[LUCENE-6826] java.lang.ClassCastException: org.apache.lucene.index.TermsEnum$2 cannot be cast to org.apache.lucene.index.MultiTermsEnum when adding indexes

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 5.2.1
• Fix Version/s: 5.4, 6.0
• Component/s: core/index
• Lucene Fields: New
• Assignee: Unassigned
• Reporter: Trejkaz
• Created: 06/Oct/15 04:16
• Updated: 28/Aug/22 14:43
• Resolved: 07/Oct/15 21:07

Description

We are using addIndexes and FilterCodecReader tricks as part of index migration.

Whether FilterCodecReader tricks are required to reproduce this is uncertain, but in any case, when migrating a particular index, I saw this exception:

java.lang.ClassCastException: org.apache.lucene.index.TermsEnum$2 cannot be cast to org.apache.lucene.index.MultiTermsEnum
	at org.apache.lucene.index.MappedMultiFields$MappedMultiTerms.iterator(MappedMultiFields.java:65)
	at org.apache.lucene.codecs.blocktree.BlockTreeTermsWriter.write(BlockTreeTermsWriter.java:426)
	at org.apache.lucene.codecs.perfield.PerFieldPostingsFormat$FieldsWriter.write(PerFieldPostingsFormat.java:198)
	at org.apache.lucene.codecs.FieldsConsumer.merge(FieldsConsumer.java:105)
	at org.apache.lucene.index.SegmentMerger.mergeTerms(SegmentMerger.java:193)
	at org.apache.lucene.index.SegmentMerger.merge(SegmentMerger.java:95)
	at org.apache.lucene.index.IndexWriter.addIndexes(IndexWriter.java:2519)

TermsEnum$2 appears to be TermsEnum.EMPTY. The place where it creates it is here:

MultiTermsEnum#reset:

    if (queue.size() == 0) {
      return TermsEnum.EMPTY;   // <- this is not a MultiTermsEnum
    } else {
      return this;
    }

A quick hack would be for MappedMultiFields to check for TermsEnum.EMPTY specifically before casting, but there might be some way to avoid the cast entirely and that would obviously be a better idea.