Fix FiltersFunctionScoreQuery highlighting (#21827)

This is a cleanup of the fix pushed in https://github.com/elastic/elasticsearch/pull/20400.
FiltersFunctionScoreQuery sub query should be extracted in CustomQueryScorer.extract (and not in CustomQueryScorer.extractUnknownQuery).
This does not fix any bug in this branch (it's just a cleanup) but the intent is first to clean up and then to backport in 2.x where there is a real bug.
The bug is in 2.x only because the backport of https://github.com/elastic/elasticsearch/pull/20400 in 2.x mistakenly renamed the FiltersFunctionScoreQuery to FunctionScoreQuery.
This leads to incorrect highlighting on FiltersFunctionScoreQuery in 2.x.

core/src/main/java/org/elasticsearch/search/fetch/subphase/highlight/CustomQueryScorer.java

@@ -24,7 +24,6 @@ import org.apache.lucene.search.Query;
 import org.apache.lucene.search.highlight.QueryScorer;
 import org.apache.lucene.search.highlight.WeightedSpanTerm;
 import org.apache.lucene.search.highlight.WeightedSpanTermExtractor;
-import org.apache.lucene.spatial.geopoint.search.GeoPointInBBoxQuery;
 import org.elasticsearch.common.lucene.search.function.FiltersFunctionScoreQuery;
 import org.elasticsearch.common.lucene.search.function.FunctionScoreQuery;
 import org.elasticsearch.index.query.HasChildQueryBuilder;
@@ -78,10 +77,7 @@ public final class CustomQueryScorer extends QueryScorer {
         @Override
         protected void extractUnknownQuery(Query query,
                                            Map<String, WeightedSpanTerm> terms) throws IOException {
-            if (query instanceof FiltersFunctionScoreQuery) {
+            if (terms.isEmpty()) {
-                query = ((FiltersFunctionScoreQuery) query).getSubQuery();
-                extract(query, 1F, terms);
-            } else if (terms.isEmpty()) {
                 extractWeightedTerms(terms, query, 1F);
             }
         }
@@ -92,6 +88,8 @@ public final class CustomQueryScorer extends QueryScorer {
                 return;
             } else if (query instanceof FunctionScoreQuery) {
                 super.extract(((FunctionScoreQuery) query).getSubQuery(), boost, terms);
+            } else if (query instanceof FiltersFunctionScoreQuery) {
+                super.extract(((FiltersFunctionScoreQuery) query).getSubQuery(), boost, terms);
             } else {
                 super.extract(query, boost, terms);
             }

core/src/test/java/org/elasticsearch/search/fetch/subphase/highlight/HighlighterSearchIT.java

@@ -39,6 +39,7 @@ import org.elasticsearch.index.query.Operator;
 import org.elasticsearch.index.query.QueryBuilder;
 import org.elasticsearch.index.query.QueryBuilders;
 import org.elasticsearch.index.query.functionscore.FunctionScoreQueryBuilder;
+import org.elasticsearch.index.query.functionscore.RandomScoreFunctionBuilder;
 import org.elasticsearch.index.search.MatchQuery;
 import org.elasticsearch.plugins.Plugin;
 import org.elasticsearch.rest.RestStatus;
@@ -2935,6 +2936,26 @@ public class HighlighterSearchIT extends ESIntegTestCase {
         assertThat(field.getFragments()[0].string(), equalTo("<em>brown</em>"));
     }
 
+    public void testFiltersFunctionScoreQueryHighlight() throws Exception {
+        client().prepareIndex("test", "type", "1")
+            .setSource(jsonBuilder().startObject().field("text", "brown").field("enable", "yes").endObject())
+            .setRefreshPolicy(WriteRequest.RefreshPolicy.IMMEDIATE)
+            .get();
+        FunctionScoreQueryBuilder.FilterFunctionBuilder filterBuilder =
+            new FunctionScoreQueryBuilder.FilterFunctionBuilder(QueryBuilders.termQuery("enable", "yes"),
+                new RandomScoreFunctionBuilder());
+        SearchResponse searchResponse = client().prepareSearch()
+            .setQuery(new FunctionScoreQueryBuilder(QueryBuilders.prefixQuery("text", "bro"),
+                new FunctionScoreQueryBuilder.FilterFunctionBuilder[] {filterBuilder}))
+            .highlighter(new HighlightBuilder()
+                .field(new Field("text")))
+            .get();
+        assertHitCount(searchResponse, 1);
+        HighlightField field = searchResponse.getHits().getAt(0).highlightFields().get("text");
+        assertThat(field.getFragments().length, equalTo(1));
+        assertThat(field.getFragments()[0].string(), equalTo("<em>brown</em>"));
+    }
+
     public void testSynonyms() throws IOException {
         Builder builder = Settings.builder()
             .put(indexSettings())