WorkerTaskMonitor: stop() waits for mainLoop to exit.

Fixes apache#2637.

indexing-service/src/main/java/io/druid/indexing/worker/WorkerTaskMonitor.java

@@ -47,8 +47,10 @@
 import java.util.Objects;
 import java.util.concurrent.BlockingQueue;
 import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.LinkedBlockingDeque;
+import java.util.concurrent.TimeUnit;
 
 /**
  * The monitor watches ZK at a specified path for new tasks to appear. Upon starting the monitor, a listener will be
@@ -57,6 +59,7 @@
 public class WorkerTaskMonitor
 {
   private static final EmittingLogger log = new EmittingLogger(WorkerTaskMonitor.class);
+  private static final int STOP_WARNING_SECONDS = 10;
 
   private final ObjectMapper jsonMapper;
   private final PathChildrenCache pathChildrenCache;
@@ -68,6 +71,7 @@ public class WorkerTaskMonitor
   private final BlockingQueue<Notice> notices = new LinkedBlockingDeque<>();
   private final Map<String, TaskDetails> running = new ConcurrentHashMap<>();
 
+  private final CountDownLatch doneStopping = new CountDownLatch(1);
   private final Object lifecycleLock = new Object();
   private volatile boolean started = false;
 
@@ -149,6 +153,9 @@ private void mainLoop()
     catch (InterruptedException e) {
       log.info("WorkerTaskMonitor interrupted, exiting.");
     }
+    finally {
+      doneStopping.countDown();
+    }
   }
 
   private void restoreRestorableTasks()
@@ -237,19 +244,27 @@ public void onFailure(Throwable t)
   }
 
   @LifecycleStop
-  public void stop()
+  public void stop() throws InterruptedException
   {
     synchronized (lifecycleLock) {
       Preconditions.checkState(started, "not started");
 
       try {
+        started = false;
         exec.shutdownNow();
         pathChildrenCache.close();
         taskRunner.stop();
 
-        started = false;
+        if (!doneStopping.await(STOP_WARNING_SECONDS, TimeUnit.SECONDS)) {
+          log.warn("WorkerTaskMonitor taking longer than %s seconds to exit. Still waiting...", STOP_WARNING_SECONDS);
+          doneStopping.await();
+        }
+
         log.info("Stopped WorkerTaskMonitor.");
       }
+      catch (InterruptedException e) {
+        throw e;
+      }
       catch (Exception e) {
         log.makeAlert(e, "Exception stopping WorkerTaskMonitor")
            .emit();