YARN-2264. Fixed a race condition in DrainDispatcher which may cause random test failures. Contributed by Li Lu

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1611126 13f79535-47bb-0310-9956-ffa450edef68

hadoop-yarn-project/CHANGES.txt

@@ -59,6 +59,9 @@ Release 2.6.0 - UNRELEASED
     YARN-2260. Fixed ResourceManager's RMNode to correctly remember containers
     when nodes resync during work-preserving RM restart. (Jian He via vinodkv)
 
+    YARN-2264. Fixed a race condition in DrainDispatcher which may cause random
+    test failures. (Li Lu via jianhe)
+
 Release 2.5.0 - UNRELEASED
 
   INCOMPATIBLE CHANGES

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-common/src/test/java/org/apache/hadoop/yarn/event/DrainDispatcher.java

@@ -28,6 +28,7 @@ public class DrainDispatcher extends AsyncDispatcher {
 // and similar grotesqueries
   private volatile boolean drained = false;
   private final BlockingQueue<Event> queue;
+  final Object mutex;
 
   public DrainDispatcher() {
     this(new LinkedBlockingQueue<Event>());
@@ -36,6 +37,7 @@ public class DrainDispatcher extends AsyncDispatcher {
   private DrainDispatcher(BlockingQueue<Event> eventQueue) {
     super(eventQueue);
     this.queue = eventQueue;
+    this.mutex = this;
   }
 
   /**
@@ -53,8 +55,10 @@ public class DrainDispatcher extends AsyncDispatcher {
       @Override
       public void run() {
         while (!Thread.currentThread().isInterrupted()) {
+          synchronized (mutex) {
             // !drained if dispatch queued new events on this dispatcher
             drained = queue.isEmpty();
+          }
           Event event;
           try {
             event = queue.take();
@@ -75,8 +79,10 @@ public class DrainDispatcher extends AsyncDispatcher {
     return new EventHandler() {
       @Override
       public void handle(Event event) {
-        drained = false;
+        synchronized (mutex) {
           actual.handle(event);
+          drained = false;
+        }
       }
     };
   }