HADOOP-10062. race condition in MetricsSystemImpl#publishMetricsNow that causes incorrect results. (Contributed by Sangjin Lee)

(cherry picked from commit 0b567f424673b5cea1c3bc23b2bd268ef6b7625f)

hadoop-common-project/hadoop-common/CHANGES.txt

@@ -485,6 +485,9 @@ Release 2.7.0 - UNRELEASED
     HADOOP-11549. flaky test detection tool failed to handle special control
     characters in test result. (Yongjun Zhang via aajisaka)
 
+    HADOOP-10062. race condition in MetricsSystemImpl#publishMetricsNow that 
+    causes incorrect results. (Sangjin Lee via junping_du)
+
 Release 2.6.1 - UNRELEASED
 
   INCOMPATIBLE CHANGES

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/metrics2/impl/MetricsSystemImpl.java

@@ -387,7 +387,7 @@ public class MetricsSystemImpl extends MetricsSystem implements MetricsSource {
    * Requests an immediate publish of all metrics from sources to sinks.
    */
   @Override
-  public void publishMetricsNow() {
+  public synchronized void publishMetricsNow() {
     if (sinks.size() > 0) {
       publishMetrics(sampleMetrics(), true);
     }    

hadoop-common-project/hadoop-common/src/test/java/org/apache/hadoop/metrics2/impl/TestMetricsSystemImpl.java

@@ -190,7 +190,7 @@ public class TestMetricsSystemImpl {
       threads[i] = new Thread(new Runnable() {
         private boolean safeAwait(int mySource, CyclicBarrier barrier) {
           try {
-            barrier1.await(2, TimeUnit.SECONDS);
+            barrier.await(2, TimeUnit.SECONDS);
           } catch (InterruptedException e) {
             results[mySource] = "Interrupted";
             return false;