HDFS-8915. TestFSNamesystem.testFSLockGetWaiterCount fails intermittently. Contributed by Masatake Iwasaki.

(cherry picked from commit 13fb1b50e608558b2970184908ee5b9fcd7eb7b6)

Conflicts:
	hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/namenode/TestFSNamesystem.java
Updated CHANGES.txt

hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt

@@ -26,6 +26,9 @@ Release 2.7.4 - UNRELEASED
     HDFS-9745. TestSecureNNWithQJM#testSecureMode sometimes fails with
     timeouts (Xiao Chen via jlowe)
 
+    HDFS-8915. TestFSNamesystem.testFSLockGetWaiterCount fails intermittently.
+    (Masatake Iwasaki via kihwal)
+
 Release 2.7.3 - UNRELEASED
 
   INCOMPATIBLE CHANGES

hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/namenode/TestFSNamesystem.java

@@ -27,6 +27,7 @@ import java.io.IOException;
 import java.net.URI;
 import java.util.Collection;
 
+import com.google.common.base.Supplier;
 import org.apache.hadoop.conf.Configuration;
 import org.apache.hadoop.fs.FileUtil;
 import org.apache.hadoop.hdfs.DFSTestUtil;
@@ -48,6 +49,7 @@ import org.mockito.internal.util.reflection.Whitebox;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
+import java.util.concurrent.TimeoutException;
 
 public class TestFSNamesystem {
 
@@ -241,8 +243,16 @@ public class TestFSNamesystem {
     }
 
     latch.await();
-    Assert.assertEquals("Expected number of blocked thread not found",
-                        threadCount, rwLock.getQueueLength());
+    try {
+      GenericTestUtils.waitFor(new Supplier<Boolean>() {
+        @Override
+        public Boolean get() {
+          return (threadCount == rwLock.getQueueLength());
+        }
+      }, 10, 1000);
+    } catch (TimeoutException e) {
+      fail("Expected number of blocked thread not found");
+    }
   }
 
   /**