Fix flaky test testFlushThrowsFlushFailedExceptionOnCorruption.

This test is flaky in two ways, one it may hit a case where a merge occurs and the hardcoded segment does not exist to delete.
Two with certain mock filesystems a corruption exception won't be thrown here, only a NoSuchFile - This doesn't change our
handling in the engine, but the test should conditionally handle if the store is marked corrupted on close.

Signed-off-by: Marc Handalian <[email protected]>

server/src/test/java/org/opensearch/index/engine/NRTReplicationEngineTests.java

@@ -617,11 +617,18 @@ public void testFlushThrowsFlushFailedExceptionOnCorruption() throws Exception {
         indexOperations(nrtEngine, operations);
         // wipe the nrt directory initially so we can sync with primary.
         cleanAndCopySegmentsFromPrimary(nrtEngine);
-        nrtEngineStore.directory().deleteFile("_0.si");
+        final Optional<String> toDelete = Set.of(nrtEngineStore.directory().listAll()).stream().filter(f -> f.endsWith(".si")).findAny();
+        assertTrue(toDelete.isPresent());
+        nrtEngineStore.directory().deleteFile(toDelete.get());
         assertThrows(FlushFailedEngineException.class, nrtEngine::flush);
-        assertTrue(nrtEngineStore.isMarkedCorrupted());
-        // store will throw when eventually closed, not handled here.
-        assertThrows(RuntimeException.class, nrtEngineStore::close);
+        nrtEngine.close();
+        if (nrtEngineStore.isMarkedCorrupted()) {
+            assertThrows(RuntimeException.class, nrtEngineStore::close);
+        } else {
+            // With certain mock directories a NoSuchFileException is thrown which is not treated as a
+            // corruption Exception. In these cases we don't expect any issue on store close.
+            nrtEngineStore.close();
+        }
     }
 
     private void copySegments(Collection<String> latestPrimaryFiles, Engine nrtEngine) throws IOException {