gosh/cmd.go - release.go.x.lib - Git at Google

 // Copyright 2015 The Vanadium 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 gosh

 import (
 	"bytes"
 	"encoding/json"
 	"errors"
 	"fmt"
 	"io"
 	"os"
 	"os/exec"
 	"path/filepath"
 	"strings"
 	"sync"
 	"time"
 )

 var (
 	errAlreadyCalledStart = errors.New("gosh: already called Cmd.Start")
 	errAlreadyCalledWait  = errors.New("gosh: already called Cmd.Wait")
 	errCloseStdout        = errors.New("gosh: use NopWriteCloser(os.Stdout) to prevent stdout from being closed")
 	errCloseStderr        = errors.New("gosh: use NopWriteCloser(os.Stderr) to prevent stderr from being closed")
 	errDidNotCallStart    = errors.New("gosh: did not call Cmd.Start")
 	errProcessExited      = errors.New("gosh: process exited")
 )

 // Cmd represents a command. Not thread-safe.
 // Public fields should not be modified after calling Start.
 type Cmd struct {
 	// Err is the most recent error from this Cmd (may be nil).
 	Err error
 	// Path is the path of the command to run.
 	Path string
 	// Vars is the map of env vars for this Cmd.
 	Vars map[string]string
 	// Args is the list of args for this Cmd.
 	Args []string
 	// PropagateOutput is inherited from Shell.Opts.PropagateChildOutput.
 	PropagateOutput bool
 	// OutputDir is inherited from Shell.Opts.ChildOutputDir.
 	OutputDir string
 	// ExitErrorIsOk specifies whether an *exec.ExitError should be reported via
 	// Shell.HandleError.
 	ExitErrorIsOk bool
 	// Stdin is a string to write to the child's stdin.
 	Stdin string
 	// Internal state.
 	sh               *Shell
 	c                *exec.Cmd
 	stdinWriteCloser io.WriteCloser // from exec.Cmd.StdinPipe
 	calledStart      bool
 	calledWait       bool
 	cond             *sync.Cond
 	waitChan         chan error
 	started          bool // protected by sh.cleanupMu
 	exited           bool // protected by cond.L
 	stdoutWriters    []io.Writer
 	stderrWriters    []io.Writer
 	closers          []io.Closer
 	recvReady        bool              // protected by cond.L
 	recvVars         map[string]string // protected by cond.L
 }

 // Clone returns a new Cmd with a copy of this Cmd's configuration.
 func (c *Cmd) Clone() *Cmd {
 	c.sh.Ok()
 	res, err := c.clone()
 	c.handleError(err)
 	return res
 }

 // StdinPipe returns a thread-safe WriteCloser backed by a buffered pipe for the
 // command's stdin. The returned pipe will be closed when the process exits, but
 // may also be closed earlier by the caller, e.g. if the command does not exit
 // until its stdin is closed. Must be called before Start. It is safe to call
 // StdinPipe multiple times; calls after the first return the pipe created by
 // the first call.
 func (c *Cmd) StdinPipe() io.WriteCloser {
 	c.sh.Ok()
 	res, err := c.stdinPipe()
 	c.handleError(err)
 	return res
 }

 // StdoutPipe returns a Reader backed by a buffered pipe for the command's
 // stdout. Must be called before Start. May be called more than once; each
 // invocation creates a new pipe.
 func (c *Cmd) StdoutPipe() io.Reader {
 	c.sh.Ok()
 	res, err := c.stdoutPipe()
 	c.handleError(err)
 	return res
 }

 // StderrPipe returns a Reader backed by a buffered pipe for the command's
 // stderr. Must be called before Start. May be called more than once; each
 // invocation creates a new pipe.
 func (c *Cmd) StderrPipe() io.Reader {
 	c.sh.Ok()
 	res, err := c.stderrPipe()
 	c.handleError(err)
 	return res
 }

 // AddStdoutWriter configures this Cmd to tee the child's stdout to the given
 // wc, which will be closed when the process exits.
 //
 // Use NopWriteCloser to extend an io.Writer to io.WriteCloser, or to prevent an
 // existing io.WriteCloser from being closed.  It is an error to pass in
 // os.Stdout or os.Stderr, since they shouldn't be closed.
 func (c *Cmd) AddStdoutWriter(wc io.WriteCloser) {
 	c.sh.Ok()
 	c.handleError(c.addStdoutWriter(wc))
 }

 // AddStderrWriter configures this Cmd to tee the child's stderr to the given
 // wc, which will be closed when the process exits.
 //
 // Use NopWriteCloser to extend an io.Writer to io.WriteCloser, or to prevent an
 // existing io.WriteCloser from being closed.  It is an error to pass in
 // os.Stdout or os.Stderr, since they shouldn't be closed.
 func (c *Cmd) AddStderrWriter(wc io.WriteCloser) {
 	c.sh.Ok()
 	c.handleError(c.addStderrWriter(wc))
 }

