exec_test.go

Documentation: os/exec

  // Copyright 2009 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.
  
  // Use an external test to avoid os/exec -> net/http -> crypto/x509 -> os/exec
  // circular dependency on non-cgo darwin.
  
  package exec_test
  
  import (
  	"bufio"
  	"bytes"
  	"context"
  	"fmt"
  	"internal/poll"
  	"internal/testenv"
  	"io"
  	"io/ioutil"
  	"log"
  	"net"
  	"net/http"
  	"net/http/httptest"
  	"os"
  	"os/exec"
  	"path/filepath"
  	"runtime"
  	"strconv"
  	"strings"
  	"testing"
  	"time"
  )
  
  func helperCommandContext(t *testing.T, ctx context.Context, s ...string) (cmd *exec.Cmd) {
  	testenv.MustHaveExec(t)
  
  	cs := []string{"-test.run=TestHelperProcess", "--"}
  	cs = append(cs, s...)
  	if ctx != nil {
  		cmd = exec.CommandContext(ctx, os.Args[0], cs...)
  	} else {
  		cmd = exec.Command(os.Args[0], cs...)
  	}
  	cmd.Env = []string{"GO_WANT_HELPER_PROCESS=1"}
  	return cmd
  }
  
  func helperCommand(t *testing.T, s ...string) *exec.Cmd {
  	return helperCommandContext(t, nil, s...)
  }
  
  func TestEcho(t *testing.T) {
  	bs, err := helperCommand(t, "echo", "foo bar", "baz").Output()
  	if err != nil {
  		t.Errorf("echo: %v", err)
  	}
  	if g, e := string(bs), "foo bar baz\n"; g != e {
  		t.Errorf("echo: want %q, got %q", e, g)
  	}
  }
  
  func TestCommandRelativeName(t *testing.T) {
  	testenv.MustHaveExec(t)
  
  	// Run our own binary as a relative path
  	// (e.g. "_test/exec.test") our parent directory.
  	base := filepath.Base(os.Args[0]) // "exec.test"
  	dir := filepath.Dir(os.Args[0])   // "/tmp/go-buildNNNN/os/exec/_test"
  	if dir == "." {
  		t.Skip("skipping; running test at root somehow")
  	}
  	parentDir := filepath.Dir(dir) // "/tmp/go-buildNNNN/os/exec"
  	dirBase := filepath.Base(dir)  // "_test"
  	if dirBase == "." {
  		t.Skipf("skipping; unexpected shallow dir of %q", dir)
  	}
  
  	cmd := exec.Command(filepath.Join(dirBase, base), "-test.run=TestHelperProcess", "--", "echo", "foo")
  	cmd.Dir = parentDir
  	cmd.Env = []string{"GO_WANT_HELPER_PROCESS=1"}
  
  	out, err := cmd.Output()
  	if err != nil {
  		t.Errorf("echo: %v", err)
  	}
  	if g, e := string(out), "foo\n"; g != e {
  		t.Errorf("echo: want %q, got %q", e, g)
  	}
  }
  
  func TestCatStdin(t *testing.T) {
  	// Cat, testing stdin and stdout.
  	input := "Input string\nLine 2"
  	p := helperCommand(t, "cat")
  	p.Stdin = strings.NewReader(input)
  	bs, err := p.Output()
  	if err != nil {
  		t.Errorf("cat: %v", err)
  	}
  	s := string(bs)
  	if s != input {
  		t.Errorf("cat: want %q, got %q", input, s)
  	}
  }
  
  func TestEchoFileRace(t *testing.T) {
  	cmd := helperCommand(t, "echo")
  	stdin, err := cmd.StdinPipe()
  	if err != nil {
  		t.Fatalf("StdinPipe: %v", err)
  	}
  	if err := cmd.Start(); err != nil {
  		t.Fatalf("Start: %v", err)
  	}
  	wrote := make(chan bool)
  	go func() {
  		defer close(wrote)
  		fmt.Fprint(stdin, "echo\n")
  	}()
  	if err := cmd.Wait(); err != nil {
  		t.Fatalf("Wait: %v", err)
  	}
  	<-wrote
  }
  
