websocket/compression.go

// Copyright 2016 The Gorilla WebSocket Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package websocket

import (
	"compress/flate"
	"errors"
	"io"
	"strings"
)

func decompressNoContextTakeover(r io.Reader) io.Reader {
	const tail =
	// Add four bytes as specified in RFC
	"\x00\x00\xff\xff" +
		// Add final block to squelch unexpected EOF error from flate reader.
		"\x01\x00\x00\xff\xff"

	return flate.NewReader(io.MultiReader(r, strings.NewReader(tail)))
}

func compressNoContextTakeover(w io.WriteCloser) (io.WriteCloser, error) {
	tw := &truncWriter{w: w}
	fw, err := flate.NewWriter(tw, 3)
	return &flateWrapper{fw: fw, tw: tw}, err
}

// truncWriter is an io.Writer that writes all but the last four bytes of the
// stream to another io.Writer.
type truncWriter struct {
	w io.WriteCloser
	n int
	p [4]byte
}

func (w *truncWriter) Write(p []byte) (int, error) {
	n := 0

	// fill buffer first for simplicity.
	if w.n < len(w.p) {
		n = copy(w.p[w.n:], p)
		p = p[n:]
		w.n += n
		if len(p) == 0 {
			return n, nil
		}
	}

	m := len(p)
	if m > len(w.p) {
		m = len(w.p)
	}

	if nn, err := w.w.Write(w.p[:m]); err != nil {
		return n + nn, err
	}

	copy(w.p[:], w.p[m:])
	copy(w.p[len(w.p)-m:], p[len(p)-m:])
	nn, err := w.w.Write(p[:len(p)-m])
	return n + nn, err
}

type flateWrapper struct {
	fw *flate.Writer
	tw *truncWriter
}

func (w *flateWrapper) Write(p []byte) (int, error) {
	return w.fw.Write(p)
}

func (w *flateWrapper) Close() error {
	err1 := w.fw.Flush()
	if w.tw.p != [4]byte{0, 0, 0xff, 0xff} {
		return errors.New("websocket: internal error, unexpected bytes at end of flate stream")
	}
	err2 := w.tw.w.Close()
	if err1 != nil {
		return err1
	}
	return err2
}