Fix concurrent list in merge test (#131186)

Fixes #131165

Author: kingherc

muted-tests.yml

@@ -421,9 +421,6 @@ tests:
 - class: org.elasticsearch.xpack.esql.qa.multi_node.EsqlSpecIT
   method: test {lookup-join.MvJoinKeyOnFromAfterStats ASYNC}
   issue: https://github.com/elastic/elasticsearch/issues/131148
-- class: org.elasticsearch.index.engine.ThreadPoolMergeExecutorServiceDiskSpaceTests
-  method: testEnqueuedMergeTasksAreUnblockedWhenEstimatedMergeSizeChanges
-  issue: https://github.com/elastic/elasticsearch/issues/131165
 - class: org.elasticsearch.xpack.esql.ccq.MultiClustersIT
   method: testLookupJoinAliases
   issue: https://github.com/elastic/elasticsearch/issues/131166

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

@@ -14,6 +14,7 @@
 import org.elasticsearch.common.settings.ClusterSettings;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.unit.ByteSizeValue;
+import org.elasticsearch.common.util.concurrent.ConcurrentCollections;
 import org.elasticsearch.common.util.concurrent.EsExecutors;
 import org.elasticsearch.core.PathUtils;
 import org.elasticsearch.core.PathUtilsForTesting;
@@ -40,6 +41,7 @@
 import java.util.IdentityHashMap;
 import java.util.LinkedHashSet;
 import java.util.List;
+import java.util.Set;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.Executor;
 import java.util.concurrent.TimeUnit;
@@ -898,8 +900,8 @@ public void testEnqueuedMergeTasksAreUnblockedWhenEstimatedMergeSizeChanges() th
             assertBusy(
                 () -> assertThat(threadPoolMergeExecutorService.getDiskSpaceAvailableForNewMergeTasks(), is(expectedAvailableBudget.get()))
             );
-            List<ThreadPoolMergeScheduler.MergeTask> tasksRunList = new ArrayList<>();
-            List<ThreadPoolMergeScheduler.MergeTask> tasksAbortList = new ArrayList<>();
+            Set<ThreadPoolMergeScheduler.MergeTask> tasksRunList = ConcurrentCollections.newConcurrentSet();
+            Set<ThreadPoolMergeScheduler.MergeTask> tasksAbortList = ConcurrentCollections.newConcurrentSet();
             int submittedMergesCount = randomIntBetween(1, 5);
             long[] mergeSizeEstimates = new long[submittedMergesCount];
             for (int i = 0; i < submittedMergesCount; i++) {