juicedata/juicefs · error

first key should be empty string, but got %s

Error message

first key should be empty string, but got %s

What it means

Raised in the objbench 'list objects' filesystem case: the first entry returned by the list must be the directory placeholder with an empty-string key, but a non-empty key was returned. Filesystem-style ObjectStorage implementations are required to include the "" entry representing the listed directory itself.

Source

Thrown at cmd/objbench.go:896

			return fmt.Errorf("deleting a non-existent object returns an error %v", err)
		}
		return nil
	})

	runCase("list objects", func(blob object.ObjectStorage) error {
		br := []byte("hello")
		if err := blob.Put(ctx, key, bytes.NewReader(br)); err != nil {
			return fmt.Errorf("put object failed: %s", err)
		}
		defer blob.Delete(ctx, key) //nolint:errcheck
		if isFileSystem {
			objs, err := listAll(ctx, blob, "", "", 2)
			if err == nil {
				if len(objs) != 2 {
					return fmt.Errorf("list should return 2 keys, but got %d", len(objs))
				}
				if objs[0].Key() != "" {
					return fmt.Errorf("first key should be empty string, but got %s", objs[0].Key())
				}
				if objs[0].Size() != 0 {
					return fmt.Errorf("first object size should be 0, but got %d", objs[0].Size())
				}
				if objs[1].Key() != key {
					return fmt.Errorf("first key should be test, but got %s", objs[1].Key())
				}
				if objs[1].Size() != 5 {
					return fmt.Errorf("size of first key shold be 5, but got %v", objs[1].Size())
				}
				now := time.Now()
				if objs[1].Mtime().Before(now.Add(-30*time.Second)) || objs[1].Mtime().After(now.Add(time.Second*30)) {
					return fmt.Errorf("mtime of key should be within 30 seconds, but got %s", objs[1].Mtime().Sub(now))
				}
			} else {
				return fmt.Errorf("list failed: %s", err)
			}

View on GitHub (pinned to c9a67b23e8)

Solutions

1. Fix the backend's List to emit the directory entry "" first when listing without a prefix.
2. Clear the test directory of extra keys that could alter ordering.
3. Compare behavior with the reference file:// implementation in pkg/object and align it.
4. Pin/upgrade JuiceFS so the backend and objbench come from the same version.
5. Rerun objbench against a clean directory.

Example fix

// before
return sortedEntries, nil // "" entry may be missing or misordered
// after
entries := append([]Object{&entry{key: "", size: 0}}, sortedEntries...)
return entries, nil

objs, _ := listAll(ctx, blob, "", "", 2)
if len(objs) > 0 && objs[0].Key() != "" {
    log.Printf("first entry is not the directory placeholder: %q", objs[0].Key())
}

func isFirstDirEntry(objs []object.Object) bool {
    return len(objs) > 0 && objs[0].Key() == ""
}

objs, err := listAll(ctx, blob, "", "", 2)
if err == nil && len(objs) > 0 && objs[0].Key() != "" {
    // handle non-conformant backend: normalize or reject
}

When it happens

Trigger: listAll(ctx, blob, "", "", 2) returned 2 objects, but objs[0].Key() != "" — i.e. the backend ordered entries differently or did not synthesize the directory entry at position 0.

Common situations: A custom file backend that sorts entries lexicographically and puts a real object before the "" entry; a backend that never emits the directory placeholder; testing with leftover keys that sort before "test".

Understand the failure class

Background: "invalid response format", "malformed payload", "missing data field": when an API returns 200 but the response shape is wrong — this error's family across 23 libraries.

Related errors

• list should return 2 keys, but got %d
• first object size should be 0, but got %d
• first key should be test, but got %s
• size of first key shold be 5, but got %v
• list failed: %s

AI-assisted analysis of juicedata/juicefs@c9a67b23e8 (2026-09-06). Data as JSON: /api/errors/33d6b20b621a4942. Report an issue: GitHub.