Fix BatchedQueryPhaseIT.testNumReducePhases (elastic#135074)

The test should only run when the batched query execution feature flag
is enabled, otherwise  we run this when the setting is off and the
assumptions in this test no longer hold.

Closes elastic#134943

Author: cbuescher

muted-tests.yml

@@ -549,9 +549,6 @@ tests:
 - class: org.elasticsearch.xpack.ml.integration.InferenceIT
   method: testInferRegressionModel
   issue: https://github.com/elastic/elasticsearch/issues/133603
-- class: org.elasticsearch.action.search.BatchedQueryPhaseIT
-  method: testNumReducePhases
-  issue: https://github.com/elastic/elasticsearch/issues/134943
 - class: org.elasticsearch.reservedstate.service.RepositoriesFileSettingsIT
   method: testSettingsApplied
   issue: https://github.com/elastic/elasticsearch/issues/126748

server/src/internalClusterTest/java/org/elasticsearch/action/search/BatchedQueryPhaseIT.java

@@ -16,6 +16,7 @@
 import org.elasticsearch.cluster.routing.IndexShardRoutingTable;
 import org.elasticsearch.cluster.routing.ShardRouting;
 import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.search.SearchService;
 import org.elasticsearch.test.ESIntegTestCase;
 
 import java.util.Collections;
@@ -31,6 +32,10 @@
 public class BatchedQueryPhaseIT extends ESIntegTestCase {
 
     public void testNumReducePhases() {
+        assumeTrue(
+            "test skipped because batched query execution disabled by feature flag",
+            SearchService.BATCHED_QUERY_PHASE_FEATURE_FLAG.isEnabled()
+        );
         String indexName = "test-idx";
         assertAcked(
             prepareCreate(indexName).setMapping("title", "type=keyword")

server/src/main/java/org/elasticsearch/search/SearchService.java

@@ -298,7 +298,7 @@ public class SearchService extends AbstractLifecycleComponent implements IndexEv
         Setting.Property.NodeScope
     );
 
-    private static final boolean BATCHED_QUERY_PHASE_FEATURE_FLAG = new FeatureFlag("batched_query_phase").isEnabled();
+    public static final FeatureFlag BATCHED_QUERY_PHASE_FEATURE_FLAG = new FeatureFlag("batched_query_phase");
 
     /**
      * The size of the buffer used for memory accounting.
@@ -433,7 +433,7 @@ public SearchService(
         clusterService.getClusterSettings().addSettingsUpdateConsumer(SEARCH_WORKER_THREADS_ENABLED, this::setEnableSearchWorkerThreads);
 
         enableQueryPhaseParallelCollection = QUERY_PHASE_PARALLEL_COLLECTION_ENABLED.get(settings);
-        if (BATCHED_QUERY_PHASE_FEATURE_FLAG) {
+        if (BATCHED_QUERY_PHASE_FEATURE_FLAG.isEnabled()) {
             batchQueryPhase = BATCHED_QUERY_PHASE.get(settings);
         } else {
             batchQueryPhase = false;