  func TestCatGoodAndBadFile(t *testing.T) {
  	// Testing combined output and error values.
  	bs, err := helperCommand(t, "cat", "/bogus/file.foo", "exec_test.go").CombinedOutput()
  	if _, ok := err.(*exec.ExitError); !ok {
  		t.Errorf("expected *exec.ExitError from cat combined; got %T: %v", err, err)
  	}
  	s := string(bs)
  	sp := strings.SplitN(s, "\n", 2)
  	if len(sp) != 2 {
  		t.Fatalf("expected two lines from cat; got %q", s)
  	}
  	errLine, body := sp[0], sp[1]
  	if !strings.HasPrefix(errLine, "Error: open /bogus/file.foo") {
  		t.Errorf("expected stderr to complain about file; got %q", errLine)
  	}
  	if !strings.Contains(body, "func TestHelperProcess(t *testing.T)") {
  		t.Errorf("expected test code; got %q (len %d)", body, len(body))
  	}
  }
  
  func TestNoExistBinary(t *testing.T) {
  	// Can't run a non-existent binary
  	err := exec.Command("/no-exist-binary").Run()
  	if err == nil {
  		t.Error("expected error from /no-exist-binary")
  	}
  }
  
  func TestExitStatus(t *testing.T) {
  	// Test that exit values are returned correctly
  	cmd := helperCommand(t, "exit", "42")
  	err := cmd.Run()
  	want := "exit status 42"
  	switch runtime.GOOS {
  	case "plan9":
  		want = fmt.Sprintf("exit status: '%s %d: 42'", filepath.Base(cmd.Path), cmd.ProcessState.Pid())
  	}
  	if werr, ok := err.(*exec.ExitError); ok {
  		if s := werr.Error(); s != want {
  			t.Errorf("from exit 42 got exit %q, want %q", s, want)
  		}
  	} else {
  		t.Fatalf("expected *exec.ExitError from exit 42; got %T: %v", err, err)
  	}
  }
  
  func TestPipes(t *testing.T) {
  	check := func(what string, err error) {
  		if err != nil {
  			t.Fatalf("%s: %v", what, err)
  		}
  	}
  	// Cat, testing stdin and stdout.
  	c := helperCommand(t, "pipetest")
  	stdin, err := c.StdinPipe()
  	check("StdinPipe", err)
  	stdout, err := c.StdoutPipe()
  	check("StdoutPipe", err)
  	stderr, err := c.StderrPipe()
  	check("StderrPipe", err)
  
  	outbr := bufio.NewReader(stdout)
  	errbr := bufio.NewReader(stderr)
  	line := func(what string, br *bufio.Reader) string {
  		line, _, err := br.ReadLine()
  		if err != nil {
  			t.Fatalf("%s: %v", what, err)
  		}
  		return string(line)
  	}
  
  	err = c.Start()
  	check("Start", err)
  
  	_, err = stdin.Write([]byte("O:I am output\n"))
  	check("first stdin Write", err)
  	if g, e := line("first output line", outbr), "O:I am output"; g != e {
  		t.Errorf("got %q, want %q", g, e)
  	}
  
  	_, err = stdin.Write([]byte("E:I am error\n"))
  	check("second stdin Write", err)
  	if g, e := line("first error line", errbr), "E:I am error"; g != e {
  		t.Errorf("got %q, want %q", g, e)
  	}
  
  	_, err = stdin.Write([]byte("O:I am output2\n"))
  	check("third stdin Write 3", err)
  	if g, e := line("second output line", outbr), "O:I am output2"; g != e {
  		t.Errorf("got %q, want %q", g, e)
  	}
  
  	stdin.Close()
  	err = c.Wait()
  	check("Wait", err)
  }
  
  const stdinCloseTestString = "Some test string."
  