 // Start starts the command.
 func (c *Cmd) Start() {
 	c.sh.Ok()
 	c.handleError(c.start())
 }

 // AwaitReady waits for the child process to call SendReady. Must not be called
 // before Start or after Wait.
 func (c *Cmd) AwaitReady() {
 	c.sh.Ok()
 	c.handleError(c.awaitReady())
 }

 // AwaitVars waits for the child process to send values for the given vars
 // (using SendVars). Must not be called before Start or after Wait.
 func (c *Cmd) AwaitVars(keys ...string) map[string]string {
 	c.sh.Ok()
 	res, err := c.awaitVars(keys...)
 	c.handleError(err)
 	return res
 }

 // Wait waits for the command to exit.
 func (c *Cmd) Wait() {
 	c.sh.Ok()
 	c.handleError(c.wait())
 }

 // Kill causes the process to exit immediately.
 func (c *Cmd) Kill() {
 	c.sh.Ok()
 	c.handleError(c.kill())
 }

 // Signal sends a signal to the process.
 func (c *Cmd) Signal(sig os.Signal) {
 	c.sh.Ok()
 	c.handleError(c.signal(sig))
 }

 // Shutdown sends a signal to the process, then waits for it to exit.
 func (c *Cmd) Shutdown(sig os.Signal) {
 	c.sh.Ok()
 	c.handleError(c.shutdown(sig))
 }

 // Run calls Start followed by Wait.
 func (c *Cmd) Run() {
 	c.sh.Ok()
 	c.handleError(c.run())
 }

 // Stdout calls Start followed by Wait, then returns the command's stdout.
 func (c *Cmd) Stdout() string {
 	c.sh.Ok()
 	res, err := c.stdout()
 	c.handleError(err)
 	return res
 }

 // StdoutStderr calls Start followed by Wait, then returns the command's stdout
 // and stderr.
 func (c *Cmd) StdoutStderr() (string, string) {
 	c.sh.Ok()
 	stdout, stderr, err := c.stdoutStderr()
 	c.handleError(err)
 	return stdout, stderr
 }

 // Pid returns the command's PID, or -1 if the command has not been started.
 func (c *Cmd) Pid() int {
 	if !c.started {
 		return -1
 	}
 	return c.c.Process.Pid
 }

 ////////////////////////////////////////
 // Internals

 func newCmdInternal(sh *Shell, vars map[string]string, path string, args []string) (*Cmd, error) {
 	c := &Cmd{
 		Path:     path,
 		Vars:     vars,
 		Args:     args,
 		sh:       sh,
 		c:        &exec.Cmd{},
 		cond:     sync.NewCond(&sync.Mutex{}),
 		waitChan: make(chan error, 1),
 		recvVars: map[string]string{},
 	}
 	// Protect against concurrent signal-triggered Shell.cleanup().
 	sh.cleanupMu.Lock()
 	defer sh.cleanupMu.Unlock()
 	if sh.calledCleanup {
 		return nil, errAlreadyCalledCleanup
 	}
 	sh.cmds = append(sh.cmds, c)
 	return c, nil
 }

 func newCmd(sh *Shell, vars map[string]string, name string, args ...string) (*Cmd, error) {
 	// Mimics https://golang.org/src/os/exec/exec.go Command.
 	if filepath.Base(name) == name {
 		if lp, err := exec.LookPath(name); err != nil {
 			return nil, err
 		} else {
 			name = lp
 		}
 	}
 	return newCmdInternal(sh, vars, name, args)
 }

 func (c *Cmd) errorIsOk(err error) bool {
 	if c.ExitErrorIsOk {
 		if _, ok := err.(*exec.ExitError); ok {
 			return true
 		}
 	}
 	return err == nil
 }

 func (c *Cmd) handleError(err error) {
 	c.Err = err
 	if !c.errorIsOk(err) {
 		c.sh.HandleError(err)
 	}
 }

 func (c *Cmd) addWriter(writers *[]io.Writer, w io.Writer) {
 	*writers = append(*writers, w)
 }

 func (c *Cmd) closeClosers() {
 	for _, c := range c.closers {
 		c.Close() // best-effort; ignore returned error
 	}
 }

 func (c *Cmd) isRunning() bool {
 	if !c.started {
 		return false
 	}
 	c.cond.L.Lock()
 	defer c.cond.L.Unlock()
 	return !c.exited
 }

