Fix parsing single `rescore` element in SearchSourceBuilder

We are currently only parsing the array-syntax for the rescore part
in SearchSourceBuilder ("rescore" : [ {...}, {...} ]) . We also need
to support "rescore" : {...}

Closes #18439

core/src/main/java/org/elasticsearch/search/builder/SearchSourceBuilder.java

@@ -1033,6 +1033,9 @@ public final class SearchSourceBuilder extends ToXContentToBytes implements Writ
                     suggestBuilder = SuggestBuilder.fromXContent(context, suggesters);
                 } else if (context.getParseFieldMatcher().match(currentFieldName, SORT_FIELD)) {
                     sorts = new ArrayList<>(SortBuilder.fromXContent(context));
+                } else if (context.getParseFieldMatcher().match(currentFieldName, RESCORE_FIELD)) {
+                    rescoreBuilders = new ArrayList<>();
+                    rescoreBuilders.add(RescoreBuilder.parseFromXContent(context));
                 } else if (context.getParseFieldMatcher().match(currentFieldName, EXT_FIELD)) {
                     XContentBuilder xContentBuilder = XContentFactory.jsonBuilder().copyCurrentStructure(parser);
                     ext = xContentBuilder.bytes();

core/src/test/java/org/elasticsearch/search/builder/SearchSourceBuilderTests.java

@@ -72,6 +72,7 @@ import org.elasticsearch.search.aggregations.AggregatorParsers;
 import org.elasticsearch.search.fetch.source.FetchSourceContext;
 import org.elasticsearch.search.highlight.HighlightBuilderTests;
 import org.elasticsearch.search.rescore.QueryRescoreBuilderTests;
+import org.elasticsearch.search.rescore.QueryRescorerBuilder;
 import org.elasticsearch.search.searchafter.SearchAfterBuilder;
 import org.elasticsearch.search.sort.FieldSortBuilder;
 import org.elasticsearch.search.sort.ScoreSortBuilder;
@@ -561,6 +562,57 @@ public class SearchSourceBuilderTests extends ESTestCase {
         }
     }
 
+    /**
+     * test that we can parse the `rescore` element either as single object or as array
+     */
+    public void testParseRescore() throws IOException {
+        {
+            String restContent = "{\n" +
+                "    \"query\" : {\n" +
+                "        \"match\": { \"content\": { \"query\": \"foo bar\" }}\n" +
+                "     },\n" +
+                "    \"rescore\": {" +
+                "        \"window_size\": 50,\n" +
+                "        \"query\": {\n" +
+                "            \"rescore_query\" : {\n" +
+                "                \"match\": { \"content\": { \"query\": \"baz\" } }\n" +
+                "            }\n" +
+                "        }\n" +
+                "    }\n" +
+                "}\n";
+            try (XContentParser parser = XContentFactory.xContent(restContent).createParser(restContent)) {
+                SearchSourceBuilder searchSourceBuilder = SearchSourceBuilder.fromXContent(createParseContext(parser),
+                        aggParsers, suggesters);
+                assertEquals(1, searchSourceBuilder.rescores().size());
+                assertEquals(new QueryRescorerBuilder(QueryBuilders.matchQuery("content", "baz")).windowSize(50),
+                        searchSourceBuilder.rescores().get(0));
+            }
+        }
+
+        {
+            String restContent = "{\n" +
+                "    \"query\" : {\n" +
+                "        \"match\": { \"content\": { \"query\": \"foo bar\" }}\n" +
+                "     },\n" +
+                "    \"rescore\": [ {" +
+                "        \"window_size\": 50,\n" +
+                "        \"query\": {\n" +
+                "            \"rescore_query\" : {\n" +
+                "                \"match\": { \"content\": { \"query\": \"baz\" } }\n" +
+                "            }\n" +
+                "        }\n" +
+                "    } ]\n" +
+                "}\n";
+            try (XContentParser parser = XContentFactory.xContent(restContent).createParser(restContent)) {
+                SearchSourceBuilder searchSourceBuilder = SearchSourceBuilder.fromXContent(createParseContext(parser),
+                        aggParsers, suggesters);
+                assertEquals(1, searchSourceBuilder.rescores().size());
+                assertEquals(new QueryRescorerBuilder(QueryBuilders.matchQuery("content", "baz")).windowSize(50),
+                        searchSourceBuilder.rescores().get(0));
+            }
+        }
+    }
+
     public void testEmptyPostFilter() throws IOException {
         SearchSourceBuilder builder = new SearchSourceBuilder();
         builder.postFilter(new EmptyQueryBuilder());