test: wait until threads are ready in MonitoringBulkTests#testConcurrentRequests

This commit synchronizes the start of the threads that are executing monitoring bulk requests concurrently
to ensure all threads are ready before starting. Without this some threads will execute requests while
other threads are still being constructed.

Original commit: elastic/x-pack-elasticsearch@e777fb5c28

elasticsearch/x-pack/marvel/src/test/java/org/elasticsearch/marvel/action/MonitoringBulkTests.java

@@ -17,6 +17,7 @@ import org.elasticsearch.search.SearchHit;
 import java.util.List;
 import java.util.Map;
 import java.util.concurrent.CopyOnWriteArrayList;
+import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.atomic.AtomicInteger;
 
 import static org.elasticsearch.common.xcontent.XContentFactory.jsonBuilder;
@@ -64,7 +65,9 @@ public class MonitoringBulkTests extends MarvelIntegTestCase {
      * This test creates N threads that execute a random number of monitoring bulk requests.
      */
     public void testConcurrentRequests() throws Exception {
-        final Thread[] threads = new Thread[3 + randomInt(7)];
+        final int numberThreads = randomIntBetween(3, 10);
+        final Thread[] threads = new Thread[numberThreads];
+        final CountDownLatch latch = new CountDownLatch(numberThreads + 1);
         final List<Throwable> exceptions = new CopyOnWriteArrayList<>();
 
         AtomicInteger total = new AtomicInteger(0);
@@ -82,6 +85,8 @@ public class MonitoringBulkTests extends MarvelIntegTestCase {
 
                 @Override
                 protected void doRun() throws Exception {
+                    latch.countDown();
+                    latch.await();
                     for (int j = 0; j < nbRequests; j++) {
                         MonitoringBulkRequestBuilder requestBuilder = monitoringClient().prepareMonitoringBulk();
 
@@ -102,6 +107,11 @@ public class MonitoringBulkTests extends MarvelIntegTestCase {
             threads[i].start();
         }
 
+        // wait for all threads to be ready
+        latch.countDown();
+        latch.await();
+
+        // wait for all threads to finish
         for (Thread thread : threads) {
             thread.join();
         }