 // recvWriter listens for gosh messages from a child process.
 type recvWriter struct {
 	c          *Cmd
 	buf        bytes.Buffer
 	readPrefix bool // if true, we've read len(msgPrefix) for the current line
 	skipLine   bool // if true, ignore bytes until next '\n'
 }

 func (w *recvWriter) Write(p []byte) (n int, err error) {
 	for _, b := range p {
 		if b == '\n' {
 			if w.readPrefix && !w.skipLine {
 				m := msg{}
 				if err := json.Unmarshal(w.buf.Bytes(), &m); err != nil {
 					return 0, err
 				}
 				switch m.Type {
 				case typeReady:
 					w.c.cond.L.Lock()
 					w.c.recvReady = true
 					w.c.cond.Signal()
 					w.c.cond.L.Unlock()
 				case typeVars:
 					w.c.cond.L.Lock()
 					w.c.recvVars = mergeMaps(w.c.recvVars, m.Vars)
 					w.c.cond.Signal()
 					w.c.cond.L.Unlock()
 				default:
 					return 0, fmt.Errorf("unknown message type: %q", m.Type)
 				}
 			}
 			// Reset state for next line.
 			w.readPrefix, w.skipLine = false, false
 			w.buf.Reset()
 		} else if !w.skipLine {
 			w.buf.WriteByte(b)
 			if !w.readPrefix && w.buf.Len() == len(msgPrefix) {
 				w.readPrefix = true
 				prefix := string(w.buf.Next(len(msgPrefix)))
 				if prefix != msgPrefix {
 					w.skipLine = true
 				}
 			}
 		}
 	}
 	return len(p), nil
 }

 func (c *Cmd) makeMultiWriter(stdout bool, t string) (io.Writer, error) {
 	std, writers := os.Stderr, &c.stderrWriters
 	if stdout {
 		std, writers = os.Stdout, &c.stdoutWriters
 		c.addWriter(writers, &recvWriter{c: c})
 	}
 	if c.PropagateOutput {
 		c.addWriter(writers, std)
 		// Don't add std to c.closers, since we don't want to close os.Stdout or
 		// os.Stderr for the entire address space when c finishes.
 	}
 	if c.OutputDir != "" {
 		suffix := "stderr"
 		if stdout {
 			suffix = "stdout"
 		}
 		name := filepath.Join(c.OutputDir, filepath.Base(c.Path)+"."+t+"."+suffix)
 		file, err := os.OpenFile(name, os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0600)
 		if err != nil {
 			return nil, err
 		}
 		c.addWriter(writers, file)
 		c.closers = append(c.closers, file)
 	}
 	return io.MultiWriter(*writers...), nil
 }

 func (c *Cmd) clone() (*Cmd, error) {
 	vars := make(map[string]string, len(c.Vars))
 	for k, v := range c.Vars {
 		vars[k] = v
 	}
 	args := make([]string, len(c.Args))
 	copy(args, c.Args)
 	res, err := newCmdInternal(c.sh, vars, c.Path, args)
 	if err != nil {
 		return nil, err
 	}
 	res.PropagateOutput = c.PropagateOutput
 	res.OutputDir = c.OutputDir
 	res.ExitErrorIsOk = c.ExitErrorIsOk
 	res.Stdin = c.Stdin
 	return res, nil
 }

 func (c *Cmd) stdinPipe() (io.WriteCloser, error) {
 	if c.calledStart {
 		return nil, errAlreadyCalledStart
 	}
 	if c.stdinWriteCloser != nil {
 		return c.stdinWriteCloser, nil
 	}
 	var err error
 	c.stdinWriteCloser, err = c.c.StdinPipe()
 	return c.stdinWriteCloser, err
 }

 func (c *Cmd) stdoutPipe() (io.Reader, error) {
 	if c.calledStart {
 		return nil, errAlreadyCalledStart
 	}
 	p := NewBufferedPipe()
 	c.addWriter(&c.stdoutWriters, p)
 	c.closers = append(c.closers, p)
 	return p, nil
 }

 func (c *Cmd) stderrPipe() (io.Reader, error) {
 	if c.calledStart {
 		return nil, errAlreadyCalledStart
 	}
 	p := NewBufferedPipe()
 	c.addWriter(&c.stderrWriters, p)
 	c.closers = append(c.closers, p)
 	return p, nil
 }

 func (c *Cmd) addStdoutWriter(wc io.WriteCloser) error {
 	switch {
 	case c.calledStart:
 		return errAlreadyCalledStart
 	case wc == os.Stdout:
 		return errCloseStdout
 	case wc == os.Stderr:
 		return errCloseStderr
 	}
 	c.addWriter(&c.stdoutWriters, wc)
 	c.closers = append(c.closers, wc)
 	return nil
 }

 func (c *Cmd) addStderrWriter(wc io.WriteCloser) error {
 	switch {
 	case c.calledStart:
 		return errAlreadyCalledStart
 	case wc == os.Stdout:
 		return errCloseStdout
 	case wc == os.Stderr:
 		return errCloseStderr
 	}
 	c.addWriter(&c.stderrWriters, wc)
 	c.closers = append(c.closers, wc)
 	return nil
 }

 // TODO(sadovsky): Maybe wrap every child process with a "supervisor" process
 // that calls WatchParent().

