websocket.go

// Package websocket implements a WebSocket server by executing
// a command and piping its input and output through the WebSocket
// connection.
package websocket

import (
	"bufio"
	"bytes"
	"io"
	"net"
	"net/http"
	"os"
	"os/exec"
	"strings"
	"time"

	"github.com/gorilla/websocket"
	"github.com/mholt/caddy/caddyhttp/httpserver"
)

const (
	// Time allowed to write a message to the peer.
	writeWait = 10 * time.Second

	// Time allowed to read the next pong message from the peer.
	pongWait = 60 * time.Second

	// Send pings to peer with this period. Must be less than pongWait.
	pingPeriod = (pongWait * 9) / 10

	// Maximum message size allowed from peer.
	maxMessageSize = 1024 * 1024 * 10 // 10 MB default.
)

var (
	// GatewayInterface is the dialect of CGI being used by the server
	// to communicate with the script.  See CGI spec, 4.1.4
	GatewayInterface string

	// ServerSoftware is the name and version of the information server
	// software making the CGI request.  See CGI spec, 4.1.17
	ServerSoftware string
)

type (
	// WebSocket is a type that holds configuration for the
	// websocket middleware generally, like a list of all the
	// websocket endpoints.
	WebSocket struct {
		// Next is the next HTTP handler in the chain for when the path doesn't match
		Next httpserver.Handler

		// Sockets holds all the web socket endpoint configurations
		Sockets []Config
	}

	// Config holds the configuration for a single websocket
	// endpoint which may serve multiple websocket connections.
	Config struct {
		Path      string
		Command   string
		Arguments []string
		Respawn   bool // TODO: Not used, but parser supports it until we decide on it
	}
)

// ServeHTTP converts the HTTP request to a WebSocket connection and serves it up.
func (ws WebSocket) ServeHTTP(w http.ResponseWriter, r *http.Request) (int, error) {
	for _, sockconfig := range ws.Sockets {
		if httpserver.Path(r.URL.Path).Matches(sockconfig.Path) {
			return serveWS(w, r, &sockconfig)
		}
	}

	// Didn't match a websocket path, so pass-through
	return ws.Next.ServeHTTP(w, r)
}

// serveWS is used for setting and upgrading the HTTP connection to a websocket connection.
// It also spawns the child process that is associated with matched HTTP path/url.
func serveWS(w http.ResponseWriter, r *http.Request, config *Config) (int, error) {
	upgrader := websocket.Upgrader{
		ReadBufferSize:  1024,
		WriteBufferSize: 1024,
		CheckOrigin:     func(r *http.Request) bool { return true },
	}
	conn, err := upgrader.Upgrade(w, r, nil)
	if err != nil {
		return http.StatusBadRequest, err
	}
	defer conn.Close()

	cmd := exec.Command(config.Command, config.Arguments...)

	stdout, err := cmd.StdoutPipe()
	if err != nil {
		return http.StatusBadGateway, err
	}
	defer stdout.Close()

	stdin, err := cmd.StdinPipe()
	if err != nil {
		return http.StatusBadGateway, err
	}
	defer stdin.Close()

	metavars, err := buildEnv(cmd.Path, r)
	if err != nil {
		return http.StatusBadGateway, err
	}

	cmd.Env = metavars

	if err := cmd.Start(); err != nil {
		return http.StatusBadGateway, err
	}

	done := make(chan struct{})
	go pumpStdout(conn, stdout, done)
	pumpStdin(conn, stdin)

	stdin.Close() // close stdin to end the process

	if err := cmd.Process.Signal(os.Interrupt); err != nil { // signal an interrupt to kill the process
		return http.StatusInternalServerError, err
	}

	select {
	case <-done:
	case <-time.After(time.Second):
		// terminate with extreme prejudice.
		if err := cmd.Process.Signal(os.Kill); err != nil {
			return http.StatusInternalServerError, err
		}
		<-done
	}

