[jvm-packages] Deterministically XGBoost training on exception (#2405)

Previously the code relied on the tracker process being terminated
by the OS, which was not the case on Windows.

Closes #2394

jvm-packages/xgboost4j/src/main/java/ml/dmlc/xgboost4j/java/IRabitTracker.java

@@ -22,7 +22,7 @@ import java.util.concurrent.TimeUnit;
  * brokers connections between workers.
  */
 public interface IRabitTracker extends Thread.UncaughtExceptionHandler {
-  public enum TrackerStatus {
+  enum TrackerStatus {
     SUCCESS(0), INTERRUPTED(1), TIMEOUT(2), FAILURE(3);
 
     private int statusCode;
@@ -38,6 +38,7 @@ public interface IRabitTracker extends Thread.UncaughtExceptionHandler {
 
   Map<String, String> getWorkerEnvs();
   boolean start(long workerConnectionTimeout);
+  void stop();
   // taskExecutionTimeout has no effect in current version of XGBoost.
   int waitFor(long taskExecutionTimeout);
 }

jvm-packages/xgboost4j/src/main/java/ml/dmlc/xgboost4j/java/RabitTracker.java

@@ -139,7 +139,7 @@ public class RabitTracker implements IRabitTracker {
     }
   }
 
-  private void stop() {
+  public void stop() {
     if (trackerProcess.get() != null) {
       trackerProcess.get().destroy();
     }

jvm-packages/xgboost4j/src/main/scala/ml/dmlc/xgboost4j/scala/rabit/RabitTracker.scala

@@ -127,6 +127,12 @@ private[scala] class RabitTracker(numWorkers: Int, port: Option[Int] = None,
     }
   }
 
+  def stop(): Unit = {
+    if (!system.isTerminated) {
+      system.shutdown()
+    }
+  }
+
   /**
     * Get a Map of necessary environment variables to initiate Rabit workers.
     *