Restore method getMasterService() to return MasterService

Signed-off-by: Tianli Feng <ftianli@amazon.com>

server/src/main/java/org/opensearch/cluster/service/ClusterService.java

@@ -55,6 +55,7 @@
 
 import java.util.Collections;
 import java.util.Map;
+import java.util.Objects;
 
 /**
  * Main Cluster Service
@@ -63,6 +64,8 @@
  */
 public class ClusterService extends AbstractLifecycleComponent {
     private final ClusterManagerService clusterManagerService;
+    @Deprecated
+    private MasterService masterService = null;
 
     private final ClusterApplierService clusterApplierService;
 
@@ -114,6 +117,20 @@ public ClusterService(
         this.clusterApplierService = clusterApplierService;
     }
 
+    /**
+     * @deprecated As of 2.2, because supporting inclusive language.
+     */
+    @Deprecated
+    public ClusterService(
+        Settings settings,
+        ClusterSettings clusterSettings,
+        MasterService masterService,
+        ClusterApplierService clusterApplierService
+    ) {
+        this(settings, clusterSettings, (ClusterManagerService) masterService, clusterApplierService);
+        this.masterService = masterService;
+    }
+
     public synchronized void setNodeConnectionsService(NodeConnectionsService nodeConnectionsService) {
         clusterApplierService.setNodeConnectionsService(nodeConnectionsService);
     }
@@ -218,10 +235,21 @@ public void addLocalNodeMasterListener(LocalNodeClusterManagerListener listener)
         clusterApplierService.addLocalNodeMasterListener(listener);
     }
 
-    public ClusterManagerService getMasterService() {
+    public ClusterManagerService getClusterManagerService() {
         return clusterManagerService;
     }
 
+    /**
+     * @deprecated As of 2.2, because supporting inclusive language, replaced by {@link #getClusterManagerService()}
+     */
+    @Deprecated
+    public MasterService getMasterService() {
+        return Objects.requireNonNullElseGet(
+            masterService,
+            () -> new MasterService(settings, clusterSettings, clusterManagerService.threadPool)
+        );
+    }
+
     /**
      * Getter and Setter for IndexingPressureService, This method exposes IndexingPressureService stats to other plugins for usage.
      * Although Indexing Pressure instances can be accessed via Node and NodeService class but none of them are

server/src/test/java/org/opensearch/cluster/service/ClusterServiceTests.java

@@ -0,0 +1,29 @@
+/*
+ * 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.cluster.service;
+
+import org.hamcrest.Matchers;
+import org.opensearch.common.settings.ClusterSettings;
+import org.opensearch.common.settings.Settings;
+import org.opensearch.test.OpenSearchTestCase;
+
+public class ClusterServiceTests extends OpenSearchTestCase {
+    public void testDeprecatedGetMasterServiceWhenUsingMasterServiceToInitializeClusterService() {
+        ClusterSettings clusterSettings = new ClusterSettings(Settings.EMPTY, ClusterSettings.BUILT_IN_CLUSTER_SETTINGS);
+        MasterService masterService = new MasterService(Settings.EMPTY, clusterSettings, null);
+        ClusterService clusterServiceWithMasterService = new ClusterService(Settings.EMPTY, clusterSettings, masterService, null);
+        assertThat(clusterServiceWithMasterService.getMasterService(), Matchers.equalTo(masterService));
+    }
+
+    public void testDeprecatedGetMasterServiceWithoutAssigningMasterService() {
+        ClusterSettings clusterSettings = new ClusterSettings(Settings.EMPTY, ClusterSettings.BUILT_IN_CLUSTER_SETTINGS);
+        ClusterService clusterService = new ClusterService(Settings.EMPTY, clusterSettings, null);
+        assertThat(clusterService.getMasterService(), Matchers.instanceOf(MasterService.class));
+    }
+}