Fix ThreadPoolMergeSchedulerTests testSchedulerCloseWaitsForRunningMerge (#130078)

This fixes a race condition in the test scenario, between the merge
scheduler closing and the merge task being scheduled to run. The test
scenario expects that the merge task runs when the scheduler is closed.
If the merge scheduler is closed before the merge task is scheduled, the
merge task will instead be scheduled as aborted.

Fixes: #125236

Author: albertzaharovits

muted-tests.yml

@@ -278,9 +278,6 @@ tests:
 - class: org.elasticsearch.search.basic.SearchWithRandomDisconnectsIT
   method: testSearchWithRandomDisconnects
   issue: https://github.com/elastic/elasticsearch/issues/122707
-- class: org.elasticsearch.index.engine.ThreadPoolMergeSchedulerTests
-  method: testSchedulerCloseWaitsForRunningMerge
-  issue: https://github.com/elastic/elasticsearch/issues/125236
 - class: org.elasticsearch.packaging.test.DockerTests
   method: test020PluginsListWithNoPlugins
   issue: https://github.com/elastic/elasticsearch/issues/126232

server/src/test/java/org/elasticsearch/index/engine/ThreadPoolMergeSchedulerTests.java

@@ -612,11 +612,12 @@ public void testSchedulerCloseWaitsForRunningMerge() throws Exception {
                         fail(e);
                     }
                 });
+                // test expects that there definitely is a running merge before closing the merge scheduler
+                mergeRunningLatch.await();
+                // closes the merge scheduler
                 t.start();
                 try {
                     assertTrue(t.isAlive());
-                    // wait for the merge to actually run
-                    mergeRunningLatch.await();
                     // ensure the merge scheduler is effectively "closed"
                     assertBusy(() -> {
                         MergeSource mergeSource2 = mock(MergeSource.class);