websocketwriter.go

/*
Copyright 2015 Gravitational, Inc.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package utils

import (
	"io"
	"sync"

	"github.com/gravitational/trace"
	"golang.org/x/net/websocket"
	"golang.org/x/text/encoding"
	"golang.org/x/text/encoding/unicode"

	log "github.com/Sirupsen/logrus"
)

// WebSockWrapper wraps the raw websocket and converts Write() calls
// to proper websocket.Send() working in binary or text mode. If text
// mode is selected, it converts the data passed to Write() into UTF8 bytes
//
// We need this to make sure that the entire buffer in io.Writer.Write(buffer)
// is delivered as a single chunk to the web browser, instead of being split
// into multiple frames. This wrapper basically substitues every Write() with
// Send() and every Read() with Receive()
type WebSockWrapper struct {
	io.ReadWriteCloser
	sync.Mutex

	ws   *websocket.Conn
	mode WebSocketMode

	encoder *encoding.Encoder
	decoder *encoding.Decoder

	// prefix (if set) will be prepended to every write
	prefix []byte
}

// WebSocketMode allows to create WebSocket wrappers working in text
// or binary mode
type WebSocketMode int

const (
	WebSocketBinaryMode = iota
	WebSocketTextMode
)

func NewWebSockWrapper(ws *websocket.Conn, m WebSocketMode) *WebSockWrapper {
	if ws == nil {
		return nil
	}
	return &WebSockWrapper{
		ws:      ws,
		mode:    m,
		encoder: unicode.UTF8.NewEncoder(),
		decoder: unicode.UTF8.NewDecoder(),
	}
}

// Write implements io.WriteCloser for WebSockWriter (that's the reason we're
// wrapping the websocket)
//
// It replaces raw Write() with "Message.Send()"
func (w *WebSockWrapper) Write(data []byte) (n int, err error) {
	p := w.GetPrefix()
	if len(p) > 0 {
		if w.mode == WebSocketBinaryMode {
			websocket.Message.Send(w.ws, p)
		} else {
			var utf8 string
			utf8, _ = w.encoder.String(string(p))
			websocket.Message.Send(w.ws, utf8)
		}
	}

	n = len(data)
	if w.mode == WebSocketBinaryMode {
		// binary send:
		err = websocket.Message.Send(w.ws, data)
		// text send:
	} else {
		var utf8 string
		utf8, err = w.encoder.String(string(data))
		err = websocket.Message.Send(w.ws, utf8)
	}
	if err != nil {
		log.Error(err)
		n = 0
	}
	return n, err
}

// Read does the opposite of write: it replaces websocket's raw "Read" with
//
// It replaces raw Read() with "Message.Receive()"
func (w *WebSockWrapper) Read(out []byte) (n int, err error) {
	var data []byte

	if w.mode == WebSocketBinaryMode {
		err = websocket.Message.Receive(w.ws, &data)
	} else {
		var utf8 string
		err = websocket.Message.Receive(w.ws, &utf8)
		switch err {
		case nil:
			data, err = w.decoder.Bytes([]byte(utf8))
		case io.EOF:
			return 0, io.EOF
		default:
			log.Error(err)
		}
	}
	if err != nil {
		return 0, trace.Wrap(err)
	}
	if len(out) < len(data) {
		log.Warningf("websocket failed to receive everything: %d vs %d", len(out), len(data))
	}
	return copy(out, data), nil
}

func (w *WebSockWrapper) Close() error {
	return w.ws.Close()
}

func (w *WebSockWrapper) GetPrefix() []byte {
	w.Lock()
	defer w.Unlock()
	return w.prefix
}

func (w *WebSockWrapper) SetPrefix(p []byte) {
	w.Lock()
	w.prefix = p[:]
	w.Unlock()
}