juicedata/juicefs · error

first object size should be 0, but got %d

Error message

first object size should be 0, but got %d

What it means

Raised in the objbench 'list objects' filesystem case: the first listed entry (the directory placeholder for "") must have Size() == 0, but a nonzero size was returned. This asserts that filesystem backends report directory entries with zero size, matching how object listings emulate directories.

Source

Thrown at cmd/objbench.go:899

	})

	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)
			}

			objs, err = listAll(ctx, blob, "", "test2", 1)
			if err != nil {
				return fmt.Errorf("list failed: %s", err)

View on GitHub (pinned to c9a67b23e8)

Solutions

1. In the backend's List, hardcode Size()=0 for synthesized directory entries instead of the directory's stat size.
2. Ensure the size field is reset/zeroed for placeholder entries.
3. Test against the reference file:// backend behavior in pkg/object.
4. Clean the test directory of stray keys that could be mistaken for the dir entry.
5. Rerun objbench.

Example fix

// before
return &entry{key: "", size: fi.Size()}, nil // 4096 for dirs
// after
return &entry{key: "", size: 0}, nil // dir entry must be size 0

objs, _ := listAll(ctx, blob, "", "", 2)
if len(objs) > 0 && objs[0].Key() == "" && objs[0].Size() != 0 {
    log.Printf("dir entry has nonzero size: %d", objs[0].Size())
}

func isZeroSizeDirEntry(o object.Object) bool {
    return o.Key() == "" && o.Size() == 0
}

if o := objs[0]; o.Key() == "" && o.Size() != 0 {
    log.Printf("backend reports dir size %d; treat as 0", o.Size())
}

When it happens

Trigger: After listAll returns 2 entries with the correct keys, objs[0].Size() != 0 — the backend filled in a real byte size (e.g. directory stat size like 4096, or an object's size) for the directory entry.

Common situations: A custom file backend that calls os.Stat on the directory and reports its allocated size; a backend that reuses a struct with a stale size field; listing a prefix that matches a real object with the same path.

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

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

AI-assisted analysis of juicedata/juicefs@c9a67b23e8 (2026-09-06). Data as JSON: /api/errors/d7f25ef6f4b3f792. Report an issue: GitHub.