Added deprecation warning for rescore in scroll queries

docs/reference/migration/migrate_6_5.asciidoc

@@ -5,6 +5,7 @@ This section discusses the changes that you need to be aware of when migrating
 your application to Elasticsearch 6.5.
 
 * <<breaking_65_logging_changes>>
+* <<breaking_65_search_changes>>
 * <<breaking_65_sql_changes>>
 
 See also <<release-highlights>> and <<es-release-notes>>.
@@ -25,6 +26,16 @@ will not change the logging configuration files though. You should make this
 change before 7.0 because in 7.0 Elasticsearch will no longer automatically
 add the node name to the logging configuration if it isn't already present.
 
+[[breaking_65_search_changes]]
+=== Search changes
+
+==== Scroll
+
+Using `rescore` with a scroll query now raises a deprecation warning and
+ignores the parameter.  In earlier 6.x releases, rescore on scroll queries was
+silently ignored.  In 7.0 and later, we will return a `400 - Bad Request` with
+a validation error.
+
 [[breaking_65_sql_changes]]
 === SQL plugin changes
 

server/src/main/java/org/elasticsearch/action/search/SearchRequest.java

@@ -137,6 +137,10 @@ public ActionRequestValidationException validate() {
         if (source != null && source.size() == 0 && scroll != null) {
             validationException = addValidationError("[size] cannot be [0] in a scroll context", validationException);
         }
+        if (source != null && source.rescores() != null && source.rescores().isEmpty() == false && scroll != null) {
+            DEPRECATION_LOGGER.deprecated("Using [rescore] for a scroll query is deprecated and will be ignored. From 7.0 on will " +
+                    "return a 400 error");
+        }
         return validationException;
     }
 

server/src/test/java/org/elasticsearch/search/SearchRequestTests.java

@@ -28,7 +28,9 @@
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.unit.TimeValue;
 import org.elasticsearch.common.util.ArrayUtils;
+import org.elasticsearch.index.query.QueryBuilders;
 import org.elasticsearch.search.builder.SearchSourceBuilder;
+import org.elasticsearch.search.rescore.QueryRescorerBuilder;
 
 import java.io.IOException;
 import java.util.ArrayList;
@@ -124,6 +126,17 @@ public void testValidate() throws IOException {
             assertEquals(1, validationErrors.validationErrors().size());
             assertEquals("[size] cannot be [0] in a scroll context", validationErrors.validationErrors().get(0));
         }
+        {
+            // Rescore is deprecated on scroll requests
+            SearchRequest searchRequest = createSearchRequest().source(new SearchSourceBuilder());
+            searchRequest.source().addRescorer(new QueryRescorerBuilder(QueryBuilders.matchAllQuery()));
+            searchRequest.requestCache(false);
+            searchRequest.scroll(new TimeValue(1000));
+            ActionRequestValidationException validationErrors = searchRequest.validate();
+            assertNull(validationErrors);
+            assertWarnings("Using [rescore] for a scroll query is deprecated and will be ignored. From 7.0 on will return a 400 error");
+        }
+
     }
 
     public void testEqualsAndHashcode() throws IOException {