Fix testMergeSourceWithFollowUpMergesRunSequentially (#126050)

Fixes #125639
Relates #120869

Author: albertzaharovits

muted-tests.yml

@@ -320,9 +320,6 @@ tests:
 - class: org.elasticsearch.smoketest.MlWithSecurityIT
   method: test {yaml=ml/data_frame_analytics_crud/Test get stats on newly created config}
   issue: https://github.com/elastic/elasticsearch/issues/121726
-- class: org.elasticsearch.index.engine.ThreadPoolMergeSchedulerTests
-  method: testMergeSourceWithFollowUpMergesRunSequentially
-  issue: https://github.com/elastic/elasticsearch/issues/125639
 - class: org.elasticsearch.smoketest.MlWithSecurityIT
   method: test {yaml=ml/data_frame_analytics_cat_apis/Test cat data frame analytics all jobs with header and column selection}
   issue: https://github.com/elastic/elasticsearch/issues/125641

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

@@ -390,11 +390,13 @@ public void testMergeSourceWithFollowUpMergesRunSequentially() throws Exception
                 }).when(mergeSource).merge(any(OneMerge.class));
                 // trigger run merges on the merge source
                 threadPoolMergeScheduler.merge(mergeSource, randomFrom(MergeTrigger.values()));
-                do {
+                boolean done = false;
+                while (done == false) {
                     // let merges run, but wait for the in-progress one to signal it is running
                     nextMergeSemaphore.acquire();
+                    done = runMergeIdx.get() >= followUpMergeCount;
                     runMergeSemaphore.release();
-                } while (runMergeIdx.get() < followUpMergeCount);
+                }
                 assertBusy(() -> assertTrue(threadPoolMergeExecutorService.allDone()));
             }
         }