add rejection listener unit test

Signed-off-by: Kaushal Kumar <ravi.kaushal97@gmail.com>

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

@@ -316,11 +316,6 @@ protected void doExecute(Task task, SearchRequest searchRequest, ActionListener<
                 listener
             );
         }
-
-        if (task instanceof QueryGroupTask) {
-            listener =
-        }
-
         executeRequest(task, searchRequest, this::searchAsyncAction, listener);
     }
 

server/src/main/java/org/opensearch/wlm/QueryGroupService.java

@@ -8,8 +8,10 @@
 
 package org.opensearch.wlm;
 
+import java.util.Optional;
+
 /**
- * This is stub at this point in time
+ * This is stub at this point in time and will be replace by an acutal one in couple of days
  */
 public class QueryGroupService {
     /**
@@ -25,8 +27,11 @@ public void requestFailedFor(final String queryGroupId) {
      * @param queryGroupId query group identifier
      * @return whether the queryGroup is contended and should reject new incoming requests
      */
-    public boolean shouldRejectFor(String queryGroupId) {
-        if (queryGroupId == null) return false;
-        return false;
+    public Optional<String> shouldRejectFor(String queryGroupId) {
+        if (queryGroupId == null) return Optional.empty();
+        // TODO: At this point this is dummy and we need to decide whether to cancel the request based on last
+        //  reported resource usage for the queryGroup. We also need to increment the rejection count here for the
+        //  query group
+        return Optional.of("Possible reason. ");
     }
 }

server/src/main/java/org/opensearch/wlm/listeners/QueryGroupRequestRejectionOperationListener.java

@@ -15,6 +15,8 @@
 import org.opensearch.wlm.QueryGroupService;
 import org.opensearch.wlm.QueryGroupTask;
 
+import java.util.Optional;
+
 /**
  * This listener is used to perform the rejections for incoming requests into a queryGroup
  */
@@ -35,8 +37,9 @@ public QueryGroupRequestRejectionOperationListener(QueryGroupService queryGroupS
     @Override
     protected void onRequestStart(SearchRequestContext searchRequestContext) {
         final String queryGroupId = threadPool.getThreadContext().getHeader(QueryGroupTask.QUERY_GROUP_ID_HEADER);
-        if (queryGroupService.shouldRejectFor(queryGroupId)) {
-            throw new OpenSearchRejectedExecutionException("QueryGroup " + queryGroupId + " is already contended.");
+        Optional<String> reason = queryGroupService.shouldRejectFor(queryGroupId);
+        if (reason.isPresent()) {
+            throw new OpenSearchRejectedExecutionException("QueryGroup " + queryGroupId + " is already contended." + reason.get());
         }
     }
 }

server/src/test/java/org/opensearch/wlm/listeners/QueryGroupRequestRejectionOperationListenerTests.java

@@ -0,0 +1,58 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ */
+
+package org.opensearch.wlm.listeners;
+
+import org.opensearch.core.concurrency.OpenSearchRejectedExecutionException;
+import org.opensearch.test.OpenSearchTestCase;
+import org.opensearch.threadpool.TestThreadPool;
+import org.opensearch.threadpool.ThreadPool;
+import org.opensearch.wlm.QueryGroupService;
+import org.opensearch.wlm.QueryGroupTask;
+
+import java.util.Optional;
+
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+public class QueryGroupRequestRejectionOperationListenerTests extends OpenSearchTestCase {
+    ThreadPool testThreadPool;
+    QueryGroupService queryGroupService;
+    QueryGroupRequestRejectionOperationListener sut;
+
+
+    public void setUp() throws Exception {
+        super.setUp();
+        testThreadPool = new TestThreadPool("RejectionTestThreadPool");
+    }
+
+    public void tearDown() throws Exception {
+        super.tearDown();
+        testThreadPool.shutdown();
+    }
+
+    public void testRejectionCase() {
+        queryGroupService = mock(QueryGroupService.class);
+        sut = new QueryGroupRequestRejectionOperationListener(queryGroupService, testThreadPool);
+        final String testQueryGroupId = "asdgasgkajgkw3141_3rt4t";
+        testThreadPool.getThreadContext().putHeader(QueryGroupTask.QUERY_GROUP_ID_HEADER, testQueryGroupId);
+        when(queryGroupService.shouldRejectFor(testQueryGroupId)).thenReturn(Optional.of("Test query group is contended"));
+
+        assertThrows(OpenSearchRejectedExecutionException.class, () -> sut.onRequestStart(null));
+    }
+
+    public void testNonRejectionCase() {
+        queryGroupService = mock(QueryGroupService.class);
+        sut = new QueryGroupRequestRejectionOperationListener(queryGroupService, testThreadPool);
+        final String testQueryGroupId = "asdgasgkajgkw3141_3rt4t";
+        testThreadPool.getThreadContext().putHeader(QueryGroupTask.QUERY_GROUP_ID_HEADER, testQueryGroupId);
+        when(queryGroupService.shouldRejectFor(testQueryGroupId)).thenReturn(Optional.empty());
+
+        sut.onRequestStart(null);
+    }
+}