 func (c *Cmd) start() error {
 	if c.calledStart {
 		return errAlreadyCalledStart
 	}
 	c.calledStart = true
 	// Protect against Cmd.start() writing to c.c.Process concurrently with
 	// signal-triggered Shell.cleanup() reading from it.
 	c.sh.cleanupMu.Lock()
 	defer c.sh.cleanupMu.Unlock()
 	if c.sh.calledCleanup {
 		return errAlreadyCalledCleanup
 	}
 	// Configure the command.
 	c.c.Path = c.Path
 	c.c.Env = mapToSlice(c.Vars)
 	c.c.Args = append([]string{c.Path}, c.Args...)
 	if c.Stdin != "" {
 		if c.stdinWriteCloser != nil {
 			return errors.New("gosh: cannot both set Stdin and call StdinPipe")
 		}
 		c.c.Stdin = strings.NewReader(c.Stdin)
 	}
 	t := time.Now().Format("20060102.150405.000000")
 	var err error
 	if c.c.Stdout, err = c.makeMultiWriter(true, t); err != nil {
 		return err
 	}
 	if c.c.Stderr, err = c.makeMultiWriter(false, t); err != nil {
 		return err
 	}
 	// Start the command.
 	onExit := func(err error) {
 		c.cond.L.Lock()
 		c.exited = true
 		c.cond.Signal()
 		c.cond.L.Unlock()
 		c.closeClosers()
 		c.waitChan <- err
 	}
 	if err = c.c.Start(); err != nil {
 		onExit(errors.New("gosh: start failed"))
 		return err
 	}
 	c.started = true
 	// Spawn a "waiter" goroutine that calls exec.Cmd.Wait and thus waits for the
 	// process to exit. Calling exec.Cmd.Wait here rather than in gosh.Cmd.Wait
 	// ensures that the child process is reaped once it exits. Note, gosh.Cmd.wait
 	// blocks on waitChan.
 	go func() {
 		onExit(c.c.Wait())
 	}()
 	return nil
 }

 // TODO(sadovsky): Maybe add optional timeouts for
 // Cmd.{awaitReady,awaitVars,wait}.

 func (c *Cmd) awaitReady() error {
 	if !c.started {
 		return errDidNotCallStart
 	} else if c.calledWait {
 		return errAlreadyCalledWait
 	}
 	c.cond.L.Lock()
 	defer c.cond.L.Unlock()
 	for !c.exited && !c.recvReady {
 		c.cond.Wait()
 	}
 	// Return nil error if both conditions triggered simultaneously.
 	if !c.recvReady {
 		return errProcessExited
 	}
 	return nil
 }

 func (c *Cmd) awaitVars(keys ...string) (map[string]string, error) {
 	if !c.started {
 		return nil, errDidNotCallStart
 	} else if c.calledWait {
 		return nil, errAlreadyCalledWait
 	}
 	wantKeys := map[string]bool{}
 	for _, key := range keys {
 		wantKeys[key] = true
 	}
 	res := map[string]string{}
 	updateRes := func() {
 		for k, v := range c.recvVars {
 			if _, ok := wantKeys[k]; ok {
 				res[k] = v
 			}
 		}
 	}
 	c.cond.L.Lock()
 	defer c.cond.L.Unlock()
 	updateRes()
 	for !c.exited && len(res) < len(wantKeys) {
 		c.cond.Wait()
 		updateRes()
 	}
 	// Return nil error if both conditions triggered simultaneously.
 	if len(res) < len(wantKeys) {
 		return nil, errProcessExited
 	}
 	return res, nil
 }

 func (c *Cmd) wait() error {
 	if !c.started {
 		return errDidNotCallStart
 	} else if c.calledWait {
 		return errAlreadyCalledWait
 	}
 	c.calledWait = true
 	return <-c.waitChan
 }

 // Note: We check for this particular error message to handle the unavoidable
 // race between sending a signal to a process and the process exiting.
 // https://golang.org/src/os/exec_unix.go
 // https://golang.org/src/os/exec_windows.go
 const errFinished = "os: process already finished"

 func (c *Cmd) kill() error {
 	if !c.started {
 		return errDidNotCallStart
 	}
 	if !c.isRunning() {
 		return nil
 	}
 	if err := c.c.Process.Kill(); err != nil && err.Error() != errFinished {
 		return err
 	}
 	return nil
 }

 func (c *Cmd) signal(sig os.Signal) error {
 	if !c.started {
 		return errDidNotCallStart
 	}
 	if !c.isRunning() {
 		return nil
 	}
 	if err := c.c.Process.Signal(sig); err != nil && err.Error() != errFinished {
 		return err
 	}
 	return nil
 }

 func (c *Cmd) shutdown(sig os.Signal) error {
 	if err := c.signal(sig); err != nil {
 		return err
 	}
 	if err := c.wait(); err != nil {
 		if _, ok := err.(*exec.ExitError); !ok {
 			return err
 		}
 	}
 	return nil
 }

 func (c *Cmd) run() error {
 	if err := c.start(); err != nil {
 		return err
 	}
 	return c.wait()
 }

