[LUCENE-2478] CachingWrapperFilter throws NPE when Filter.getDocIdSet() returns null

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 2.9.1, 2.9.2, 3.0, 3.0.1
• Fix Version/s: 2.9.3, 3.0.2, 3.1, 4.0-ALPHA
• Lucene Fields: New, Patch Available
• Assignee: Uwe Schindler
• Reporter: Uwe Schindler
• Created: 26/May/10 13:29
• Updated: 28/Aug/22 12:27
• Resolved: 26/May/10 14:27

Description

Followup for http://www.lucidimagination.com/search/document/1014ea92f15677bd/filter_getdocidset_returning_null_and_what_this_means_for_cachingwrapperfilter:

Daniel Noll is seeing an exception like this:

java.lang.NullPointerException
    at org.apache.lucene.search.CachingWrapperFilter.docIdSetToCache(CachingWrapperFilter.java:84)
    at org.apache.lucene.search.CachingWrapperFilter.getDocIdSet(CachingWrapperFilter.java:112)
    at com.nuix.storage.search.LazyConstantScoreQuery$LazyFilterWrapper.getDocIdSet(SourceFile:91)
    at org.apache.lucene.search.ConstantScoreQuery$ConstantScorer.<init>(ConstantScoreQuery.java:116)
    at org.apache.lucene.search.ConstantScoreQuery$ConstantWeight.scorer(ConstantScoreQuery.java:81)
    at org.apache.lucene.search.BooleanQuery$BooleanWeight.scorer(BooleanQuery.java:297)
    at org.apache.lucene.search.BooleanQuery$BooleanWeight.scorer(BooleanQuery.java:297)
    at org.apache.lucene.search.QueryWrapperFilter$2.iterator(QueryWrapperFilter.java:75)

The class of our own is just an intermediary which delays creating the Filter object...

@Override
public DocIdSet getDocIdSet(IndexReader reader) throws IOException {
            if (delegate == null) {
                delegate = factory.createFilter();
            }
            return delegate.getDocIdSet(reader);
}

Tracing through the code in CachingWrapperFilter, I can see that this NPE would occur if getDocIdSet() were to return null.

The Javadoc on Filter says that null will be returned if no documents will be accepted by the filter, but it doesn't seem that Lucene itself is handling null return values correctly, so which is correct? The code or the Javadoc? Supposing that null really is OK, does this cause any problems with how CachingWrapperFilter is implementing the caching? I notice it's calling get() and then comparing against null so it wouldn't appear that it can distinguish "the entry isn't in the cache" from "the entry is in the cache but it's null".