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// Copyright 2012 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// +build !appengine
// Package socket implements an WebSocket-based playground backend.
// Clients connect to a websocket handler and send run/kill commands, and
// the server sends the output and exit status of the running processes.
// Multiple clients running multiple processes may be served concurrently.
// The wire format is JSON and is described by the Message type.
//
// This will not run on App Engine as WebSockets are not supported there.
package socket
import (
"bytes"
"encoding/json"
"errors"
"go/parser"
"go/token"
"io"
"io/ioutil"
"log"
"os"
"os/exec"
"path/filepath"
"runtime"
"strconv"
"strings"
"sync"
"time"
"unicode/utf8"
"code.google.com/p/go.net/websocket"
)
// Handler implements a WebSocket handler for a client connection.
var Handler = websocket.Handler(socketHandler)
// Environ provides an environment when a binary, such as the go tool, is
// invoked.
var Environ func() []string = os.Environ
const (
// The maximum number of messages to send per session (avoid flooding).
msgLimit = 1000
// Batch messages sent in this interval and send as a single message.
msgDelay = 10 * time.Millisecond
)
// Message is the wire format for the websocket connection to the browser.
// It is used for both sending output messages and receiving commands, as
// distinguished by the Kind field.
type Message struct {
Id string // client-provided unique id for the process
Kind string // in: "run", "kill" out: "stdout", "stderr", "end"
Body string
Options *Options `json:",omitempty"`
}
// Options specify additional message options.
type Options struct {
Race bool // use -race flag when building code (for "run" only)
}
// socketHandler handles the websocket connection for a given present session.
// It handles transcoding Messages to and from JSON format, and starting
// and killing processes.
func socketHandler(c *websocket.Conn) {
in, out := make(chan *Message), make(chan *Message)
errc := make(chan error, 1)
// Decode messages from client and send to the in channel.
go func() {
dec := json.NewDecoder(c)
for {
var m Message
if err := dec.Decode(&m); err != nil {
errc <- err
return
}
in <- &m
}
}()
// Receive messages from the out channel and encode to the client.
go func() {
enc := json.NewEncoder(c)
for m := range out {
if err := enc.Encode(m); err != nil {
errc <- err
return
}
}
}()
// Start and kill processes and handle errors.
proc := make(map[string]*process)
for {
select {
case m := <-in:
switch m.Kind {
case "run":
proc[m.Id].Kill()
lOut := limiter(in, out)
proc[m.Id] = startProcess(m.Id, m.Body, lOut, m.Options)
case "kill":
proc[m.Id].Kill()
}
case err := <-errc:
if err != io.EOF {
// A encode or decode has failed; bail.
log.Println(err)
}
// Shut down any running processes.
for _, p := range proc {
p.Kill()
}
return
}
}
}
// process represents a running process.
type process struct {
id string
out chan<- *Message
done chan struct{} // closed when wait completes
run *exec.Cmd
bin string
}
// startProcess builds and runs the given program, sending its output
// and end event as Messages on the provided channel.
func startProcess(id, body string, out chan<- *Message, opt *Options) *process {
p := &process{
id: id,
out: out,
done: make(chan struct{}),
}
if err := p.start(body, opt); err != nil {
p.end(err)
return nil
}
go p.wait()
return p
}
// Kill stops the process if it is running and waits for it to exit.
func (p *process) Kill() {
if p == nil {
return
}
p.run.Process.Kill()
<-p.done // block until process exits
}
// start builds and starts the given program, sending its output to p.out,
// and stores the running *exec.Cmd in the run field.
func (p *process) start(body string, opt *Options) error {
// We "go build" and then exec the binary so that the
// resultant *exec.Cmd is a handle to the user's program
// (rather than the go tool process).
