YARN-5809. AsyncDispatcher possibly invokes multiple shutdown threads when handling exception (Jian He via Varun Saxena)

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-common/src/main/java/org/apache/hadoop/yarn/event/AsyncDispatcher.java

@@ -195,6 +195,7 @@ public class AsyncDispatcher extends AbstractService implements Dispatcher {
       if (exitOnDispatchException
           && (ShutdownHookManager.get().isShutdownInProgress()) == false
           && stopped == false) {
+        stopped = true;
         Thread shutDownThread = new Thread(createShutDownThread());
         shutDownThread.setName("AsyncDispatcher ShutDown handler");
         shutDownThread.start();