  // Issue 6270.
  func TestStdinClose(t *testing.T) {
  	check := func(what string, err error) {
  		if err != nil {
  			t.Fatalf("%s: %v", what, err)
  		}
  	}
  	cmd := helperCommand(t, "stdinClose")
  	stdin, err := cmd.StdinPipe()
  	check("StdinPipe", err)
  	// Check that we can access methods of the underlying os.File.`
  	if _, ok := stdin.(interface {
  		Fd() uintptr
  	}); !ok {
  		t.Error("can't access methods of underlying *os.File")
  	}
  	check("Start", cmd.Start())
  	go func() {
  		_, err := io.Copy(stdin, strings.NewReader(stdinCloseTestString))
  		check("Copy", err)
  		// Before the fix, this next line would race with cmd.Wait.
  		check("Close", stdin.Close())
  	}()
  	check("Wait", cmd.Wait())
  }
  
  // Issue 17647.
  // It used to be the case that TestStdinClose, above, would fail when
  // run under the race detector. This test is a variant of TestStdinClose
  // that also used to fail when run under the race detector.
  // This test is run by cmd/dist under the race detector to verify that
  // the race detector no longer reports any problems.
  func TestStdinCloseRace(t *testing.T) {
  	cmd := helperCommand(t, "stdinClose")
  	stdin, err := cmd.StdinPipe()
  	if err != nil {
  		t.Fatalf("StdinPipe: %v", err)
  	}
  	if err := cmd.Start(); err != nil {
  		t.Fatalf("Start: %v", err)
  	}
  	go func() {
  		// We don't check the error return of Kill. It is
  		// possible that the process has already exited, in
  		// which case Kill will return an error "process
  		// already finished". The purpose of this test is to
  		// see whether the race detector reports an error; it
  		// doesn't matter whether this Kill succeeds or not.
  		cmd.Process.Kill()
  	}()
  	go func() {
  		// Send the wrong string, so that the child fails even
  		// if the other goroutine doesn't manage to kill it first.
  		// This test is to check that the race detector does not
  		// falsely report an error, so it doesn't matter how the
  		// child process fails.
  		io.Copy(stdin, strings.NewReader("unexpected string"))
  		if err := stdin.Close(); err != nil {
  			t.Errorf("stdin.Close: %v", err)
  		}
  	}()
  	if err := cmd.Wait(); err == nil {
  		t.Fatalf("Wait: succeeded unexpectedly")
  	}
  }
  
  // Issue 5071
  func TestPipeLookPathLeak(t *testing.T) {
  	// If we are reading from /proc/self/fd we (should) get an exact result.
  	tolerance := 0
  
  	// Reading /proc/self/fd is more reliable than calling lsof, so try that
  	// first.
  	numOpenFDs := func() (int, []byte, error) {
  		fds, err := ioutil.ReadDir("/proc/self/fd")
  		if err != nil {
  			return 0, nil, err
  		}
  		return len(fds), nil, nil
  	}
  	want, before, err := numOpenFDs()
  	if err != nil {
  		// We encountered a problem reading /proc/self/fd (we might be on
  		// a platform that doesn't have it). Fall back onto lsof.
  		t.Logf("using lsof because: %v", err)
  		numOpenFDs = func() (int, []byte, error) {
  			// Android's stock lsof does not obey the -p option,
  			// so extra filtering is needed.
  			// https://golang.org/issue/10206
  			if runtime.GOOS == "android" {
  				// numOpenFDsAndroid handles errors itself and
  				// might skip or fail the test.
  				n, lsof := numOpenFDsAndroid(t)
  				return n, lsof, nil
  			}
  			lsof, err := exec.Command("lsof", "-b", "-n", "-p", strconv.Itoa(os.Getpid())).Output()
  			return bytes.Count(lsof, []byte("\n")), lsof, err
  		}
  
  		// lsof may see file descriptors associated with the fork itself,
  		// so we allow some extra margin if we have to use it.
  		// https://golang.org/issue/19243
  		tolerance = 5
  
  		// Retry reading the number of open file descriptors.
  		want, before, err = numOpenFDs()
  		if err != nil {
  			t.Log(err)
  			t.Skipf("skipping test; error finding or running lsof")
  		}
  	}
  
  	for i := 0; i < 6; i++ {
  		cmd := exec.Command("something-that-does-not-exist-binary")
  		cmd.StdoutPipe()
  		cmd.StderrPipe()
  		cmd.StdinPipe()
  		if err := cmd.Run(); err == nil {
  			t.Fatal("unexpected success")
  		}
  	}
  	got, after, err := numOpenFDs()
  	if err != nil {
  		// numOpenFDs has already succeeded once, it should work here.
  		t.Errorf("unexpected failure: %v", err)
  	}
  	if got-want > tolerance {
  		t.Errorf("number of open file descriptors changed: got %v, want %v", got, want)
  		if before != nil {
  			t.Errorf("before:\n%v\n", before)
  		}
  		if after != nil {
  			t.Errorf("after:\n%v\n", after)
  		}
  	}
  }
  
  func numOpenFDsAndroid(t *testing.T) (n int, lsof []byte) {
  	raw, err := exec.Command("lsof").Output()
  	if err != nil {
  		t.Skip("skipping test; error finding or running lsof")
  	}
  