 func (c *Cmd) stdout() (string, error) {
 	if c.calledStart {
 		return "", errAlreadyCalledStart
 	}
 	var stdout bytes.Buffer
 	c.addWriter(&c.stdoutWriters, &stdout)
 	err := c.run()
 	return stdout.String(), err
 }

 func (c *Cmd) stdoutStderr() (string, string, error) {
 	if c.calledStart {
 		return "", "", errAlreadyCalledStart
 	}
 	var stdout, stderr bytes.Buffer
 	c.addWriter(&c.stdoutWriters, &stdout)
 	c.addWriter(&c.stderrWriters, &stderr)
 	err := c.run()
 	return stdout.String(), stderr.String(), err
 }
	// Copyright 2015 The Vanadium 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 gosh

	import (
	"bytes"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"os"
	"os/exec"
	"path/filepath"
	"strings"
	"sync"
	"time"
	)

	var (
	errAlreadyCalledStart = errors.New("gosh: already called Cmd.Start")
	errAlreadyCalledWait = errors.New("gosh: already called Cmd.Wait")
	errCloseStdout = errors.New("gosh: use NopWriteCloser(os.Stdout) to prevent stdout from being closed")
	errCloseStderr = errors.New("gosh: use NopWriteCloser(os.Stderr) to prevent stderr from being closed")
	errDidNotCallStart = errors.New("gosh: did not call Cmd.Start")
	errProcessExited = errors.New("gosh: process exited")
	)

	// Cmd represents a command. Not thread-safe.
	// Public fields should not be modified after calling Start.
	type Cmd struct {
	// Err is the most recent error from this Cmd (may be nil).
	Err error
	// Path is the path of the command to run.
	Path string
	// Vars is the map of env vars for this Cmd.
	Vars map[string]string
	// Args is the list of args for this Cmd.
	Args []string
	// PropagateOutput is inherited from Shell.Opts.PropagateChildOutput.
	PropagateOutput bool
	// OutputDir is inherited from Shell.Opts.ChildOutputDir.
	OutputDir string
	// ExitErrorIsOk specifies whether an *exec.ExitError should be reported via
	// Shell.HandleError.
	ExitErrorIsOk bool
	// Stdin is a string to write to the child's stdin.
	Stdin string
	// Internal state.
	sh *Shell
	c *exec.Cmd
	stdinWriteCloser io.WriteCloser // from exec.Cmd.StdinPipe
	calledStart bool
	calledWait bool
	cond *sync.Cond
	waitChan chan error
	started bool // protected by sh.cleanupMu
	exited bool // protected by cond.L
	stdoutWriters []io.Writer
	stderrWriters []io.Writer
	closers []io.Closer
	recvReady bool // protected by cond.L
	recvVars map[string]string // protected by cond.L
	}

	// Clone returns a new Cmd with a copy of this Cmd's configuration.
	func (c Cmd) Clone() Cmd {
	c.sh.Ok()
	res, err := c.clone()
	c.handleError(err)
	return res
	}

	// StdinPipe returns a thread-safe WriteCloser backed by a buffered pipe for the
	// command's stdin. The returned pipe will be closed when the process exits, but
	// may also be closed earlier by the caller, e.g. if the command does not exit
	// until its stdin is closed. Must be called before Start. It is safe to call
	// StdinPipe multiple times; calls after the first return the pipe created by
	// the first call.
	func (c *Cmd) StdinPipe() io.WriteCloser {
	c.sh.Ok()
	res, err := c.stdinPipe()
	c.handleError(err)
	return res
	}

	// StdoutPipe returns a Reader backed by a buffered pipe for the command's
	// stdout. Must be called before Start. May be called more than once; each
	// invocation creates a new pipe.
	func (c *Cmd) StdoutPipe() io.Reader {
	c.sh.Ok()
	res, err := c.stdoutPipe()
	c.handleError(err)
	return res
	}

	// StderrPipe returns a Reader backed by a buffered pipe for the command's
	// stderr. Must be called before Start. May be called more than once; each
	// invocation creates a new pipe.
	func (c *Cmd) StderrPipe() io.Reader {
	c.sh.Ok()
	res, err := c.stderrPipe()
	c.handleError(err)
	return res
	}

	// AddStdoutWriter configures this Cmd to tee the child's stdout to the given
	// wc, which will be closed when the process exits.
	//
	// Use NopWriteCloser to extend an io.Writer to io.WriteCloser, or to prevent an
	// existing io.WriteCloser from being closed. It is an error to pass in
	// os.Stdout or os.Stderr, since they shouldn't be closed.
	func (c *Cmd) AddStdoutWriter(wc io.WriteCloser) {
	c.sh.Ok()
	c.handleError(c.addStdoutWriter(wc))
	}

	// AddStderrWriter configures this Cmd to tee the child's stderr to the given
	// wc, which will be closed when the process exits.
	//
	// Use NopWriteCloser to extend an io.Writer to io.WriteCloser, or to prevent an
	// existing io.WriteCloser from being closed. It is an error to pass in
	// os.Stdout or os.Stderr, since they shouldn't be closed.
	func (c *Cmd) AddStderrWriter(wc io.WriteCloser) {
	c.sh.Ok()
	c.handleError(c.addStderrWriter(wc))
	}

