HDFS-11041. Unable to unregister FsDatasetState MBean if DataNode is shutdown twice. Contributed by Wei-Chiu Chuang.

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/fsdataset/impl/FsDatasetImpl.java

@@ -2385,6 +2385,7 @@ class FsDatasetImpl implements FsDatasetSpi<FsVolumeImpl> {
 
     if (mbeanName != null) {
       MBeans.unregister(mbeanName);
+      mbeanName = null;
     }
     
     if (asyncDiskService != null) {

hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/datanode/SimulatedFSDataset.java

@@ -1367,7 +1367,10 @@ public class SimulatedFSDataset implements FsDatasetSpi<FsVolumeSpi> {
 
   @Override
   public void shutdown() {
-    if (mbeanName != null) MBeans.unregister(mbeanName);
+    if (mbeanName != null) {
+      MBeans.unregister(mbeanName);
+      mbeanName = null;
+    }
   }
 
   @Override