thanos-io/thanos · error

proto: negative length found during unmarshaling

Error message

proto: negative length found during unmarshaling

What it means

ErrInvalidLengthRpc is a sentinel generated into gogo-protobuf's rpc.pb.go files. It is returned when a decoded length (e.g. a string/bytes field length or a computed postIndex) is negative, which cannot happen with well-formed protobuf input. It signals that the marshaled data or offset arithmetic is corrupt.

Solutions

  1. Check the buffer slicing logic that produced the byte slice passed to Unmarshal (correct offset and length).
  2. Validate payload integrity (checksums, content-length) before unmarshaling.
  3. Ensure all services use the same generated proto code version.
  4. Return a 4xx/invalid-data error to the caller rather than retrying, since the input is malformed.

Example fix

// before
if err := proto.Unmarshal(chunk, msg); err != nil { return err }
// after
if len(chunk) == 0 { return errors.New("empty chunk") }
if err := proto.Unmarshal(chunk, msg); err != nil {
    if errors.Is(err, errpb.ErrInvalidLengthRpc) { return errors.Wrap(err, "malformed protobuf chunk") }
    return err
}
Defensive patterns

Strategy: try-catch

Validate before calling

if len(data) < 2 { return errors.New("payload too short to be valid protobuf") }

Try / catch

if err := proto.Unmarshal(data, msg); err != nil {
    if errors.Is(err, ErrInvalidLengthRpc) { return errors.Wrap(err, "corrupt protobuf: negative length") }
    return err
}

Prevention

When it happens

Trigger: Unmarshaling a message where a length varint decodes to a negative value (int overflow of the length prefix) or iNdEx + length underflows, in any generated Unmarshal/skip function referencing ErrInvalidLengthRpc.

Common situations: Corrupt object-storage payloads, byte-offset bugs when reading message slices from a larger buffer, or schema mismatch producing misaligned varints.

Understand the failure class

Background: "cannot parse invalid wire-format data", "cannot unmarshal", "failed unmarshalling": protobuf unmarshal errors explained — this error's family across 10 libraries.

Related errors

AI-assisted analysis of thanos-io/thanos@35b8b99117 (2026-09-07). Data as JSON: /api/errors/64cfe0022e0f196e. Report an issue: GitHub.

Appendix: source

Thrown at pkg/status/statuspb/rpc.pb.go:2004

			}
			depth--
		case 5:
			iNdEx += 4
		default:
			return 0, fmt.Errorf("proto: illegal wireType %d", wireType)
		}
		if iNdEx < 0 {
			return 0, ErrInvalidLengthRpc
		}
		if depth == 0 {
			return iNdEx, nil
		}
	}
	return 0, io.ErrUnexpectedEOF
}

var (
	ErrInvalidLengthRpc        = fmt.Errorf("proto: negative length found during unmarshaling")
	ErrIntOverflowRpc          = fmt.Errorf("proto: integer overflow")
	ErrUnexpectedEndOfGroupRpc = fmt.Errorf("proto: unexpected end of group")
)

View on GitHub (pinned to 35b8b99117)