TEST: Create following engines in the main thread (#33391)

There are two races in the testUpdateAndReadChangesConcurrently if the
following engines are created in the worker threads. We fixed the
translog issue in #33352, but there is still another race with
createStore.

This commit ensures that we create all engines in the main thread.

Relates #33352
Closes #33344

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

@@ -19,14 +19,12 @@
 
 package org.elasticsearch.index.engine;
 
-import java.nio.file.Path;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.core.internal.io.IOUtils;
 import org.elasticsearch.index.IndexSettings;
 import org.elasticsearch.index.VersionType;
 import org.elasticsearch.index.mapper.MapperService;
 import org.elasticsearch.index.mapper.ParsedDocument;
-import org.elasticsearch.index.store.Store;
 import org.elasticsearch.index.translog.SnapshotMatchers;
 import org.elasticsearch.index.translog.Translog;
 import org.elasticsearch.test.IndexSettingsModule;
@@ -202,7 +200,7 @@ public void testUpdateAndReadChangesConcurrently() throws Exception {
         CountDownLatch readyLatch = new CountDownLatch(followers.length + 1);
         AtomicBoolean isDone = new AtomicBoolean();
         for (int i = 0; i < followers.length; i++) {
-            followers[i] = new Follower(engine, isDone, readyLatch, createTempDir());
+            followers[i] = new Follower(engine, isDone, readyLatch);
             followers[i].start();
         }
         boolean onPrimary = randomBoolean();
@@ -228,28 +226,30 @@ public void testUpdateAndReadChangesConcurrently() throws Exception {
             operations.add(op);
         }
         readyLatch.countDown();
+        readyLatch.await();
         concurrentlyApplyOps(operations, engine);
         assertThat(engine.getLocalCheckpointTracker().getCheckpoint(), equalTo(operations.size() - 1L));
         isDone.set(true);
         for (Follower follower : followers) {
             follower.join();
+            IOUtils.close(follower.engine, follower.engine.store);
         }
     }
 
     class Follower extends Thread {
         private final Engine leader;
+        private final InternalEngine engine;
         private final TranslogHandler translogHandler;
         private final AtomicBoolean isDone;
         private final CountDownLatch readLatch;
-        private final Path translogPath;
 
-        Follower(Engine leader, AtomicBoolean isDone, CountDownLatch readLatch, Path translogPath) {
+        Follower(Engine leader, AtomicBoolean isDone, CountDownLatch readLatch) throws IOException {
             this.leader = leader;
             this.isDone = isDone;
             this.readLatch = readLatch;
             this.translogHandler = new TranslogHandler(xContentRegistry(), IndexSettingsModule.newIndexSettings(shardId.getIndexName(),
-                engine.engineConfig.getIndexSettings().getSettings()));
-            this.translogPath = translogPath;
+                leader.engineConfig.getIndexSettings().getSettings()));
+            this.engine = createEngine(createStore(), createTempDir());
         }
 
         void pullOperations(Engine follower) throws IOException {
@@ -267,16 +267,15 @@ void pullOperations(Engine follower) throws IOException {
 
         @Override
         public void run() {
-            try (Store store = createStore();
-                 InternalEngine follower = createEngine(store, translogPath)) {
+            try {
                 readLatch.countDown();
                 readLatch.await();
                 while (isDone.get() == false ||
-                    follower.getLocalCheckpointTracker().getCheckpoint() < leader.getLocalCheckpoint()) {
-                    pullOperations(follower);
+                    engine.getLocalCheckpointTracker().getCheckpoint() < leader.getLocalCheckpoint()) {
+                    pullOperations(engine);
                 }
-                assertConsistentHistoryBetweenTranslogAndLuceneIndex(follower, mapperService);
-                assertThat(getDocIds(follower, true), equalTo(getDocIds(leader, true)));
+                assertConsistentHistoryBetweenTranslogAndLuceneIndex(engine, mapperService);
+                assertThat(getDocIds(engine, true), equalTo(getDocIds(leader, true)));
             } catch (Exception ex) {
                 throw new AssertionError(ex);
             }