	// Start starts the command.
	func (c *Cmd) Start() {
	c.sh.Ok()
	c.handleError(c.start())
	}

	// AwaitReady waits for the child process to call SendReady. Must not be called
	// before Start or after Wait.
	func (c *Cmd) AwaitReady() {
	c.sh.Ok()
	c.handleError(c.awaitReady())
	}

	// AwaitVars waits for the child process to send values for the given vars
	// (using SendVars). Must not be called before Start or after Wait.
	func (c *Cmd) AwaitVars(keys ...string) map[string]string {
	c.sh.Ok()
	res, err := c.awaitVars(keys...)
	c.handleError(err)
	return res
	}

	// Wait waits for the command to exit.
	func (c *Cmd) Wait() {
	c.sh.Ok()
	c.handleError(c.wait())
	}

	// Kill causes the process to exit immediately.
	func (c *Cmd) Kill() {
	c.sh.Ok()
	c.handleError(c.kill())
	}

	// Signal sends a signal to the process.
	func (c *Cmd) Signal(sig os.Signal) {
	c.sh.Ok()
	c.handleError(c.signal(sig))
	}

	// Shutdown sends a signal to the process, then waits for it to exit.
	func (c *Cmd) Shutdown(sig os.Signal) {
	c.sh.Ok()
	c.handleError(c.shutdown(sig))
	}

	// Run calls Start followed by Wait.
	func (c *Cmd) Run() {
	c.sh.Ok()
	c.handleError(c.run())
	}

	// Stdout calls Start followed by Wait, then returns the command's stdout.
	func (c *Cmd) Stdout() string {
	c.sh.Ok()
	res, err := c.stdout()
	c.handleError(err)
	return res
	}

	// StdoutStderr calls Start followed by Wait, then returns the command's stdout
	// and stderr.
	func (c *Cmd) StdoutStderr() (string, string) {
	c.sh.Ok()
	stdout, stderr, err := c.stdoutStderr()
	c.handleError(err)
	return stdout, stderr
	}

	// Pid returns the command's PID, or -1 if the command has not been started.
	func (c *Cmd) Pid() int {
	if !c.started {
	return -1
	}
	return c.c.Process.Pid
	}

	////////////////////////////////////////
	// Internals

	func newCmdInternal(sh Shell, vars map[string]string, path string, args []string) (Cmd, error) {
	c := &Cmd{
	Path: path,
	Vars: vars,
	Args: args,
	sh: sh,
	c: &exec.Cmd{},
	cond: sync.NewCond(&sync.Mutex{}),
	waitChan: make(chan error, 1),
	recvVars: map[string]string{},
	}
	// Protect against concurrent signal-triggered Shell.cleanup().
	sh.cleanupMu.Lock()
	defer sh.cleanupMu.Unlock()
	if sh.calledCleanup {
	return nil, errAlreadyCalledCleanup
	}
	sh.cmds = append(sh.cmds, c)
	return c, nil
	}

	func newCmd(sh Shell, vars map[string]string, name string, args ...string) (Cmd, error) {
	// Mimics https://golang.org/src/os/exec/exec.go Command.
	if filepath.Base(name) == name {
	if lp, err := exec.LookPath(name); err != nil {
	return nil, err
	} else {
	name = lp
	}
	}
	return newCmdInternal(sh, vars, name, args)
	}

	func (c *Cmd) errorIsOk(err error) bool {
	if c.ExitErrorIsOk {
	if _, ok := err.(*exec.ExitError); ok {
	return true
	}
	}
	return err == nil
	}

	func (c *Cmd) handleError(err error) {
	c.Err = err
	if !c.errorIsOk(err) {
	c.sh.HandleError(err)
	}
	}

	func (c Cmd) addWriter(writers []io.Writer, w io.Writer) {
	writers = append(writers, w)
	}

	func (c *Cmd) closeClosers() {
	for _, c := range c.closers {
	c.Close() // best-effort; ignore returned error
	}
	}

	func (c *Cmd) isRunning() bool {
	if !c.started {
	return false
	}
	c.cond.L.Lock()
	defer c.cond.L.Unlock()
	return !c.exited
	}

