data_reader.go

package telnet


import (
	"bufio"
	"errors"
	"io"
)


var (
	errCorrupted = errors.New("Corrupted")
)


// An internalDataReader deals with "un-escaping" according to the TELNET protocol.
//
// In the TELNET protocol byte value 255 is special.
//
// The TELNET protocol calls byte value 255: "IAC". Which is short for "interpret as command".
//
// The TELNET protocol also has a distinction between 'data' and 'commands'.
//
//(DataReader is targetted toward TELNET 'data', not TELNET 'commands'.)
//
// If a byte with value 255 (=IAC) appears in the data, then it must be escaped.
//
// Escaping byte value 255 (=IAC) in the data is done by putting 2 of them in a row.
//
// So, for example:
//
//	[]byte{255} -> []byte{255, 255}
//
// Or, for a more complete example, if we started with the following:
//
//	[]byte{1, 55, 2, 155, 3, 255, 4, 40, 255, 30, 20}
//
// ... TELNET escaping would produce the following:
//
//	[]byte{1, 55, 2, 155, 3, 255, 255, 4, 40, 255, 255, 30, 20}
//
// (Notice that each "255" in the original byte array became 2 "255"s in a row.)
//
// DataReader deals with "un-escaping". In other words, it un-does what was shown
// in the examples.
//
// So, for example, it does this:
//
//	[]byte{255, 255} -> []byte{255}
//
// And, for example, goes from this:
//
//	[]byte{1, 55, 2, 155, 3, 255, 255, 4, 40, 255, 255, 30, 20}
//
// ... to this:
//
//	[]byte{1, 55, 2, 155, 3, 255, 4, 40, 255, 30, 20}
type internalDataReader struct {
	wrapped  io.Reader
	buffered  *bufio.Reader
}


// newDataReader creates a new DataReader reading from 'r'.
func newDataReader(r io.Reader) *internalDataReader {
	buffered := bufio.NewReader(r)

	reader := internalDataReader{
		wrapped:r,
		buffered:buffered,
	}

	return &reader
}


// Read reads the TELNET escaped data from the  wrapped io.Reader, and "un-escapes" it into 'data'.
func (r *internalDataReader) Read(data []byte) (n int, err error) {

	const IAC = 255

	const SB = 250
	const SE = 240

	const WILL = 251
	const WONT = 252
	const DO   = 253
	const DONT = 254

	p := data

	for len(p) > 0 {
		var b byte

		b, err = r.buffered.ReadByte()
		if nil != err {
			return n, err
		}

		if IAC == b {
			var peeked []byte

			peeked, err = r.buffered.Peek(1)
			if nil != err {
				return n, err
			}

			switch peeked[0] {
			case WILL, WONT, DO, DONT:
				_, err = r.buffered.Discard(2)
				if nil != err {
					return n, err
				}
			case IAC:
				p[0] = IAC
				n++
				p = p[1:]

				_, err = r.buffered.Discard(1)
				if nil != err {
					return n, err
				}
			case SB:
				for {
					var b2 byte
					b2, err = r.buffered.ReadByte()
					if nil != err {
						return n, err
					}

					if IAC == b2 {
						peeked, err = r.buffered.Peek(1)
						if nil != err {
							return n, err
						}

						if IAC == peeked[0] {
							_, err = r.buffered.Discard(1)
							if nil != err {
								return n, err
							}
						}

						if SE == peeked[0] {
							_, err = r.buffered.Discard(1)
							if nil != err {
								return n, err
							}
							break
						}
					}
				}
			case SE:
				_, err = r.buffered.Discard(1)
				if nil != err {
					return n, err
				}
			default:
				// If we get in here, this is not following the TELNET protocol.
//@TODO: Make a better error.
				err = errCorrupted
				return n, err
			}
		} else {

			p[0] = b
			n++
			p = p[1:]
		}
	}

	return n, nil
}