testConcurrentWriteViewsAndSnapshot shouldn't flush concurrently

Fixes #24933

core/src/test/java/org/elasticsearch/index/translog/TranslogTests.java

@@ -682,6 +682,8 @@ public class TranslogTests extends ESTestCase {
         // a signal for all threads to stop
         final AtomicBoolean run = new AtomicBoolean(true);
 
+        final Object flushMutex = new Object();
+
         // any errors on threads
         final List<Exception> errors = new CopyOnWriteArrayList<>();
         logger.debug("using [{}] readers. [{}] writers. flushing every ~[{}] ops.", readers.length, writers.length, flushEveryOps);
@@ -721,7 +723,11 @@ public class TranslogTests extends ESTestCase {
                             translog.ensureSynced(location);
                         }
                         if (id % flushEveryOps == 0) {
-                            translog.commit(translog.currentFileGeneration());
+                            synchronized (flushMutex) {
+                                // we need not do this concurrently as we need to make sure that the generation
+                                // we're committing - translog.currentFileGeneration() - is still present when we're committing
+                                translog.commit(translog.currentFileGeneration());
+                            }
                         }
                         if (id % 7 == 0) {
                             synchronized (signalReaderSomeDataWasIndexed) {