HDFS-15457. TestFsDatasetImpl fails intermittently (#2407)

(cherry picked from commit 98097b8f19789605b9697f6a959da57261e0fe19)

hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/datanode/fsdataset/impl/TestFsDatasetImpl.java

@@ -17,6 +17,8 @@
  */
 package org.apache.hadoop.hdfs.server.datanode.fsdataset.impl;
 
+import java.util.Arrays;
+import java.util.Collections;
 import java.util.function.Supplier;
 import com.google.common.collect.Lists;
 
@@ -197,9 +199,9 @@ public class TestFsDatasetImpl {
     assertEquals(0, dataset.getNumFailedVolumes());
   }
 
-  @Test
+  @Test(timeout=10000)
   public void testReadLockEnabledByDefault()
-      throws IOException, InterruptedException {
+      throws Exception {
     final FsDatasetSpi ds = dataset;
     AtomicBoolean accessed = new AtomicBoolean(false);
     CountDownLatch latch = new CountDownLatch(1);
@@ -209,7 +211,8 @@ public class TestFsDatasetImpl {
       public void run() {
         try (AutoCloseableLock l = ds.acquireDatasetReadLock()) {
           latch.countDown();
-          sleep(10000);
+          // wait for the waiter thread to access the lock.
+          waiterLatch.await();
         } catch (Exception e) {
         }
       }
@@ -217,29 +220,33 @@ public class TestFsDatasetImpl {
 
     Thread waiter = new Thread() {
       public void run() {
-        try (AutoCloseableLock l = ds.acquireDatasetReadLock()) {
+        try {
+          latch.await();
+        } catch (InterruptedException e) {
           waiterLatch.countDown();
+          return;
+        }
+        try (AutoCloseableLock l = ds.acquireDatasetReadLock()) {
           accessed.getAndSet(true);
+          // signal the holder thread.
+          waiterLatch.countDown();
         } catch (Exception e) {
         }
       }
     };
-
-    holder.start();
-    latch.await();
     waiter.start();
-    waiterLatch.await();
+    holder.start();
+    holder.join();
+    waiter.join();
     // The holder thread is still holding the lock, but the waiter can still
     // run as the lock is a shared read lock.
     assertEquals(true, accessed.get());
     holder.interrupt();
-    holder.join();
-    waiter.join();
   }
 
   @Test(timeout=10000)
   public void testReadLockCanBeDisabledByConfig()
-      throws IOException, InterruptedException {
+      throws Exception {
     HdfsConfiguration conf = new HdfsConfiguration();
     conf.setBoolean(
         DFSConfigKeys.DFS_DATANODE_LOCK_READ_WRITE_ENABLED_KEY, false);
@@ -252,41 +259,52 @@ public class TestFsDatasetImpl {
 
       CountDownLatch latch = new CountDownLatch(1);
       CountDownLatch waiterLatch = new CountDownLatch(1);
-      AtomicBoolean accessed = new AtomicBoolean(false);
+      // create a synchronized list and verify the order of elements.
+      List<Integer> syncList =
+          Collections.synchronizedList(new ArrayList<>());
+
 
       Thread holder = new Thread() {
         public void run() {
+          latch.countDown();
           try (AutoCloseableLock l = ds.acquireDatasetReadLock()) {
-            latch.countDown();
-            sleep(10000);
+            syncList.add(0);
           } catch (Exception e) {
+            return;
+          }
+          try {
+            waiterLatch.await();
+            syncList.add(2);
+          } catch (InterruptedException e) {
           }
         }
       };
 
       Thread waiter = new Thread() {
         public void run() {
+          try {
+            // wait for holder to get into the critical section.
+            latch.await();
+          } catch (InterruptedException e) {
+            waiterLatch.countDown();
+          }
           try (AutoCloseableLock l = ds.acquireDatasetReadLock()) {
-            accessed.getAndSet(true);
+            syncList.add(1);
             waiterLatch.countDown();
           } catch (Exception e) {
           }
         }
       };
-
-      holder.start();
-      latch.await();
       waiter.start();
-      Thread.sleep(200);
-      // Waiting thread should not have been able to update the variable
-      // as the read lock is disabled and hence an exclusive lock.
-      assertEquals(false, accessed.get());
-      holder.interrupt();
-      holder.join();
-      waiterLatch.await();
-      // After the holder thread exits, the variable is updated.
-      assertEquals(true, accessed.get());
+      holder.start();
+
       waiter.join();
+      holder.join();
+
+      // verify that the synchronized list has the correct sequence.
+      assertEquals(
+          "The sequence of checkpoints does not correspond to shared lock",
+          syncList, Arrays.asList(0, 1, 2));
     } finally {
       cluster.shutdown();
     }