LocalNodeMasterListener is a regular listener (#62485)

This commit makes the LocalNodeMasterListener interface extend the
ClusterStateListener interface and use a default implementation for
detecting whether the local node master status changed.

Backport of #62422

server/src/main/java/org/elasticsearch/cluster/LocalNodeMasterListener.java

@@ -22,7 +22,7 @@ package org.elasticsearch.cluster;
  * Enables listening to master changes events of the local node (when the local node becomes the master, and when the local
  * node cease being a master).
  */
-public interface LocalNodeMasterListener {
+public interface LocalNodeMasterListener extends ClusterStateListener {
 
     /**
      * Called when local node is elected to be the master
@@ -33,5 +33,16 @@ public interface LocalNodeMasterListener {
      * Called when the local node used to be the master, a new master was elected and it's no longer the local node.
      */
     void offMaster();
+
+    @Override
+    default void clusterChanged(ClusterChangedEvent event) {
+        final boolean wasMaster = event.previousState().nodes().isLocalNodeElectedMaster();
+        final boolean isMaster = event.localNodeMaster();
+        if (wasMaster == false && isMaster) {
+            onMaster();
+        } else if (wasMaster && isMaster == false) {
+            offMaster();
+        }
+    }
 }
 

server/src/main/java/org/elasticsearch/cluster/service/ClusterApplierService.java

@@ -51,7 +51,6 @@ import org.elasticsearch.threadpool.ThreadPool;
 
 import java.util.Arrays;
 import java.util.Collection;
-import java.util.List;
 import java.util.Map;
 import java.util.Objects;
 import java.util.concurrent.ConcurrentHashMap;
@@ -92,8 +91,6 @@ public class ClusterApplierService extends AbstractLifecycleComponent implements
     private final Collection<ClusterStateListener> clusterStateListeners = new CopyOnWriteArrayList<>();
     private final Map<TimeoutClusterStateListener, NotifyTimeout> timeoutClusterStateListeners = new ConcurrentHashMap<>();
 
-    private final LocalNodeMasterListeners localNodeMasterListeners;
-
     private final AtomicReference<ClusterState> state; // last applied state
 
     private final String nodeName;
@@ -104,7 +101,6 @@ public class ClusterApplierService extends AbstractLifecycleComponent implements
         this.clusterSettings = clusterSettings;
         this.threadPool = threadPool;
         this.state = new AtomicReference<>();
-        this.localNodeMasterListeners = new LocalNodeMasterListeners();
         this.nodeName = nodeName;
 
         this.slowTaskLoggingThreshold = CLUSTER_SERVICE_SLOW_TASK_LOGGING_THRESHOLD_SETTING.get(settings);
@@ -134,7 +130,6 @@ public class ClusterApplierService extends AbstractLifecycleComponent implements
     protected synchronized void doStart() {
         Objects.requireNonNull(nodeConnectionsService, "please set the node connection service before starting");
         Objects.requireNonNull(state.get(), "please set initial state before starting");
-        addListener(localNodeMasterListeners);
         threadPoolExecutor = createThreadPoolExecutor();
     }
 
@@ -179,7 +174,6 @@ public class ClusterApplierService extends AbstractLifecycleComponent implements
             }
         }
         ThreadPool.terminate(threadPoolExecutor, 10, TimeUnit.SECONDS);
-        removeListener(localNodeMasterListeners);
     }
 
     @Override
@@ -255,7 +249,7 @@ public class ClusterApplierService extends AbstractLifecycleComponent implements
      * Add a listener for on/off local node master events
      */
     public void addLocalNodeMasterListener(LocalNodeMasterListener listener) {
-        localNodeMasterListeners.add(listener);
+        addListener(listener);
     }
 
     /**
@@ -606,43 +600,6 @@ public class ClusterApplierService extends AbstractLifecycleComponent implements
         }
     }
 
-    private static class LocalNodeMasterListeners implements ClusterStateListener {
-
-        private final List<LocalNodeMasterListener> listeners = new CopyOnWriteArrayList<>();
-        private volatile boolean master = false;
-
-        private LocalNodeMasterListeners() {
-        }
-
-        @Override
-        public void clusterChanged(ClusterChangedEvent event) {
-            if (!master && event.localNodeMaster()) {
-                master = true;
-                for (LocalNodeMasterListener listener : listeners) {
-                    try {
-                        listener.onMaster();
-                    } catch (Exception e) {
-                        logger.warn("failed to notify LocalNodeMasterListener", e);
-                    }
-                }
-            } else if (master && !event.localNodeMaster()) {
-                master = false;
-                for (LocalNodeMasterListener listener : listeners) {
-                    try {
-                        listener.offMaster();
-                    } catch (Exception e) {
-                        logger.warn("failed to notify LocalNodeMasterListener", e);
-                    }
-                }
-            }
-        }
-
-        private void add(LocalNodeMasterListener listener) {
-            listeners.add(listener);
-        }
-
-    }
-
     // this one is overridden in tests so we can control time
     protected long currentTimeInMillis() {
         return threadPool.relativeTimeInMillis();