package pktline
import (
"errors"
"io"
"github.com/go-git/go-git/v5/utils/trace"
)
const (
lenSize = 4
)
// ErrInvalidPktLen is returned by Err() when an invalid pkt-len is found.
var ErrInvalidPktLen = errors.New("invalid pkt-len found")
// Scanner provides a convenient interface for reading the payloads of a
// series of pkt-lines. It takes an io.Reader providing the source,
// which then can be tokenized through repeated calls to the Scan
// method.
//
// After each Scan call, the Bytes method will return the payload of the
// corresponding pkt-line on a shared buffer, which will be 65516 bytes
// or smaller. Flush pkt-lines are represented by empty byte slices.
//
// Scanning stops at EOF or the first I/O error.
type Scanner struct {
r io.Reader // The reader provided by the client
err error // Sticky error
payload []byte // Last pkt-payload
len [lenSize]byte // Last pkt-len
}
// NewScanner returns a new Scanner to read from r.
func NewScanner(r io.Reader) *Scanner {
return &Scanner{
r: r,
}
}
// Err returns the first error encountered by the Scanner.
func (s *Scanner) Err() error {
return s.err
}
// Scan advances the Scanner to the next pkt-line, whose payload will
// then be available through the Bytes method. Scanning stops at EOF
// or the first I/O error. After Scan returns false, the Err method
// will return any error that occurred during scanning, except that if
// it was io.EOF, Err will return nil.
func (s *Scanner) Scan() bool {
var l int
l, s.err = s.readPayloadLen()
if s.err == io.EOF {
s.err = nil
return false
}
if s.err != nil {
return false
}
if cap(s.payload) < l {
s.payload = make([]byte, 0, l)
}
if _, s.err = io.ReadFull(s.r, s.payload[:l]); s.err != nil {
return false
}
s.payload = s.payload[:l]
trace.Packet.Printf("packet: < %04x %s", l, s.payload)
return true
}
// Bytes returns the most recent payload generated by a call to Scan.
// The underlying array may point to data that will be overwritten by a
// subsequent call to Scan. It does no allocation.
func (s *Scanner) Bytes() []byte {
return s.payload
}
// Method readPayloadLen returns the payload length by reading the
// pkt-len and subtracting the pkt-len size.
func (s *Scanner) readPayloadLen() (int, error) {
if _, err := io.ReadFull(s.r, s.len[:]); err != nil {
if err == io.ErrUnexpectedEOF {
return 0, ErrInvalidPktLen
}
return 0, err
}
n, err := hexDecode(s.len)
if err != nil {
return 0, err
}
switch {
case n == 0:
return 0, nil
case n <= lenSize:
return 0, ErrInvalidPktLen
case n > OversizePayloadMax+lenSize:
return 0, ErrInvalidPktLen
default:
return n - lenSize, nil
}
}
// Turns the hexadecimal representation of a number in a byte slice into
// a number. This function substitute strconv.ParseUint(string(buf), 16,
// 16) and/or hex.Decode, to avoid generating new strings, thus helping the
// GC.
func hexDecode(buf [lenSize]byte) (int, error) {
var ret int
for i := 0; i < lenSize; i++ {
n, err := asciiHexToByte(buf[i])
if err != nil {
return 0, ErrInvalidPktLen
}
ret = 16*ret + int(n)
}
return ret, nil
}
// turns the hexadecimal ascii representation of a byte into its
// numerical value. Example: from 'b' to 11 (0xb).
func asciiHexToByte(b byte) (byte, error) {
switch {
case b >= '0' && b <= '9':
return b - '0', nil
case b >= 'a' && b <= 'f':
return b - 'a' + 10, nil
default:
return 0, ErrInvalidPktLen
}
}