  	// First find the PID column index by parsing the first line, and
  	// select lines containing pid in the column.
  	pid := []byte(strconv.Itoa(os.Getpid()))
  	pidCol := -1
  
  	s := bufio.NewScanner(bytes.NewReader(raw))
  	for s.Scan() {
  		line := s.Bytes()
  		fields := bytes.Fields(line)
  		if pidCol < 0 {
  			for i, v := range fields {
  				if bytes.Equal(v, []byte("PID")) {
  					pidCol = i
  					break
  				}
  			}
  			lsof = append(lsof, line...)
  			continue
  		}
  		if bytes.Equal(fields[pidCol], pid) {
  			lsof = append(lsof, '\n')
  			lsof = append(lsof, line...)
  		}
  	}
  	if pidCol < 0 {
  		t.Fatal("error processing lsof output: unexpected header format")
  	}
  	if err := s.Err(); err != nil {
  		t.Fatalf("error processing lsof output: %v", err)
  	}
  	return bytes.Count(lsof, []byte("\n")), lsof
  }
  
  var testedAlreadyLeaked = false
  
  // basefds returns the number of expected file descriptors
  // to be present in a process at start.
  // stdin, stdout, stderr, epoll/kqueue, maybe testlog
  func basefds() uintptr {
  	n := os.Stderr.Fd() + 1
  	for _, arg := range os.Args {
  		if strings.HasPrefix(arg, "-test.testlogfile=") {
  			n++
  		}
  	}
  	return n
  }
  
  func closeUnexpectedFds(t *testing.T, m string) {
  	for fd := basefds(); fd <= 101; fd++ {
  		if fd == poll.PollDescriptor() {
  			continue
  		}
  		err := os.NewFile(fd, "").Close()
  		if err == nil {
  			t.Logf("%s: Something already leaked - closed fd %d", m, fd)
  		}
  	}
  }
  
  func TestExtraFilesFDShuffle(t *testing.T) {
  	t.Skip("flaky test; see https://golang.org/issue/5780")
  	switch runtime.GOOS {
  	case "darwin":
  		// TODO(cnicolaou): https://golang.org/issue/2603
  		// leads to leaked file descriptors in this test when it's
  		// run from a builder.
  		closeUnexpectedFds(t, "TestExtraFilesFDShuffle")
  	case "netbsd":
  		// https://golang.org/issue/3955
  		closeUnexpectedFds(t, "TestExtraFilesFDShuffle")
  	case "windows":
  		t.Skip("no operating system support; skipping")
  	}
  
  	// syscall.StartProcess maps all the FDs passed to it in
  	// ProcAttr.Files (the concatenation of stdin,stdout,stderr and
  	// ExtraFiles) into consecutive FDs in the child, that is:
  	// Files{11, 12, 6, 7, 9, 3} should result in the file
  	// represented by FD 11 in the parent being made available as 0
  	// in the child, 12 as 1, etc.
  	//
  	// We want to test that FDs in the child do not get overwritten
  	// by one another as this shuffle occurs. The original implementation
  	// was buggy in that in some data dependent cases it would overwrite
  	// stderr in the child with one of the ExtraFile members.
  	// Testing for this case is difficult because it relies on using
  	// the same FD values as that case. In particular, an FD of 3
  	// must be at an index of 4 or higher in ProcAttr.Files and
  	// the FD of the write end of the Stderr pipe (as obtained by
  	// StderrPipe()) must be the same as the size of ProcAttr.Files;
  	// therefore we test that the read end of this pipe (which is what
  	// is returned to the parent by StderrPipe() being one less than
  	// the size of ProcAttr.Files, i.e. 3+len(cmd.ExtraFiles).
  	//
  	// Moving this test case around within the overall tests may
  	// affect the FDs obtained and hence the checks to catch these cases.
  	npipes := 2
  	c := helperCommand(t, "extraFilesAndPipes", strconv.Itoa(npipes+1))
  	rd, wr, _ := os.Pipe()
  	defer rd.Close()
  	if rd.Fd() != 3 {
  		t.Errorf("bad test value for test pipe: fd %d", rd.Fd())
  	}
  	stderr, _ := c.StderrPipe()
  	wr.WriteString("_LAST")
  	wr.Close()
  
