YARN-6563 ConcurrentModificationException in TimelineCollectorManager while stopping RM (Contributed by Haibo Chen via Vrushali C)

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-timelineservice/src/main/java/org/apache/hadoop/yarn/server/timelineservice/collector/TimelineCollectorManager.java

@@ -219,8 +219,10 @@ public class TimelineCollectorManager extends AbstractService {
   @Override
   protected void serviceStop() throws Exception {
     if (collectors != null && collectors.size() > 1) {
-      for (TimelineCollector c : collectors.values()) {
-        c.serviceStop();
+      synchronized (collectors) {
+        for (TimelineCollector c : collectors.values()) {
+          c.serviceStop();
+        }
       }
     }
     // stop the flusher first