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package styxproto
import (
var (
errFillOverflow = errors.New("cannot fill buffer past maxInt")
// Design goals of the parser:
// - minimize allocations
// - resilient to malicious input (invalid/overlarge sizes)
// - streaming: a 4GB (max uint32) Twrite should not take 4G of memory
// NewDecoder returns a Decoder with an internal buffer of size
// DefaultBufSize.
func NewDecoder(r io.Reader) *Decoder {
return NewDecoderSize(r, DefaultBufSize)
// NewDecoderSize returns a Decoder with an internal buffer of size
// max(MinBufSize, bufsize) bytes. A Decoder with a larger buffer can
// provide more 9P messages at once, if they are available. This may
// improve performance on connections that are heavily multiplexed,
// where there messages from independent sessions that can be handled
// in any order.
func NewDecoderSize(r io.Reader, bufsize int) *Decoder {
if bufsize < MinBufSize {
bufsize = MinBufSize
return &Decoder{r: r, br: bufio.NewReaderSize(r, bufsize), MaxSize: -1}
// A Decoder provides an interface for reading a stream of 9P
// messages from an io.Reader. Successive calls to the Next
// method of a Decoder will fetch and validate 9P messages
// from the input stream, until EOF is encountered, or another
// error is encountered.
// A Decoder is not safe for concurrent use. Usage of any Decoder
// method should be delegated to a single thread of execution or
// protected by a mutex.
type Decoder struct {
// MaxSize is the maximum size message that a Decoder will accept. If
// MaxSize is -1, a Decoder will accept any size message.
MaxSize int64
// input source. we need to expose this so we can stitch together
// an io.Reader for large Twrite/Rread messages.
r io.Reader
// internal buffer is used to store messages
br *bufio.Reader
// current selection in the buffered data
start, pos int
// Last fetched message. slices on br's internal buffer, so
// only valid until next call to br.Read
msg Msg
// Last error encountered when reading from r
// or during parsing
err error
// Reset resets a Decoder with a new io.Reader.
func (s *Decoder) Reset(r io.Reader) {
s.MaxSize = -1
s.r = r
s.start = 0
s.pos = 0
s.msg = nil
s.err = nil
// Err returns the first error encountered during parsing.
// If the underyling io.Reader was closed in the middle of
// a message, Err will return io.ErrUnexpectedEOF. Otherwise,
// io.EOF is not considered to be an error, and is not relayed
// by Err.
// Invalid messages are not considered errors, and are
// represented in the Messages slice as values of type BadMessage.
// Only problems with the underlying io.Reader are
// considered errors.
func (s *Decoder) Err() error {
if s.err == io.EOF {
return nil
return s.err
// Msg returns the last 9P message decoded in the stream. It
// returns a non-nil message if and only if the last call to the
// Decoder's Next method returned true. The return value of
// Msg is only valid until the next call to a decoder's Next method.
func (s *Decoder) Msg() Msg {
return s.msg
// Next fetches the next 9P message from the Decoder's underlying
// io.Reader. If an error is encountered reading from the underlying
// stream, Next will return false, and the Decoder's Err method will return
// the first error encountered.
// If Next returns true, the Msg method of the Decoder will return the
// decoded 9P message.
func (s *Decoder) Next() bool {
if s.msg != nil {
// Exhaust any Rread/Twrite messages to move the stream to the
// next message boundary.
if r, ok := s.msg.(io.Reader); ok {
_, s.err = io.Copy(ioutil.Discard, r)
s.err = discard(, s.msg.nbytes())
s.msg = nil
if s.err != nil {
s.msg = nil
return false
s.msg, s.err = s.fetchMessage()
return s.msg != nil
// A bufio.Reader is not just a way to smooth out I/O performance;
// it can also be used as a "sliding window" over a byte stream.
// If the terminology below seems odd, it is inspired by the sam
// text editor, where "dot" refers to the current text selection.
func (s *Decoder) dot() []byte {
if s.pos > {
panic("decoder position out of bounds")
buf, err :=
if err != nil {
panic(err) // this should never happen, it's buffered
return buf[s.start:]
func (s *Decoder) resetdot() {
s.start, s.pos = 0, 0
func (s *Decoder) advance(n int) {
if s.buflen() < n {
panic("advance decoder out of bounds")
s.pos += n
func (s *Decoder) shrinkdot(n int) {
if s.dotlen() < n {
panic("shrink dot out of bounds")
s.pos -= n
// advance start of dot to end of dot
func (s *Decoder) mark() {
s.start = s.pos
// number of bytes buffered after dot
func (s *Decoder) buflen() int {
return - s.pos
func (s *Decoder) dotlen() int {
return s.pos - s.start
// extends dot to be n bytes long, performing
// IO if necessary. returns dot
func (s *Decoder) growdot(n int) ([]byte, error) {
if err := s.fill(n - s.dotlen()); err != nil {
return nil, err
s.advance(n - s.dotlen())
return, nil
// guarantees that s.buflen() >= n if error is nil
func (s *Decoder) fill(n int) error {
if maxInt-n < s.pos {
return errFillOverflow
_, err := + n)
return err
func discard(r *bufio.Reader, n int64) error {
_, err := io.CopyN(ioutil.Discard, r, n)
return err