go.tools/playground/socket: add Native Client wrapper handling

LGTM=adg
R=adg, dave, rsc, minux
CC=golang-codereviews
https://golang.org/cl/77240043

playground/socket/socket.go

@@ -268,7 +268,14 @@ func (p *process) start(body string, opt *Options) error {
 	}
 
 	// run x
-	cmd = p.cmd("", bin)
+	if isNacl() {
+		cmd, err = p.naclCmd(bin)
+		if err != nil {
+			return err
+		}
+	} else {
+		cmd = p.cmd("", bin)
+	}
 	if opt != nil && opt.Race {
 		cmd.Env = append(cmd.Env, "GOMAXPROCS=2")
 	}
@@ -317,6 +324,44 @@ func (p *process) cmd(dir string, args ...string) *exec.Cmd {
 	return cmd
 }
 
+func isNacl() bool {
+	for _, v := range append(Environ(), os.Environ()...) {
+		if v == "GOOS=nacl" {
+			return true
+		}
+	}
+	return false
+}
+
+// naclCmd returns an *exec.Cmd that executes bin under native client.
+func (p *process) naclCmd(bin string) (*exec.Cmd, error) {
+	pwd, err := os.Getwd()
+	if err != nil {
+		return nil, err
+	}
+	var ldr string
+	env := []string{
+		"NACLENV_GOOS=" + runtime.GOOS,
+		"NACLENV_GOROOT=/go",
+		"NACLENV_NACLPWD=" + strings.Replace(pwd, runtime.GOROOT(), "/go", 1),
+	}
+	switch runtime.GOARCH {
+	case "amd64":
+		env = append(env, "NACLENV_GOARCH=amd64")
+		ldr = "sel_ldr_x86_64"
+	case "386":
+		env = append(env, "NACLENV_GOARCH=386")
+		ldr = "sel_ldr_x86_32"
+	default:
+		return nil, errors.New("native client does not support GOARCH=" + runtime.GOARCH)
+	}
+
+	cmd := p.cmd("", ldr, "-l", "/dev/null", "-S", "-e", bin)
+	cmd.Env = append(cmd.Env, env...)
+
+	return cmd, nil
+}
+
 func packageName(body string) (string, error) {
 	f, err := parser.ParseFile(token.NewFileSet(), "prog.go",
 		strings.NewReader(body), parser.PackageClauseOnly)