Fix unhandled exception when blobstore repository contains unexpected file (elastic#93914)

If there's any file with the `index-` prefix but not a number after that at the repo root we
must not throw here. If we do, we will end up throwing an unexpected exception that is not
properly handled by `org.elasticsearch.snapshots.SnapshotsService#failAllListenersOnMasterFailOver`,
leading to the repository generation not getting correctly set in the cluster state down the line.

Author: original-brownbear

docs/changelog/93914.yaml

@@ -0,0 +1,5 @@
+pr: 93914
+summary: Fix unhandled exception when blobstore repository contains unexpected file
+area: Snapshot/Restore
+type: bug
+issues: []

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

@@ -31,6 +31,7 @@
 import org.elasticsearch.repositories.ShardGeneration;
 import org.elasticsearch.repositories.ShardGenerations;
 import org.elasticsearch.repositories.blobstore.BlobStoreRepository;
+import org.elasticsearch.repositories.fs.FsRepository;
 import org.elasticsearch.threadpool.ThreadPool;
 import org.elasticsearch.xcontent.XContentFactory;
 
@@ -814,6 +815,19 @@ public void testSnapshotWithMissingShardLevelIndexFile() throws Exception {
         assertEquals(0, restoreSnapshotResponse.getRestoreInfo().failedShards());
     }
 
+    public void testDeletesWithUnexpectedIndexBlob() throws Exception {
+        Path repo = randomRepoPath();
+        final String repoName = "test-repo";
+        createRepository(repoName, FsRepository.TYPE, repo);
+        final String snapshot = "first-snapshot";
+        createFullSnapshot(repoName, snapshot);
+        // create extra file with the index prefix
+        final var extraFile = Files.createFile(repo.resolve(BlobStoreRepository.INDEX_FILE_PREFIX + randomAlphaOfLength(5)));
+        assertAcked(startDeleteSnapshot(repoName, snapshot).get());
+        // delete file again so repo consistency checks pass
+        Files.delete(extraFile);
+    }
+
     private void assertRepositoryBlocked(String repo, String existingSnapshot) {
         if (getRepositoryMetadata(repo).generation() == RepositoryData.CORRUPTED_REPO_GEN) return;
 

server/src/main/java/org/elasticsearch/repositories/blobstore/BlobStoreRepository.java

@@ -1298,7 +1298,12 @@ private static List<String> staleRootBlobs(RepositoryData repositoryData, Set<St
                 return allSnapshotIds.contains(foundUUID) == false;
             } else if (blob.startsWith(INDEX_FILE_PREFIX)) {
                 // TODO: Include the current generation here once we remove keeping index-(N-1) around from #writeIndexGen
-                return repositoryData.getGenId() > Long.parseLong(blob.substring(INDEX_FILE_PREFIX.length()));
+                try {
+                    return repositoryData.getGenId() > Long.parseLong(blob.substring(INDEX_FILE_PREFIX.length()));
+                } catch (NumberFormatException nfe) {
+                    // odd case of an extra file with the index- prefix that we can't identify
+                    return false;
+                }
             }
             return false;
         }).toList();