LUCENE-7520: WSTE shouldn't expand MTQ if its field doesn't match filter

lucene/CHANGES.txt

@@ -111,6 +111,10 @@ Improvements
 
 * LUCENE-7496: Better toString for SweetSpotSimilarity (janhoy)
 
+* LUCENE-7520: Highlighter's WeightedSpanTermExtractor shouldn't attempt to expand a MultiTermQuery
+  when its field doesn't match the field the extraction is scoped to.
+  (Cao Manh Dat via David Smiley)
+
 Optimizations
 
 * LUCENE-7501: BKDReader should not store the split dimension explicitly in the

lucene/highlighter/src/java/org/apache/lucene/search/highlight/WeightedSpanTermExtractor.java

@@ -217,13 +217,14 @@ public class WeightedSpanTermExtractor {
     } else if (isQueryUnsupported(query.getClass())) {
       // nothing
     } else {
+      if (query instanceof MultiTermQuery &&
+          (!expandMultiTermQuery || !fieldNameComparator(((MultiTermQuery)query).getField()))) {
+        return;
+      }
       Query origQuery = query;
       final IndexReader reader = getLeafContext().reader();
       Query rewritten;
       if (query instanceof MultiTermQuery) {
-        if (!expandMultiTermQuery) {
-          return;
-        }
         rewritten = MultiTermQuery.SCORING_BOOLEAN_REWRITE.rewrite(reader, (MultiTermQuery) query);
       } else {
         rewritten = origQuery.rewrite(reader);
@@ -508,11 +509,7 @@ public class WeightedSpanTermExtractor {
    */
   public Map<String,WeightedSpanTerm> getWeightedSpanTerms(Query query, float boost, TokenStream tokenStream,
       String fieldName) throws IOException {
-    if (fieldName != null) {
     this.fieldName = fieldName;
-    } else {
-      this.fieldName = null;
-    }
 
     Map<String,WeightedSpanTerm> terms = new PositionCheckingMap<>();
     this.tokenStream = tokenStream;

lucene/highlighter/src/test/org/apache/lucene/search/highlight/HighlighterTest.java

@@ -33,6 +33,7 @@ import java.util.StringTokenizer;
 import org.apache.lucene.analysis.Analyzer;
 import org.apache.lucene.analysis.BaseTokenStreamTestCase;
 import org.apache.lucene.analysis.CachingTokenFilter;
+import org.apache.lucene.analysis.CannedTokenStream;
 import org.apache.lucene.analysis.MockAnalyzer;
 import org.apache.lucene.analysis.MockPayloadAnalyzer;
 import org.apache.lucene.analysis.MockTokenFilter;
@@ -1339,6 +1340,22 @@ public class HighlighterTest extends BaseTokenStreamTestCase implements Formatte
 
   }
 
+  public void testNotRewriteMultiTermQuery() throws IOException {
+    // field "bar": (not the field we ultimately want to extract)
+    MultiTermQuery mtq = new TermRangeQuery("bar", new BytesRef("aa"), new BytesRef("zz"), true, true) ;
+    WeightedSpanTermExtractor extractor = new WeightedSpanTermExtractor() {
+      @Override
+      protected void extract(Query query, float boost, Map<String, WeightedSpanTerm> terms) throws IOException {
+        assertEquals(mtq, query);
+        super.extract(query, boost, terms);
+      }
+    };
+    extractor.setExpandMultiTermQuery(true);
+    extractor.setMaxDocCharsToAnalyze(51200);
+    extractor.getWeightedSpanTerms(
+        mtq, 3, new CannedTokenStream(new Token("aa",0,2), new Token("bb", 2,4)), "foo"); // field "foo"
+  }
+
   public void testGetBestSingleFragmentWithWeights() throws Exception {
 
     TestHighlightRunner helper = new TestHighlightRunner() {