  	pipes := make([]struct {
  		r, w *os.File
  	}, npipes)
  	data := []string{"a", "b"}
  
  	for i := 0; i < npipes; i++ {
  		r, w, err := os.Pipe()
  		if err != nil {
  			t.Fatalf("unexpected error creating pipe: %s", err)
  		}
  		pipes[i].r = r
  		pipes[i].w = w
  		w.WriteString(data[i])
  		c.ExtraFiles = append(c.ExtraFiles, pipes[i].r)
  		defer func() {
  			r.Close()
  			w.Close()
  		}()
  	}
  	// Put fd 3 at the end.
  	c.ExtraFiles = append(c.ExtraFiles, rd)
  
  	stderrFd := int(stderr.(*os.File).Fd())
  	if stderrFd != ((len(c.ExtraFiles) + 3) - 1) {
  		t.Errorf("bad test value for stderr pipe")
  	}
  
  	expected := "child: " + strings.Join(data, "") + "_LAST"
  
  	err := c.Start()
  	if err != nil {
  		t.Fatalf("Run: %v", err)
  	}
  	ch := make(chan string, 1)
  	go func(ch chan string) {
  		buf := make([]byte, 512)
  		n, err := stderr.Read(buf)
  		if err != nil {
  			t.Errorf("Read: %s", err)
  			ch <- err.Error()
  		} else {
  			ch <- string(buf[:n])
  		}
  		close(ch)
  	}(ch)
  	select {
  	case m := <-ch:
  		if m != expected {
  			t.Errorf("Read: '%s' not '%s'", m, expected)
  		}
  	case <-time.After(5 * time.Second):
  		t.Errorf("Read timedout")
  	}
  	c.Wait()
  }
  
  func TestExtraFiles(t *testing.T) {
  	testenv.MustHaveExec(t)
  
  	if runtime.GOOS == "windows" {
  		t.Skipf("skipping test on %q", runtime.GOOS)
  	}
  
  	// Ensure that file descriptors have not already been leaked into
  	// our environment.
  	if !testedAlreadyLeaked {
  		testedAlreadyLeaked = true
  		closeUnexpectedFds(t, "TestExtraFiles")
  	}
  
  	// Force network usage, to verify the epoll (or whatever) fd
  	// doesn't leak to the child,
  	ln, err := net.Listen("tcp", "127.0.0.1:0")
  	if err != nil {
  		t.Fatal(err)
  	}
  	defer ln.Close()
  
  	// Make sure duplicated fds don't leak to the child.
  	f, err := ln.(*net.TCPListener).File()
  	if err != nil {
  		t.Fatal(err)
  	}
  	defer f.Close()
  	ln2, err := net.FileListener(f)
  	if err != nil {
  		t.Fatal(err)
  	}
  	defer ln2.Close()
  
  	// Force TLS root certs to be loaded (which might involve
  	// cgo), to make sure none of that potential C code leaks fds.
  	ts := httptest.NewUnstartedServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {}))
  	// quiet expected TLS handshake error "remote error: bad certificate"
  	ts.Config.ErrorLog = log.New(ioutil.Discard, "", 0)
  	ts.StartTLS()
  	defer ts.Close()
  	_, err = http.Get(ts.URL)
  	if err == nil {
  		t.Errorf("success trying to fetch %s; want an error", ts.URL)
  	}
  
  	tf, err := ioutil.TempFile("", "")
  	if err != nil {
  		t.Fatalf("TempFile: %v", err)
  	}
  	defer os.Remove(tf.Name())
  	defer tf.Close()
  
