Relax cleanup check in SnapshotStressTestsIT (#106569)

We can't assert no leaked blobs here because today the first cleanup
leaves the original `RepositoryData` in place so the second cleanup is
not a no-op.

Relates #100718
Backport of #100855 to 7.17

server/src/internalClusterTest/java/org/elasticsearch/snapshots/SnapshotStressTestsIT.java

@@ -809,9 +809,33 @@ private void startCleaner() {
                             Releasables.close(releaseAll);
                             logger.info("--> completed cleanup of [{}]", trackedRepository.repositoryName);
                             final RepositoryCleanupResult result = cleanupRepositoryResponse.result();
-                            assertThat(Strings.toString(result), result.blobs(), equalTo(0L));
-                            assertThat(Strings.toString(result), result.bytes(), equalTo(0L));
-                            startCleaner();
+                            if (result.bytes() > 0L || result.blobs() > 0L) {
+                                // we could legitimately run into dangling blobs as the result of a shard snapshot failing half-way
+                                // through the snapshot because of a concurrent index-close or -delete. The second round of cleanup on
+                                // the same repository however should always find no more dangling blobs and be a no-op since we block all
+                                // concurrent operations on the repository.
+                                client.admin()
+                                    .cluster()
+                                    .prepareCleanupRepository(trackedRepository.repositoryName)
+                                    .execute(mustSucceed(secondCleanupRepositoryResponse -> {
+                                        final RepositoryCleanupResult secondCleanupResult = secondCleanupRepositoryResponse.result();
+                                        if (secondCleanupResult.blobs() == 1) {
+                                            // The previous cleanup actually leaves behind a stale index-N blob, so this cleanup removes it
+                                            // and reports it in its response. When https://github.com/elastic/elasticsearch/pull/100718 is
+                                            // fixed the second cleanup will be a proper no-op and we can remove this lenience -- TODO
+                                        } else {
+                                            assertThat(Strings.toString(secondCleanupResult), secondCleanupResult.blobs(), equalTo(0L));
+                                            assertThat(Strings.toString(secondCleanupResult), secondCleanupResult.bytes(), equalTo(0L));
+                                        }
+                                        Releasables.close(releaseAll);
+                                        logger.info("--> completed second cleanup of [{}]", trackedRepository.repositoryName);
+                                        startCleaner();
+                                    }));
+                            } else {
+                                Releasables.close(releaseAll);
+                                logger.info("--> completed cleanup of [{}]", trackedRepository.repositoryName);
+                                startCleaner();
+                            }
                         }));
 
                     startedCleanup = true;