CCS: Disable minimizing round-trips when dfs is requested (#40044)

When using DFS_QUERY_THEN_FETCH search type, the dfs phase is run and
its results are used in the query phase to make scoring accurate.
When using CCS, depending on whether the DFS phase runs in the CCS
coordinating node (like if all shards were local) or in each remote
cluster (when minimizing round-trips), scoring will differ.

This commit disables minimizing round-trips whenever DFS is requested,
as it is not currently  possible to ensure that scoring is accurate in
that case.

Relates to #32125

Author: javanna

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

@@ -80,6 +80,7 @@
 import java.util.function.Function;
 import java.util.function.LongSupplier;
 
+import static org.elasticsearch.action.search.SearchType.DFS_QUERY_THEN_FETCH;
 import static org.elasticsearch.action.search.SearchType.QUERY_THEN_FETCH;
 
 public class TransportSearchAction extends HandledTransportAction<SearchRequest, SearchResponse> {
@@ -248,6 +249,9 @@ static boolean shouldMinimizeRoundtrips(SearchRequest searchRequest) {
         if (searchRequest.scroll() != null) {
             return false;
         }
+        if (searchRequest.searchType() == DFS_QUERY_THEN_FETCH) {
+            return false;
+        }
         SearchSourceBuilder source = searchRequest.source();
         return source == null || source.collapse() == null || source.collapse().getInnerHits() == null ||
             source.collapse().getInnerHits().isEmpty();

server/src/test/java/org/elasticsearch/action/search/TransportSearchActionTests.java

@@ -812,6 +812,11 @@ public void testShouldMinimizeRoundtrips() throws Exception {
             collapseBuilder.setInnerHits(new InnerHitBuilder("inner"));
             assertFalse(TransportSearchAction.shouldMinimizeRoundtrips(searchRequest));
         }
+        {
+            SearchRequest searchRequest = new SearchRequest();
+            searchRequest.searchType(SearchType.DFS_QUERY_THEN_FETCH);
+            assertFalse(TransportSearchAction.shouldMinimizeRoundtrips(searchRequest));
+        }
         {
             SearchRequestTests searchRequestTests = new SearchRequestTests();
             searchRequestTests.setUp();