  	const text = "Hello, fd 3!"
  	_, err = tf.Write([]byte(text))
  	if err != nil {
  		t.Fatalf("Write: %v", err)
  	}
  	_, err = tf.Seek(0, io.SeekStart)
  	if err != nil {
  		t.Fatalf("Seek: %v", err)
  	}
  
  	c := helperCommand(t, "read3")
  	var stdout, stderr bytes.Buffer
  	c.Stdout = &stdout
  	c.Stderr = &stderr
  	c.ExtraFiles = []*os.File{tf}
  	err = c.Run()
  	if err != nil {
  		t.Fatalf("Run: %v; stdout %q, stderr %q", err, stdout.Bytes(), stderr.Bytes())
  	}
  	if stdout.String() != text {
  		t.Errorf("got stdout %q, stderr %q; want %q on stdout", stdout.String(), stderr.String(), text)
  	}
  }
  
  func TestExtraFilesRace(t *testing.T) {
  	if runtime.GOOS == "windows" {
  		t.Skip("no operating system support; skipping")
  	}
  	listen := func() net.Listener {
  		ln, err := net.Listen("tcp", "127.0.0.1:0")
  		if err != nil {
  			t.Fatal(err)
  		}
  		return ln
  	}
  	listenerFile := func(ln net.Listener) *os.File {
  		f, err := ln.(*net.TCPListener).File()
  		if err != nil {
  			t.Fatal(err)
  		}
  		return f
  	}
  	runCommand := func(c *exec.Cmd, out chan<- string) {
  		bout, err := c.CombinedOutput()
  		if err != nil {
  			out <- "ERROR:" + err.Error()
  		} else {
  			out <- string(bout)
  		}
  	}
  
  	for i := 0; i < 10; i++ {
  		la := listen()
  		ca := helperCommand(t, "describefiles")
  		ca.ExtraFiles = []*os.File{listenerFile(la)}
  		lb := listen()
  		cb := helperCommand(t, "describefiles")
  		cb.ExtraFiles = []*os.File{listenerFile(lb)}
  		ares := make(chan string)
  		bres := make(chan string)
  		go runCommand(ca, ares)
  		go runCommand(cb, bres)
  		if got, want := <-ares, fmt.Sprintf("fd3: listener %s\n", la.Addr()); got != want {
  			t.Errorf("iteration %d, process A got:\n%s\nwant:\n%s\n", i, got, want)
  		}
  		if got, want := <-bres, fmt.Sprintf("fd3: listener %s\n", lb.Addr()); got != want {
  			t.Errorf("iteration %d, process B got:\n%s\nwant:\n%s\n", i, got, want)
  		}
  		la.Close()
  		lb.Close()
  		for _, f := range ca.ExtraFiles {
  			f.Close()
  		}
  		for _, f := range cb.ExtraFiles {
  			f.Close()
  		}
  
  	}
  }
  
  // TestHelperProcess isn't a real test. It's used as a helper process
  // for TestParameterRun.
  func TestHelperProcess(*testing.T) {
  	if os.Getenv("GO_WANT_HELPER_PROCESS") != "1" {
  		return
  	}
  	defer os.Exit(0)
  
  	// Determine which command to use to display open files.
  	ofcmd := "lsof"
  	switch runtime.GOOS {
  	case "dragonfly", "freebsd", "netbsd", "openbsd":
  		ofcmd = "fstat"
  	case "plan9":
  		ofcmd = "/bin/cat"
  	}
  
  	args := os.Args
  	for len(args) > 0 {
  		if args[0] == "--" {
  			args = args[1:]
  			break
  		}
  		args = args[1:]
  	}
  	if len(args) == 0 {
  		fmt.Fprintf(os.Stderr, "No command\n")
  		os.Exit(2)
  	}
  
