HDFS-15796. ConcurrentModificationException error happens on NameNode occasionally. Contributed by Daniel Ma

(cherry picked from commit b6c06c4b76ca5c206639e2ddea4f3bef393dbb11)

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/PendingReconstructionBlocks.java

@@ -333,7 +333,7 @@ class PendingReconstructionBlocks {
     synchronized (pendingReconstructions) {
       PendingBlockInfo found = pendingReconstructions.get(block);
       if (found != null) {
-        return found.targets;
+        return new ArrayList<>(found.targets);
       }
     }
     return null;