
>             Fix For: 5.0, 4.5
>
>         Attachments: LUCENE-5152.patch, LUCENE-5152.patch, LUCENE-5152.patch
>
>
> a spinnoff from LUCENE-5120 where the analyzing suggester modified a returned output
from and FST (BytesRef) which caused sideffects in later execution. 
> I added an assertion into the FST that checks if a cached root arc is modified and in-fact
this happens for instance in our MemoryPostingsFormat and I bet we find more places. We need
to think about how to make this less trappy since it can cause bugs that are super hard to
find.

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