Scroll queries asking for rescore are considered invalid

docs/reference/migration/migrate_7_0/search.asciidoc

@@ -54,6 +54,12 @@ 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 and will now return a `400 - Bad request`.  Allowing
+rescore on scroll queries would break the scroll sort.  In 6, the rescore
+clause was silently ignored.
+
 ==== 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

@@ -188,6 +188,10 @@ 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

@@ -26,9 +26,15 @@
 import org.elasticsearch.common.io.stream.BytesStreamOutput;
 import org.elasticsearch.common.io.stream.NamedWriteableAwareStreamInput;
 import org.elasticsearch.common.io.stream.StreamInput;
+import org.elasticsearch.common.io.stream.StreamOutput;
 import org.elasticsearch.common.unit.TimeValue;
 import org.elasticsearch.common.util.ArrayUtils;
+import org.elasticsearch.common.xcontent.XContentBuilder;
+import org.elasticsearch.index.query.QueryRewriteContext;
+import org.elasticsearch.index.query.QueryShardContext;
 import org.elasticsearch.search.builder.SearchSourceBuilder;
+import org.elasticsearch.search.rescore.RescorerBuilder;
+import org.elasticsearch.search.rescore.RescoreContext;
 
 import java.io.IOException;
 import java.util.ArrayList;
@@ -123,6 +129,17 @@ 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 NoOpRescorerBuilder());
+            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 {
@@ -164,4 +181,33 @@ private static SearchRequest copyRequest(SearchRequest searchRequest) throws IOE
         }
         return result;
     }
+
+    private static class NoOpRescorerBuilder extends RescorerBuilder<NoOpRescorerBuilder> {
+        NoOpRescorerBuilder() throws IOException {
+        }
+
+        @Override
+        public String getWriteableName() {
+            return "test";
+        }
+
+        @Override
+        public RescorerBuilder<NoOpRescorerBuilder> rewrite(QueryRewriteContext ctx) throws IOException {
+            return this;
+        }
+
+        @Override
+        protected void doWriteTo(StreamOutput out) throws IOException {
+        }
+
+        @Override
+        protected void doXContent(XContentBuilder builder, Params params) throws IOException {
+        }
+
+        @Override
+        public RescoreContext innerBuildContext(int windowSize, QueryShardContext context) throws IOException {
+            return null;
+        }
+    }
+
 }