Check if event is actually added before releasing semaphore

Author: andrewor14

core/src/main/scala/org/apache/spark/scheduler/LiveListenerBus.scala

@@ -75,13 +75,14 @@ private[spark] class LiveListenerBus extends SparkListenerBus with Logging {
 
   def post(event: SparkListenerEvent) {
     val eventAdded = eventQueue.offer(event)
-    if (!eventAdded && !queueFullErrorMessageLogged) {
+    if (eventAdded) {
+      eventLock.release()
+    } else if (!queueFullErrorMessageLogged) {
       logError("Dropping SparkListenerEvent because no remaining room in event queue. " +
         "This likely means one of the SparkListeners is too slow and cannot keep up with the " +
         "rate at which tasks are being started by the scheduler.")
       queueFullErrorMessageLogged = true
     }
-    eventLock.release()
   }
 
   /**