Misc cleanups in S3BlobContainerRetriesTests (#126325)

- Simplify multi-object-delete request detection
- Replace `AtomicBoolean` with volatile field

Backport of #126101 to `8.x`

Author: DaveCTurner

modules/repository-s3/src/test/java/org/elasticsearch/repositories/s3/S3BlobContainerRetriesTests.java

@@ -73,7 +73,6 @@
 import java.util.Map;
 import java.util.Objects;
 import java.util.OptionalInt;
-import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicInteger;
 import java.util.concurrent.atomic.AtomicLong;
 import java.util.regex.Pattern;
@@ -105,19 +104,19 @@
 public class S3BlobContainerRetriesTests extends AbstractBlobContainerRetriesTestCase {
 
     private S3Service service;
-    private AtomicBoolean shouldErrorOnDns;
+    private volatile boolean shouldErrorOnDns;
     private RecordingMeterRegistry recordingMeterRegistry;
 
     @Before
     public void setUp() throws Exception {
-        shouldErrorOnDns = new AtomicBoolean(false);
+        shouldErrorOnDns = false;
         service = new S3Service(Mockito.mock(Environment.class), Settings.EMPTY, Mockito.mock(ResourceWatcherService.class)) {
             @Override
             protected AmazonS3ClientBuilder buildClientBuilder(S3ClientSettings clientSettings) {
                 final AmazonS3ClientBuilder builder = super.buildClientBuilder(clientSettings);
                 final DnsResolver defaultDnsResolver = builder.getClientConfiguration().getDnsResolver();
                 builder.getClientConfiguration().setDnsResolver(host -> {
-                    if (shouldErrorOnDns.get() && randomBoolean() && randomBoolean()) {
+                    if (shouldErrorOnDns && randomBoolean() && randomBoolean()) {
                         throw new UnknownHostException(host);
                     }
                     return defaultDnsResolver.resolve(host);
@@ -653,7 +652,7 @@ public void testReadWithIndicesPurposeRetriesForever() throws IOException {
 
         final byte[] bytes = randomBlobContent(512);
 
-        shouldErrorOnDns.set(true);
+        shouldErrorOnDns = true;
         final AtomicInteger failures = new AtomicInteger();
         @SuppressForbidden(reason = "use a http server")
         class FlakyReadHandler implements HttpHandler {
@@ -776,7 +775,7 @@ public void testSuppressedDeletionErrorsAreCapped() {
         int maxBulkDeleteSize = randomIntBetween(1, 10);
         final BlobContainer blobContainer = createBlobContainer(1, readTimeout, true, null, maxBulkDeleteSize);
         httpServer.createContext("/", exchange -> {
-            if (exchange.getRequestMethod().equals("POST") && exchange.getRequestURI().toString().startsWith("/bucket/?delete")) {
+            if (isMultiDeleteRequest(exchange)) {
                 exchange.sendResponseHeaders(
                     randomFrom(
                         HttpStatus.SC_INTERNAL_SERVER_ERROR,
@@ -810,7 +809,7 @@ public void testTrimmedLogAndCappedSuppressedErrorOnMultiObjectDeletionException
 
         final Pattern pattern = Pattern.compile("<Key>(.+?)</Key>");
         httpServer.createContext("/", exchange -> {
-            if (exchange.getRequestMethod().equals("POST") && exchange.getRequestURI().toString().startsWith("/bucket/?delete")) {
+            if (isMultiDeleteRequest(exchange)) {
                 final String requestBody = Streams.copyToString(new InputStreamReader(exchange.getRequestBody(), StandardCharsets.UTF_8));
                 final var matcher = pattern.matcher(requestBody);
                 final StringBuilder deletes = new StringBuilder();
@@ -962,6 +961,10 @@ private Map<String, Object> metricAttributes(String action) {
         return Map.of("repo_type", "s3", "repo_name", "repository", "operation", "GetObject", "purpose", "Indices", "action", action);
     }
 
+    private static boolean isMultiDeleteRequest(HttpExchange exchange) {
+        return new S3HttpHandler("bucket").parseRequest(exchange).isMultiObjectDeleteRequest();
+    }
+
     /**
      * Asserts that an InputStream is fully consumed, or aborted, when it is closed
      */