Scroll queries asking for rescore are considered invalid (#32918)

This PR changes our behavior from silently ignoring rescore in a scroll query to instead report to the user that such a query is invalid.

Closes #31775

docs/reference/migration/migrate_7_0/search.asciidoc

@@ -54,6 +54,13 @@ Setting `request_cache:true` on a query that creates a scroll (`scroll=1m`)
 has been deprecated in 6 and will now return a `400 - Bad request`.
 Scroll queries are not meant to be cached.
 
+==== Scroll queries cannot use `rescore`  anymore
+Including a rescore clause on a query that creates a scroll (`scroll=1m`) has
+been deprecated in 6.5 and will now return a `400 - Bad request`.  Allowing
+rescore on scroll queries would break the scroll sort.  In the 6.x line, the
+rescore clause was silently ignored (for scroll queries), and it was allowed in
+the 5.x line.
+
 ==== Term Suggesters supported distance algorithms
 
 The following string distance algorithms were given additional names in 6.2 and

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

@@ -184,6 +184,10 @@ 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) {
+            validationException =
+                addValidationError("using [rescore] is not allowed in a scroll context", validationException);
+        }
         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;
@@ -123,6 +125,17 @@ 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 not allowed 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();
+            assertNotNull(validationErrors);
+            assertEquals(1, validationErrors.validationErrors().size());
+            assertEquals("using [rescore] is not allowed in a scroll context", validationErrors.validationErrors().get(0));
+        }
     }
 
     public void testEqualsAndHashcode() throws IOException {