	// recvWriter listens for gosh messages from a child process.
	type recvWriter struct {
	c *Cmd
	buf bytes.Buffer
	readPrefix bool // if true, we've read len(msgPrefix) for the current line
	skipLine bool // if true, ignore bytes until next '\n'
	}

	func (w *recvWriter) Write(p []byte) (n int, err error) {
	for _, b := range p {
	if b == '\n' {
	if w.readPrefix && !w.skipLine {
	m := msg{}
	if err := json.Unmarshal(w.buf.Bytes(), &m); err != nil {
	return 0, err
	}
	switch m.Type {
	case typeReady:
	w.c.cond.L.Lock()
	w.c.recvReady = true
	w.c.cond.Signal()
	w.c.cond.L.Unlock()
	case typeVars:
	w.c.cond.L.Lock()
	w.c.recvVars = mergeMaps(w.c.recvVars, m.Vars)
	w.c.cond.Signal()
	w.c.cond.L.Unlock()
	default:
	return 0, fmt.Errorf("unknown message type: %q", m.Type)
	}
	}
	// Reset state for next line.
	w.readPrefix, w.skipLine = false, false
	w.buf.Reset()
	} else if !w.skipLine {
	w.buf.WriteByte(b)
	if !w.readPrefix && w.buf.Len() == len(msgPrefix) {
	w.readPrefix = true
	prefix := string(w.buf.Next(len(msgPrefix)))
	if prefix != msgPrefix {
	w.skipLine = true
	}
	}
	}
	}
	return len(p), nil
	}

	func (c *Cmd) makeMultiWriter(stdout bool, t string) (io.Writer, error) {
	std, writers := os.Stderr, &c.stderrWriters
	if stdout {
	std, writers = os.Stdout, &c.stdoutWriters
	c.addWriter(writers, &recvWriter{c: c})
	}
	if c.PropagateOutput {
	c.addWriter(writers, std)
	// Don't add std to c.closers, since we don't want to close os.Stdout or
	// os.Stderr for the entire address space when c finishes.
	}
	if c.OutputDir != "" {
	suffix := "stderr"
	if stdout {
	suffix = "stdout"
	}
	name := filepath.Join(c.OutputDir, filepath.Base(c.Path)+"."+t+"."+suffix)
	file, err := os.OpenFile(name, os.O_WRONLY\|os.O_CREATE\|os.O_EXCL, 0600)
	if err != nil {
	return nil, err
	}
	c.addWriter(writers, file)
	c.closers = append(c.closers, file)
	}
	return io.MultiWriter(*writers...), nil
	}

	func (c Cmd) clone() (Cmd, error) {
	vars := make(map[string]string, len(c.Vars))
	for k, v := range c.Vars {
	vars[k] = v
	}
	args := make([]string, len(c.Args))
	copy(args, c.Args)
	res, err := newCmdInternal(c.sh, vars, c.Path, args)
	if err != nil {
	return nil, err
	}
	res.PropagateOutput = c.PropagateOutput
	res.OutputDir = c.OutputDir
	res.ExitErrorIsOk = c.ExitErrorIsOk
	res.Stdin = c.Stdin
	return res, nil
	}

	func (c *Cmd) stdinPipe() (io.WriteCloser, error) {
	if c.calledStart {
	return nil, errAlreadyCalledStart
	}
	if c.stdinWriteCloser != nil {
	return c.stdinWriteCloser, nil
	}
	var err error
	c.stdinWriteCloser, err = c.c.StdinPipe()
	return c.stdinWriteCloser, err
	}

	func (c *Cmd) stdoutPipe() (io.Reader, error) {
	if c.calledStart {
	return nil, errAlreadyCalledStart
	}
	p := NewBufferedPipe()
	c.addWriter(&c.stdoutWriters, p)
	c.closers = append(c.closers, p)
	return p, nil
	}

	func (c *Cmd) stderrPipe() (io.Reader, error) {
	if c.calledStart {
	return nil, errAlreadyCalledStart
	}
	p := NewBufferedPipe()
	c.addWriter(&c.stderrWriters, p)
	c.closers = append(c.closers, p)
	return p, nil
	}

	func (c *Cmd) addStdoutWriter(wc io.WriteCloser) error {
	switch {
	case c.calledStart:
	return errAlreadyCalledStart
	case wc == os.Stdout:
	return errCloseStdout
	case wc == os.Stderr:
	return errCloseStderr
	}
	c.addWriter(&c.stdoutWriters, wc)
	c.closers = append(c.closers, wc)
	return nil
	}

	func (c *Cmd) addStderrWriter(wc io.WriteCloser) error {
	switch {
	case c.calledStart:
	return errAlreadyCalledStart
	case wc == os.Stdout:
	return errCloseStdout
	case wc == os.Stderr:
	return errCloseStderr
	}
	c.addWriter(&c.stderrWriters, wc)
	c.closers = append(c.closers, wc)
	return nil
	}

	// TODO(sadovsky): Maybe wrap every child process with a "supervisor" process
	// that calls WatchParent().