  	cmd, args := args[0], args[1:]
  	switch cmd {
  	case "echo":
  		iargs := []interface{}{}
  		for _, s := range args {
  			iargs = append(iargs, s)
  		}
  		fmt.Println(iargs...)
  	case "echoenv":
  		for _, s := range args {
  			fmt.Println(os.Getenv(s))
  		}
  		os.Exit(0)
  	case "cat":
  		if len(args) == 0 {
  			io.Copy(os.Stdout, os.Stdin)
  			return
  		}
  		exit := 0
  		for _, fn := range args {
  			f, err := os.Open(fn)
  			if err != nil {
  				fmt.Fprintf(os.Stderr, "Error: %v\n", err)
  				exit = 2
  			} else {
  				defer f.Close()
  				io.Copy(os.Stdout, f)
  			}
  		}
  		os.Exit(exit)
  	case "pipetest":
  		bufr := bufio.NewReader(os.Stdin)
  		for {
  			line, _, err := bufr.ReadLine()
  			if err == io.EOF {
  				break
  			} else if err != nil {
  				os.Exit(1)
  			}
  			if bytes.HasPrefix(line, []byte("O:")) {
  				os.Stdout.Write(line)
  				os.Stdout.Write([]byte{'\n'})
  			} else if bytes.HasPrefix(line, []byte("E:")) {
  				os.Stderr.Write(line)
  				os.Stderr.Write([]byte{'\n'})
  			} else {
  				os.Exit(1)
  			}
  		}
  	case "stdinClose":
  		b, err := ioutil.ReadAll(os.Stdin)
  		if err != nil {
  			fmt.Fprintf(os.Stderr, "Error: %v\n", err)
  			os.Exit(1)
  		}
  		if s := string(b); s != stdinCloseTestString {
  			fmt.Fprintf(os.Stderr, "Error: Read %q, want %q", s, stdinCloseTestString)
  			os.Exit(1)
  		}
  		os.Exit(0)
  	case "read3": // read fd 3
  		fd3 := os.NewFile(3, "fd3")
  		bs, err := ioutil.ReadAll(fd3)
  		if err != nil {
  			fmt.Printf("ReadAll from fd 3: %v", err)
  			os.Exit(1)
  		}
  		switch runtime.GOOS {
  		case "dragonfly":
  			// TODO(jsing): Determine why DragonFly is leaking
  			// file descriptors...
  		case "darwin":
  			// TODO(bradfitz): broken? Sometimes.
  			// https://golang.org/issue/2603
  			// Skip this additional part of the test for now.
  		case "netbsd":
  			// TODO(jsing): This currently fails on NetBSD due to
  			// the cloned file descriptors that result from opening
  			// /dev/urandom.
  			// https://golang.org/issue/3955
  		case "solaris":
  			// TODO(aram): This fails on Solaris because libc opens
  			// its own files, as it sees fit. Darwin does the same,
  			// see: https://golang.org/issue/2603
  		default:
  			// Now verify that there are no other open fds.
  			var files []*os.File
  			for wantfd := basefds() + 1; wantfd <= 100; wantfd++ {
  				if wantfd == poll.PollDescriptor() {
  					continue
  				}
  				f, err := os.Open(os.Args[0])
  				if err != nil {
  					fmt.Printf("error opening file with expected fd %d: %v", wantfd, err)
  					os.Exit(1)
  				}
  				if got := f.Fd(); got != wantfd {
  					fmt.Printf("leaked parent file. fd = %d; want %d\n", got, wantfd)
  					var args []string
  					switch runtime.GOOS {
  					case "plan9":
  						args = []string{fmt.Sprintf("/proc/%d/fd", os.Getpid())}
  					default:
  						args = []string{"-p", fmt.Sprint(os.Getpid())}
  					}
  					out, _ := exec.Command(ofcmd, args...).CombinedOutput()
  					fmt.Print(string(out))
  					os.Exit(1)
  				}
  				files = append(files, f)
  			}
  			for _, f := range files {
  				f.Close()
  			}
  		}
  		// Referring to fd3 here ensures that it is not
  		// garbage collected, and therefore closed, while
  		// executing the wantfd loop above. It doesn't matter
  		// what we do with fd3 as long as we refer to it;
  		// closing it is the easy choice.
  		fd3.Close()
  		os.Stdout.Write(bs)
  	case "exit":
  		n, _ := strconv.Atoi(args[0])
  		os.Exit(n)
  	case "describefiles":
  		f := os.NewFile(3, fmt.Sprintf("fd3"))
  		ln, err := net.FileListener(f)
  		if err == nil {
  			fmt.Printf("fd3: listener %s\n", ln.Addr())
  			ln.Close()
  		}
  		os.Exit(0)
  	case "extraFilesAndPipes":
  		n, _ := strconv.Atoi(args[0])
  		pipes := make([]*os.File, n)
  		for i := 0; i < n; i++ {
  			pipes[i] = os.NewFile(uintptr(3+i), strconv.Itoa(i))
  		}
  		response := ""
  		for i, r := range pipes {
  			ch := make(chan string, 1)
  			go func(c chan string) {
  				buf := make([]byte, 10)
  				n, err := r.Read(buf)
  				if err != nil {
  					fmt.Fprintf(os.Stderr, "Child: read error: %v on pipe %d\n", err, i)
  					os.Exit(1)
  				}
  				c <- string(buf[:n])
  				close(c)
  			}(ch)
  			select {
  			case m := <-ch:
  				response = response + m
  			case <-time.After(5 * time.Second):
  				fmt.Fprintf(os.Stderr, "Child: Timeout reading from pipe: %d\n", i)
  				os.Exit(1)
  			}
  		}
  		fmt.Fprintf(os.Stderr, "child: %s", response)
  		os.Exit(0)
  	case "exec":
  		cmd := exec.Command(args[1])
  		cmd.Dir = args[0]
  		output, err := cmd.CombinedOutput()
  		if err != nil {
  			fmt.Fprintf(os.Stderr, "Child: %s %s", err, string(output))
  			os.Exit(1)
  		}
  		fmt.Printf("%s", string(output))
  		os.Exit(0)
  	case "lookpath":
  		p, err := exec.LookPath(args[0])
  		if err != nil {
  			fmt.Fprintf(os.Stderr, "LookPath failed: %v\n", err)
  			os.Exit(1)
  		}
  		fmt.Print(p)
  		os.Exit(0)
  	case "stderrfail":
  		fmt.Fprintf(os.Stderr, "some stderr text\n")
  		os.Exit(1)
  	case "sleep":
  		time.Sleep(3 * time.Second)
  		os.Exit(0)
  	default:
  		fmt.Fprintf(os.Stderr, "Unknown command %q\n", cmd)
  		os.Exit(2)
  	}
  }
  
