use another thread to control spark job

jvm-packages/xgboost4j-spark/src/main/scala/ml/dmlc/xgboost4j/scala/spark/XGBoost.scala

@@ -62,14 +62,28 @@ object XGBoost extends Serializable {
     require(tracker.start(), "FAULT: Failed to start tracker")
     boosters = buildDistributedBoosters(trainingData, configMap,
       tracker.getWorkerEnvs.asScala, numWorkers, round, obj, eval)
+    @volatile var booster: Booster = null
+    val sparkJobThread = new Thread() {
+      override def run() {
         // force the job
         boosters.foreachPartition(_ => ())
-    val booster = boosters.first()
+      }
+    }
+    sparkJobThread.start()
     val returnVal = tracker.waitFor()
     logger.info(s"Rabit returns with exit code $returnVal")
     if (returnVal == 0) {
+      booster = boosters.first()
       Some(booster)
     } else {
+      try {
+        if (sparkJobThread.isAlive) {
+          sparkJobThread.interrupt()
+        }
+      } catch {
+        case ie: InterruptedException =>
+          logger.info("spark job thread is interrupted")
+      }
       None
     }
   }