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From "Robert Muir (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (LUCENE-7914) Add safeguards to RegExp.toAutomaton and Operations
Date Wed, 02 Aug 2017 03:32:01 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-7914?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16110223#comment-16110223
] 

Robert Muir commented on LUCENE-7914:
-------------------------------------

Patch looks good to me. I think there are some spurious imports ({{import org.apache.lucene.util.fst.Util}})
added in some of the tests that can be removed.

> Add safeguards to RegExp.toAutomaton and Operations
> ---------------------------------------------------
>
>                 Key: LUCENE-7914
>                 URL: https://issues.apache.org/jira/browse/LUCENE-7914
>             Project: Lucene - Core
>          Issue Type: Bug
>            Reporter: Jim Ferenczi
>         Attachments: LUCENE-7914.patch, LUCENE-7914.patch, LUCENE-7914.patch, LUCENE-7914.patch,
LUCENE-7914.patch
>
>
> When creating an automaton from a regexp, some operators can create more states than
maxDeterminizedStates:
> {code}
> a{10000}
> {code}
> The example above <b>creates</b> a single path with 10k states already determinized
so maxDeterminizedStates is not checked. 
> Some operations on automaton like Operations.isFinite or Operations.topoSortStates are
recursive and the maximum level of recursion depends on the longest path in the automaton.
So a large automaton like above can exceed java's stack.
> In most of the cases we are covered by maxDeterminizedStates but there will always be
adversarial cases where a large automaton is created from a small input so I think we should
also have safeguards in the recursive methods. 
> I've attached a patch that adds a max recursion level to Operations.isFinite and Operations.topoSortStates
in order to limit stack overflows. The limit is set to 1000 so any automaton with a path bigger
than 1000 would throw an IllegalStateException.
> The patch also uses maxDeterminizedStates to limit the number of states that a repeat
operator can create and throw a TooComplex..Exception when this limit is reached.
> Finally the patch adds the ability to skip Operations.isFinite on AutomatonQuery and
uses this as an optimization for PrefixQuery that uses infinite automatons only.



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