percolator: Make explain use the two phase iterator

So that we don't eveluate percolator queries that don't match.

Closes #17314

core/src/main/java/org/elasticsearch/index/query/PercolatorQuery.java

@@ -135,9 +135,12 @@ public final class PercolatorQuery extends Query implements Accountable {
             public Explanation explain(LeafReaderContext leafReaderContext, int docId) throws IOException {
                 Scorer scorer = scorer(leafReaderContext);
                 if (scorer != null) {
-                    int result = scorer.iterator().advance(docId);
+                    TwoPhaseIterator twoPhaseIterator = scorer.twoPhaseIterator();
+                    int result = twoPhaseIterator.approximation().advance(docId);
                     if (result == docId) {
-                        return Explanation.match(scorer.score(), "PercolatorQuery");
+                        if (twoPhaseIterator.matches()) {
+                            return Explanation.match(scorer.score(), "PercolatorQuery");
+                        }
                     }
                 }
                 return Explanation.noMatch("PercolatorQuery");

core/src/test/java/org/elasticsearch/index/query/PercolatorQueryTests.java

@@ -36,6 +36,7 @@ import org.apache.lucene.queries.BlendedTermQuery;
 import org.apache.lucene.queries.CommonTermsQuery;
 import org.apache.lucene.search.BooleanClause;
 import org.apache.lucene.search.BooleanQuery;
+import org.apache.lucene.search.Explanation;
 import org.apache.lucene.search.IndexSearcher;
 import org.apache.lucene.search.MatchAllDocsQuery;
 import org.apache.lucene.search.PhraseQuery;
@@ -60,6 +61,7 @@ import java.util.HashMap;
 import java.util.Map;
 
 import static org.hamcrest.Matchers.equalTo;
+import static org.hamcrest.Matchers.is;
 
 public class PercolatorQueryTests extends ESTestCase {
 
@@ -147,10 +149,38 @@ public class PercolatorQueryTests extends ESTestCase {
         assertThat(topDocs.totalHits, equalTo(5));
         assertThat(topDocs.scoreDocs.length, equalTo(5));
         assertThat(topDocs.scoreDocs[0].doc, equalTo(0));
+        Explanation explanation = shardSearcher.explain(builder.build(), 0);
+        assertThat(explanation.isMatch(), is(true));
+        assertThat(explanation.getValue(), equalTo(topDocs.scoreDocs[0].score));
+
+        explanation = shardSearcher.explain(builder.build(), 1);
+        assertThat(explanation.isMatch(), is(false));
+
         assertThat(topDocs.scoreDocs[1].doc, equalTo(2));
+        explanation = shardSearcher.explain(builder.build(), 2);
+        assertThat(explanation.isMatch(), is(true));
+        assertThat(explanation.getValue(), equalTo(topDocs.scoreDocs[1].score));
+
         assertThat(topDocs.scoreDocs[2].doc, equalTo(3));
+        explanation = shardSearcher.explain(builder.build(), 3);
+        assertThat(explanation.isMatch(), is(true));
+        assertThat(explanation.getValue(), equalTo(topDocs.scoreDocs[2].score));
+
+        explanation = shardSearcher.explain(builder.build(), 4);
+        assertThat(explanation.isMatch(), is(false));
+
         assertThat(topDocs.scoreDocs[3].doc, equalTo(5));
+        explanation = shardSearcher.explain(builder.build(), 5);
+        assertThat(explanation.isMatch(), is(true));
+        assertThat(explanation.getValue(), equalTo(topDocs.scoreDocs[3].score));
+
+        explanation = shardSearcher.explain(builder.build(), 6);
+        assertThat(explanation.isMatch(), is(false));
+
         assertThat(topDocs.scoreDocs[4].doc, equalTo(7));
+        explanation = shardSearcher.explain(builder.build(), 7);
+        assertThat(explanation.isMatch(), is(true));
+        assertThat(explanation.getValue(), equalTo(topDocs.scoreDocs[4].score));
     }
 
     public void testDuel() throws Exception {
@@ -236,11 +266,14 @@ public class PercolatorQueryTests extends ESTestCase {
                 new MatchAllDocsQuery()
         );
         TopDocs topDocs2 = shardSearcher.search(builder2.build(), 10);
-
         assertThat(topDocs1.totalHits, equalTo(topDocs2.totalHits));
         assertThat(topDocs1.scoreDocs.length, equalTo(topDocs2.scoreDocs.length));
         for (int j = 0; j < topDocs1.scoreDocs.length; j++) {
             assertThat(topDocs1.scoreDocs[j].doc, equalTo(topDocs2.scoreDocs[j].doc));
+            assertThat(topDocs1.scoreDocs[j].score, equalTo(topDocs2.scoreDocs[j].score));
+            Explanation explain1 = shardSearcher.explain(builder1.build(), topDocs1.scoreDocs[j].doc);
+            Explanation explain2 = shardSearcher.explain(builder2.build(), topDocs2.scoreDocs[j].doc);
+            assertThat(explain1.toHtml(), equalTo(explain2.toHtml()));
         }
     }