HDFS-14677. TestDataNodeHotSwapVolumes#testAddVolumesConcurrently fails intermittently in trunk. Contributed by Chen Zhang.

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

@@ -549,7 +549,8 @@ public class TestDataNodeHotSwapVolumes {
     dn.data = Mockito.spy(data);
 
     final int newVolumeCount = 40;
-    List<Thread> addVolumeDelayedThreads = new ArrayList<>();
+    List<Thread> addVolumeDelayedThreads =
+        Collections.synchronizedList(new ArrayList<>());
     AtomicBoolean addVolumeError = new AtomicBoolean(false);
     AtomicBoolean listStorageError = new AtomicBoolean(false);
     CountDownLatch addVolumeCompletionLatch =