NullReferenceException in DrillSideways.Search - ReqExclScorer.GetCost

We have migrated all our Lucene 3.0 code to Lucene 4.8. However
when searching with DrillSideways.Search we sometimes get a NullReferenceException with this stacktrace:

System.NullReferenceException: Object reference not set to an instance of an object.
at Lucene.Net.Search.ReqExclScorer.GetCost() in C:\BuildAgent\work\b1b63ca15b99dddb\src\Lucene.Net\Search\ReqExclScorer.cs:line 148
at Lucene.Net.Facet.DrillSidewaysScorer.Score(ICollector collector, Int32 maxDoc) in C:\BuildAgent\work\b1b63ca15b99dddb\src\Lucene.Net.Facet\DrillSidewaysScorer.cs:line 139
at Lucene.Net.Search.IndexSearcher.Search(IList`1 leaves, Weight weight, ICollector collector) in C:\BuildAgent\work\b1b63ca15b99dddb\src\Lucene.Net\Search\IndexSearcher.cs:line 649
at Lucene.Net.Facet.DrillSideways.Search(DrillDownQuery query, ICollector hitCollector) in C:\BuildAgent\work\b1b63ca15b99dddb\src\Lucene.Net.Facet\DrillSideways.cs:line 194
at Lucene.Net.Facet.DrillSideways.Search(ScoreDoc after, DrillDownQuery query, Int32 topN) in C:\BuildAgent\work\b1b63ca15b99dddb\src\Lucene.Net.Facet\DrillSideways.cs:line 249

I managed to reproduce this in an unit test. If you add this unit test for example to TestDrillSideways.cs it will throw a NullReferenceException when running. This unit test should give 0 results because the criteria "Age != 23 AND Name == e" matches nothing. However I sometimes have the same issue when the query returns multiple results but that is currently a bit harder to reproduce in a unit test.

[Test]
public virtual void TestFacetNRE()
{
    Directory dir = NewDirectory();
    Directory taxoDir = NewDirectory();

    // Writes facet ords to a separate directory from the
    // main index:
    var taxoWriter = new DirectoryTaxonomyWriter(taxoDir, OpenMode.CREATE);

    FacetsConfig config = new FacetsConfig();

    RandomIndexWriter writer = new RandomIndexWriter(Random(), dir, Similarity, TimeZone);

    Document doc = new Document();
    doc.Add(new Field("Name", "John", Documents.StringField.TYPE_STORED));
    doc.Add(new Field("Age", "19", Documents.StringField.TYPE_STORED));
    doc.Add(new FacetField("Function", "Developer"));
    writer.AddDocument(config.Build(taxoWriter, doc));

    doc = new Document();
    doc.Add(new Field("Name", "Steven", Documents.StringField.TYPE_STORED));
    doc.Add(new Field("Age", "23", Documents.StringField.TYPE_STORED));
    doc.Add(new FacetField("Function", "Sales"));
    writer.AddDocument(config.Build(taxoWriter, doc));

    // NRT open
    IndexSearcher searcher = NewSearcher(writer.Reader);

    // NRT open
    var taxoReader = new DirectoryTaxonomyReader(taxoWriter);

    DrillSideways ds = new DrillSideways(searcher, config, taxoReader);

    var query = new BooleanQuery(true);
    query.Add(new TermQuery(new Term("Age", "23")), Occur.MUST_NOT);
    query.Add(new WildcardQuery(new Term("Name", "*e*")), Occur.MUST);

    var mydrillDownQuery = new DrillDownQuery(config, query);
    mydrillDownQuery.Add("Function", "Developer");

    var z = ds.Search(mydrillDownQuery, null, null, 10, null, false, false);

    IOUtils.Dispose(searcher.IndexReader, taxoReader, writer, taxoWriter, dir, taxoDir);
}

JIRA link - [LUCENENET-598] created by harold.harkema

bongohrtech / lucenenet

NullReferenceException in DrillSideways.Search - ReqExclScorer.GetCost #132