	func (c *Cmd) start() error {
	if c.calledStart {
	return errAlreadyCalledStart
	}
	c.calledStart = true
	// Protect against Cmd.start() writing to c.c.Process concurrently with
	// signal-triggered Shell.cleanup() reading from it.
	c.sh.cleanupMu.Lock()
	defer c.sh.cleanupMu.Unlock()
	if c.sh.calledCleanup {
	return errAlreadyCalledCleanup
	}
	// Configure the command.
	c.c.Path = c.Path
	c.c.Env = mapToSlice(c.Vars)
	c.c.Args = append([]string{c.Path}, c.Args...)
	if c.Stdin != "" {
	if c.stdinWriteCloser != nil {
	return errors.New("gosh: cannot both set Stdin and call StdinPipe")
	}
	c.c.Stdin = strings.NewReader(c.Stdin)
	}
	t := time.Now().Format("20060102.150405.000000")
	var err error
	if c.c.Stdout, err = c.makeMultiWriter(true, t); err != nil {
	return err
	}
	if c.c.Stderr, err = c.makeMultiWriter(false, t); err != nil {
	return err
	}
	// Start the command.
	onExit := func(err error) {
	c.cond.L.Lock()
	c.exited = true
	c.cond.Signal()
	c.cond.L.Unlock()
	c.closeClosers()
	c.waitChan <- err
	}
	if err = c.c.Start(); err != nil {
	onExit(errors.New("gosh: start failed"))
	return err
	}
	c.started = true
	// Spawn a "waiter" goroutine that calls exec.Cmd.Wait and thus waits for the
	// process to exit. Calling exec.Cmd.Wait here rather than in gosh.Cmd.Wait
	// ensures that the child process is reaped once it exits. Note, gosh.Cmd.wait
	// blocks on waitChan.
	go func() {
	onExit(c.c.Wait())
	}()
	return nil
	}

	// TODO(sadovsky): Maybe add optional timeouts for
	// Cmd.{awaitReady,awaitVars,wait}.

	func (c *Cmd) awaitReady() error {
	if !c.started {
	return errDidNotCallStart
	} else if c.calledWait {
	return errAlreadyCalledWait
	}
	c.cond.L.Lock()
	defer c.cond.L.Unlock()
	for !c.exited && !c.recvReady {
	c.cond.Wait()
	}
	// Return nil error if both conditions triggered simultaneously.
	if !c.recvReady {
	return errProcessExited
	}
	return nil
	}

	func (c *Cmd) awaitVars(keys ...string) (map[string]string, error) {
	if !c.started {
	return nil, errDidNotCallStart
	} else if c.calledWait {
	return nil, errAlreadyCalledWait
	}
	wantKeys := map[string]bool{}
	for _, key := range keys {
	wantKeys[key] = true
	}
	res := map[string]string{}
	updateRes := func() {
	for k, v := range c.recvVars {
	if _, ok := wantKeys[k]; ok {
	res[k] = v
	}
	}
	}
	c.cond.L.Lock()
	defer c.cond.L.Unlock()
	updateRes()
	for !c.exited && len(res) < len(wantKeys) {
	c.cond.Wait()
	updateRes()
	}
	// Return nil error if both conditions triggered simultaneously.
	if len(res) < len(wantKeys) {
	return nil, errProcessExited
	}
	return res, nil
	}

	func (c *Cmd) wait() error {
	if !c.started {
	return errDidNotCallStart
	} else if c.calledWait {
	return errAlreadyCalledWait
	}
	c.calledWait = true
	return <-c.waitChan
	}

	// Note: We check for this particular error message to handle the unavoidable
	// race between sending a signal to a process and the process exiting.
	// https://golang.org/src/os/exec_unix.go
	// https://golang.org/src/os/exec_windows.go
	const errFinished = "os: process already finished"

	func (c *Cmd) kill() error {
	if !c.started {
	return errDidNotCallStart
	}
	if !c.isRunning() {
	return nil
	}
	if err := c.c.Process.Kill(); err != nil && err.Error() != errFinished {
	return err
	}
	return nil
	}

	func (c *Cmd) signal(sig os.Signal) error {
	if !c.started {
	return errDidNotCallStart
	}
	if !c.isRunning() {
	return nil
	}
	if err := c.c.Process.Signal(sig); err != nil && err.Error() != errFinished {
	return err
	}
	return nil
	}

	func (c *Cmd) shutdown(sig os.Signal) error {
	if err := c.signal(sig); err != nil {
	return err
	}
	if err := c.wait(); err != nil {
	if _, ok := err.(*exec.ExitError); !ok {
	return err
	}
	}
	return nil
	}

	func (c *Cmd) run() error {
	if err := c.start(); err != nil {
	return err
	}
	return c.wait()
	}

	func (c *Cmd) stdout() (string, error) {
	if c.calledStart {
	return "", errAlreadyCalledStart
	}
	var stdout bytes.Buffer
	c.addWriter(&c.stdoutWriters, &stdout)
	err := c.run()
	return stdout.String(), err
	}

	func (c *Cmd) stdoutStderr() (string, string, error) {
	if c.calledStart {
	return "", "", errAlreadyCalledStart
	}
	var stdout, stderr bytes.Buffer
	c.addWriter(&c.stdoutWriters, &stdout)
	c.addWriter(&c.stderrWriters, &stderr)
	err := c.run()
	return stdout.String(), stderr.String(), err
	}