juicedata/juicefs · error

failed to read segment type: %v

Error message

failed to read segment type: %v

What it means

BakSegment.Unmarshal reads the 1-byte segment type from the backup stream with binary.Read. This error is thrown when that read fails — almost always io.EOF or io.ErrUnexpectedEOF, meaning the stream ended before a segment type byte could be read. The caller (ReadFooter) treats errBakEOF (returned when typ == BakEOS) as a normal end marker; this error instead means the stream is truncated or unreadable without a valid EOS.

Source

Thrown at pkg/meta/backup.go:351

	data, err := proto.Marshal(s.val)
	if err != nil {
		return 0, fmt.Errorf("failed to marshal segment message %s : %w", s, err)
	}
	s.len = uint64(len(data))
	if err := binary.Write(w, binary.BigEndian, s.len); err != nil {
		return 0, fmt.Errorf("failed to write segment length %s: %w", s, err)
	}

	if n, err := w.Write(data); err != nil || n != len(data) {
		return 0, fmt.Errorf("failed to write segment data %s: err %w, write len %d, expect len %d", s, err, n, len(data))
	}

	return binary.Size(s.typ) + binary.Size(s.len) + len(data), nil
}

func (s *BakSegment) Unmarshal(r io.Reader) error {
	if err := binary.Read(r, binary.BigEndian, &s.typ); err != nil {
		return fmt.Errorf("failed to read segment type: %v", err)
	}

	if s.typ == BakEOS {
		return errBakEOF
	}

	if err := binary.Read(r, binary.BigEndian, &s.len); err != nil {
		return fmt.Errorf("failed to read segment %s length: %v", s, err)
	}
	data := make([]byte, s.len)
	n, err := r.Read(data)
	if err != nil && n != int(s.len) {
		return fmt.Errorf("failed to read segment value: err %v, read len %d, expect len %d", err, n, s.len)
	}

	msg, err := getMessageFromType(int(s.typ))
	if err != nil {
		return fmt.Errorf("failed to create message by type %d: %w", s.typ, err)

View on GitHub (pinned to c9a67b23e8)

Solutions

1. Verify the backup file is complete and non-empty (compare size against the original backup output or re-run the backup)
2. Check read permissions on the backup file and that the handle is open/valid
3. If the file is truncated, regenerate the backup rather than trying to salvage the footer
4. If EOF at a clean boundary is expected, handle errBakEOF, not this error

fi, err := os.Stat(backupPath)
if err != nil || fi.Size() == 0 { return fmt.Errorf("backup file missing or empty: %s", backupPath) }

if err := seg.Unmarshal(r); err != nil {
    switch {
    case errors.Is(err, errBakEOF): // clean end
    case errors.Is(err, io.ErrUnexpectedEOF), errors.Is(err, io.EOF):
        return fmt.Errorf("backup file truncated: %w", err)
    default:
        return err
    }
}

When it happens

Trigger: Calling ReadFooter on a backup file/stream that is empty, truncated (partial write during backup), or whose read fails with an I/O error (bad file descriptor, read permission, corrupted medium).

Common situations: Loading a metadata backup file that was cut short by a failed/partial backup write; a corrupted or zero-byte backup file; reading from a closed or unreadable file handle.

Understand the failure class

Background: "failed to read file", EACCES, ENOENT and "could not read <path>" errors: when a program can't read a file from disk — this error's family across 49 libraries.

Related errors

• failed to read segment %s length: %v
• failed to read segment value: err %v, read len %d, expect le
• Unable to skip %s bytes (position=%s, fileSize=%s): %s
• failed to write EOS: err %w, write len %d, expect len 4
• failed to write footer data: err %w, write len %d, expect le

AI-assisted analysis of juicedata/juicefs@c9a67b23e8 (2026-09-06). Data as JSON: /api/errors/263c32a66f7be542. Report an issue: GitHub.