	// not sure what we want to do here.
	// status for an "exited" process is greater
	// than 0, but isn't really an error per se.
	// just going to ignore it for now.
	cmd.Wait()

	return 0, nil
}

// buildEnv creates the meta-variables for the child process according
// to the CGI 1.1 specification: http://tools.ietf.org/html/rfc3875#section-4.1
// cmdPath should be the path of the command being run.
// The returned string slice can be set to the command's Env property.
func buildEnv(cmdPath string, r *http.Request) (metavars []string, err error) {
	if !strings.Contains(r.RemoteAddr, ":") {
		r.RemoteAddr += ":"
	}
	remoteHost, remotePort, err := net.SplitHostPort(r.RemoteAddr)
	if err != nil {
		return
	}

	if !strings.Contains(r.Host, ":") {
		r.Host += ":"
	}
	serverHost, serverPort, err := net.SplitHostPort(r.Host)
	if err != nil {
		return
	}

	metavars = []string{
		`AUTH_TYPE=`,      // Not used
		`CONTENT_LENGTH=`, // Not used
		`CONTENT_TYPE=`,   // Not used
		`GATEWAY_INTERFACE=` + GatewayInterface,
		`PATH_INFO=`,       // TODO
		`PATH_TRANSLATED=`, // TODO
		`QUERY_STRING=` + r.URL.RawQuery,
		`REMOTE_ADDR=` + remoteHost,
		`REMOTE_HOST=` + remoteHost, // Host lookups are slow - don't do them
		`REMOTE_IDENT=`,             // Not used
		`REMOTE_PORT=` + remotePort,
		`REMOTE_USER=`, // Not used,
		`REQUEST_METHOD=` + r.Method,
		`REQUEST_URI=` + r.RequestURI,
		`SCRIPT_NAME=` + cmdPath, // path of the program being executed
		`SERVER_NAME=` + serverHost,
		`SERVER_PORT=` + serverPort,
		`SERVER_PROTOCOL=` + r.Proto,
		`SERVER_SOFTWARE=` + ServerSoftware,
	}

	// Add each HTTP header to the environment as well
	for header, values := range r.Header {
		value := strings.Join(values, ", ")
		header = strings.ToUpper(header)
		header = strings.Replace(header, "-", "_", -1)
		value = strings.Replace(value, "\n", " ", -1)
		metavars = append(metavars, "HTTP_"+header+"="+value)
	}

	return
}

// pumpStdin handles reading data from the websocket connection and writing
// it to stdin of the process.
func pumpStdin(conn *websocket.Conn, stdin io.WriteCloser) {
	// Setup our connection's websocket ping/pong handlers from our const values.
	defer conn.Close()
	conn.SetReadLimit(maxMessageSize)
	conn.SetReadDeadline(time.Now().Add(pongWait))
	conn.SetPongHandler(func(string) error { conn.SetReadDeadline(time.Now().Add(pongWait)); return nil })
	for {
		_, message, err := conn.ReadMessage()
		if err != nil {
			break
		}
		message = append(message, '\n')
		if _, err := stdin.Write(message); err != nil {
			break
		}
	}
}

// pumpStdout handles reading data from stdout of the process and writing
// it to websocket connection.
func pumpStdout(conn *websocket.Conn, stdout io.Reader, done chan struct{}) {
	go pinger(conn, done)
	defer func() {
		conn.Close()
		close(done) // make sure to close the pinger when we are done.
	}()

	s := bufio.NewScanner(stdout)
	for s.Scan() {
		conn.SetWriteDeadline(time.Now().Add(writeWait))
		if err := conn.WriteMessage(websocket.TextMessage, bytes.TrimSpace(s.Bytes())); err != nil {
			break
		}
	}
	if s.Err() != nil {
		conn.WriteControl(websocket.CloseMessage, websocket.FormatCloseMessage(websocket.CloseGoingAway, s.Err().Error()), time.Time{})
	}
}

// pinger simulates the websocket to keep it alive with ping messages.
func pinger(conn *websocket.Conn, done chan struct{}) {
	ticker := time.NewTicker(pingPeriod)
	defer ticker.Stop()

	for { // blocking loop with select to wait for stimulation.
		select {
		case <-ticker.C:
			if err := conn.WriteControl(websocket.PingMessage, []byte{}, time.Now().Add(writeWait)); err != nil {
				conn.WriteControl(websocket.CloseMessage, websocket.FormatCloseMessage(websocket.CloseGoingAway, err.Error()), time.Time{})
				return
			}
		case <-done:
			return // clean up this routine.
		}
	}
}