HDFS-16293. Client sleeps and holds 'dataQueue' when DataNodes are congested. Contributed by Yuanxin Zhu.

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

@@ -687,11 +687,6 @@ public void run() {
             continue;
           }
           // get packet to be sent.
-          try {
-            backOffIfNecessary();
-          } catch (InterruptedException e) {
-            LOG.debug("Thread interrupted", e);
-          }
           one = dataQueue.getFirst(); // regular data packet
           SpanContext[] parents = one.getTraceParents();
           if (parents != null && parents.length > 0) {
@@ -704,6 +699,14 @@ public void run() {
           }
         }
 
+        // The DataStreamer has to release the dataQueue before sleeping,
+        // otherwise it will cause the ResponseProcessor to accept the ACK delay.
+        try {
+          backOffIfNecessary();
+        } catch (InterruptedException e) {
+          LOG.debug("Thread interrupted", e);
+        }
+
         // get new block from namenode.
         LOG.debug("stage={}, {}", stage, this);
 

hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/TestDFSOutputStream.java

@@ -31,6 +31,7 @@
 import java.util.Map;
 import java.util.Random;
 import java.util.concurrent.TimeoutException;
+import java.util.concurrent.atomic.AtomicBoolean;
 
 import org.apache.hadoop.conf.Configuration;
 import org.apache.hadoop.fs.CreateFlag;
@@ -300,6 +301,86 @@ public void testCongestionBackoff() throws IOException {
     Assert.assertTrue(congestedNodes.isEmpty());
   }
 
+  @Test(timeout=60000)
+  public void testCongestionAckDelay() {
+    DfsClientConf dfsClientConf = mock(DfsClientConf.class);
+    DFSClient client = mock(DFSClient.class);
+    when(client.getConf()).thenReturn(dfsClientConf);
+    when(client.getTracer()).thenReturn(FsTracer.get(new Configuration()));
+    client.clientRunning = true;
+    DataStreamer stream = new DataStreamer(
+            mock(HdfsFileStatus.class),
+            mock(ExtendedBlock.class),
+            client,
+            "foo", null, null, null, null, null, null);
+    DataOutputStream blockStream = mock(DataOutputStream.class);
+    Whitebox.setInternalState(stream, "blockStream", blockStream);
+    Whitebox.setInternalState(stream, "stage",
+            BlockConstructionStage.PIPELINE_CLOSE);
+    @SuppressWarnings("unchecked")
+    LinkedList<DFSPacket> dataQueue = (LinkedList<DFSPacket>)
+            Whitebox.getInternalState(stream, "dataQueue");
+    @SuppressWarnings("unchecked")
+    ArrayList<DatanodeInfo> congestedNodes = (ArrayList<DatanodeInfo>)
+            Whitebox.getInternalState(stream, "congestedNodes");
+    int backOffMaxTime = (int)
+            Whitebox.getInternalState(stream, "CONGESTION_BACK_OFF_MAX_TIME_IN_MS");
+    DFSPacket[] packet = new DFSPacket[100];
+    AtomicBoolean isDelay = new AtomicBoolean(true);
+
+    // ResponseProcessor needs the dataQueue for the next step.
+    new Thread(() -> {
+      for (int i = 0; i < 10; i++) {
+        // In order to ensure that other threads run for a period of time to prevent affecting
+        // the results.
+        try {
+          Thread.sleep(backOffMaxTime / 50);
+        } catch (InterruptedException e) {
+          e.printStackTrace();
+        }
+        synchronized (dataQueue) {
+          congestedNodes.add(mock(DatanodeInfo.class));
+          // The DataStreamer releases the dataQueue before sleeping, and the ResponseProcessor
+          // has time to hold the dataQueue to continuously accept ACKs and add congestedNodes
+          // to the list. Therefore, congestedNodes.size() is greater than 1.
+          if (congestedNodes.size() > 1){
+            isDelay.set(false);
+            try {
+              doThrow(new IOException()).when(blockStream).flush();
+            } catch (Exception e) {
+              e.printStackTrace();
+            }
+          }
+        }
+      }
+      try {
+        doThrow(new IOException()).when(blockStream).flush();
+      } catch (Exception e) {
+        e.printStackTrace();
+      }
+      // Prevent the DataStreamer from always waiting because the
+      // dataQueue may be empty, so that the unit test cannot exit.
+      DFSPacket endPacket = mock(DFSPacket.class);
+      dataQueue.add(endPacket);
+    }).start();
+
+    // The purpose of adding packets to the dataQueue is to make the DataStreamer run
+    // normally and judge whether to enter the sleep state according to the congestion.
+    new Thread(() -> {
+      for (int i = 0; i < 100; i++) {
+        packet[i] = mock(DFSPacket.class);
+        dataQueue.add(packet[i]);
+        try {
+          Thread.sleep(backOffMaxTime / 100);
+        } catch (InterruptedException e) {
+          e.printStackTrace();
+        }
+      }
+    }).start();
+    stream.run();
+    Assert.assertFalse(isDelay.get());
+  }
+
   @Test
   public void testNoLocalWriteFlag() throws IOException {
     DistributedFileSystem fs = cluster.getFileSystem();