HDFS-17024. Potential data race introduced by HDFS-15865 (#6223)

(cherry picked from commit 93a3c6e2cd4db4b395b3ec00a513dd3aceb3e306)

hadoop-hdfs-project/hadoop-hdfs-client/src/main/java/org/apache/hadoop/hdfs/DataStreamer.java

@@ -475,6 +475,7 @@ class DataStreamer extends Daemon {
   private DataOutputStream blockStream;
   private DataInputStream blockReplyStream;
   private ResponseProcessor response = null;
+  private final Object nodesLock = new Object();
   private volatile DatanodeInfo[] nodes = null; // list of targets for current block
   private volatile StorageType[] storageTypes = null;
   private volatile String[] storageIDs = null;
@@ -613,7 +614,9 @@ class DataStreamer extends Daemon {
 
   private void setPipeline(DatanodeInfo[] nodes, StorageType[] storageTypes,
                            String[] storageIDs) {
-    this.nodes = nodes;
+    synchronized (nodesLock) {
+      this.nodes = nodes;
+    }
     this.storageTypes = storageTypes;
     this.storageIDs = storageIDs;
   }
@@ -910,7 +913,10 @@ class DataStreamer extends Daemon {
     try (TraceScope ignored = dfsClient.getTracer().
         newScope("waitForAckedSeqno")) {
       LOG.debug("{} waiting for ack for: {}", this, seqno);
-      int dnodes = nodes != null ? nodes.length : 3;
+      int dnodes;
+      synchronized (nodesLock) {
+        dnodes = nodes != null ? nodes.length : 3;
+      }
       int writeTimeout = dfsClient.getDatanodeWriteTimeout(dnodes);
       long begin = Time.monotonicNow();
       try {