// This makes Kill work.
bin := filepath.Join(tmpdir, "compile"+strconv.Itoa(<-uniq))
src := bin + ".go"
if runtime.GOOS == "windows" {
bin += ".exe"
}
// write body to x.go
defer os.Remove(src)
err := ioutil.WriteFile(src, []byte(body), 0666)
if err != nil {
return err
}
// build x.go, creating x
p.bin = bin // to be removed by p.end
dir, file := filepath.Split(src)
args := []string{"go", "build", "-tags", "OMIT"}
if opt != nil && opt.Race {
p.out <- &Message{
Id: p.id, Kind: "stderr",
Body: "Running with race detector.\n",
}
args = append(args, "-race")
}
args = append(args, "-o", bin, file)
cmd := p.cmd(dir, args...)
cmd.Stdout = cmd.Stderr // send compiler output to stderr
if err := cmd.Run(); err != nil {
return err
}
// run x
cmd = p.cmd("", bin)
if opt != nil && opt.Race {
cmd.Env = append(cmd.Env, "GOMAXPROCS=2")
}
if err := cmd.Start(); err != nil {
// If we failed to exec, that might be because they built
// a non-main package instead of an executable.
// Check and report that.
if name, err := packageName(body); err == nil && name != "main" {
return errors.New(`executable programs must use "package main"`)
}
return err
}
p.run = cmd
return nil
}
// wait waits for the running process to complete
// and sends its error state to the client.
func (p *process) wait() {
p.end(p.run.Wait())
close(p.done) // unblock waiting Kill calls
}
// end sends an "end" message to the client, containing the process id and the
// given error value. It also removes the binary.
func (p *process) end(err error) {
if p.bin != "" {
defer os.Remove(p.bin)
}
m := &Message{Id: p.id, Kind: "end"}
if err != nil {
m.Body = err.Error()
}
// Wait for any outstanding reads to finish (potential race here).
time.AfterFunc(msgDelay, func() { p.out <- m })
}
// cmd builds an *exec.Cmd that writes its standard output and error to the
// process' output channel.
func (p *process) cmd(dir string, args ...string) *exec.Cmd {
cmd := exec.Command(args[0], args[1:]...)
cmd.Dir = dir
cmd.Env = Environ()
cmd.Stdout = &messageWriter{id: p.id, kind: "stdout", out: p.out}
cmd.Stderr = &messageWriter{id: p.id, kind: "stderr", out: p.out}
return cmd
}
func packageName(body string) (string, error) {
f, err := parser.ParseFile(token.NewFileSet(), "prog.go",
strings.NewReader(body), parser.PackageClauseOnly)
if err != nil {
return "", err
}
return f.Name.String(), nil
}
// messageWriter is an io.Writer that converts all writes to Message sends on
// the out channel with the specified id and kind.
type messageWriter struct {
id, kind string
out chan<- *Message
mu sync.Mutex
buf []byte
send *time.Timer
}
func (w *messageWriter) Write(b []byte) (n int, err error) {
// Buffer writes that occur in a short period to send as one Message.
w.mu.Lock()
w.buf = append(w.buf, b...)
if w.send == nil {
w.send = time.AfterFunc(msgDelay, w.sendNow)
}
w.mu.Unlock()
return len(b), nil
}
func (w *messageWriter) sendNow() {
w.mu.Lock()
body := safeString(w.buf)
w.buf, w.send = nil, nil
w.mu.Unlock()
w.out <- &Message{Id: w.id, Kind: w.kind, Body: body}
}
// safeString returns b as a valid UTF-8 string.
func safeString(b []byte) string {
if utf8.Valid(b) {
return string(b)
}
var buf bytes.Buffer
for len(b) > 0 {
r, size := utf8.DecodeRune(b)
b = b[size:]
buf.WriteRune(r)
}
return buf.String()
}
// limiter returns a channel that wraps dest. Messages sent to the channel are
// sent to dest. After msgLimit Messages have been passed on, a "kill" Message
// is sent to the kill channel, and only "end" messages are passed.
func limiter(kill chan<- *Message, dest chan<- *Message) chan<- *Message {
ch := make(chan *Message)
go func() {
n := 0
for m := range ch {
switch {
case n < msgLimit || m.Kind == "end":
dest <- m
if m.Kind == "end" {
return
}
case n == msgLimit:
// process produced too much output. Kill it.
kill <- &Message{Id: m.Id, Kind: "kill"}
}
n++
}
}()
return ch
}
var tmpdir string
func init() {
// find real path to temporary directory
var err error
tmpdir, err = filepath.EvalSymlinks(os.TempDir())
if err != nil {
log.Fatal(err)
}
}
var uniq = make(chan int) // a source of numbers for naming temporary files
func init() {
go func() {
for i := 0; ; i++ {
uniq <- i
}
}()
}