  type delayedInfiniteReader struct{}
  
  func (delayedInfiniteReader) Read(b []byte) (int, error) {
  	time.Sleep(100 * time.Millisecond)
  	for i := range b {
  		b[i] = 'x'
  	}
  	return len(b), nil
  }
  
  // Issue 9173: ignore stdin pipe writes if the program completes successfully.
  func TestIgnorePipeErrorOnSuccess(t *testing.T) {
  	testenv.MustHaveExec(t)
  
  	// We really only care about testing this on Unixy and Windowsy things.
  	if runtime.GOOS == "plan9" {
  		t.Skipf("skipping test on %q", runtime.GOOS)
  	}
  
  	testWith := func(r io.Reader) func(*testing.T) {
  		return func(t *testing.T) {
  			cmd := helperCommand(t, "echo", "foo")
  			var out bytes.Buffer
  			cmd.Stdin = r
  			cmd.Stdout = &out
  			if err := cmd.Run(); err != nil {
  				t.Fatal(err)
  			}
  			if got, want := out.String(), "foo\n"; got != want {
  				t.Errorf("output = %q; want %q", got, want)
  			}
  		}
  	}
  	t.Run("10MB", testWith(strings.NewReader(strings.Repeat("x", 10<<20))))
  	t.Run("Infinite", testWith(delayedInfiniteReader{}))
  }
  
  type badWriter struct{}
  
  func (w *badWriter) Write(data []byte) (int, error) {
  	return 0, io.ErrUnexpectedEOF
  }
  
  func TestClosePipeOnCopyError(t *testing.T) {
  	testenv.MustHaveExec(t)
  
  	if runtime.GOOS == "windows" || runtime.GOOS == "plan9" {
  		t.Skipf("skipping test on %s - no yes command", runtime.GOOS)
  	}
  	cmd := exec.Command("yes")
  	cmd.Stdout = new(badWriter)
  	c := make(chan int, 1)
  	go func() {
  		err := cmd.Run()
  		if err == nil {
  			t.Errorf("yes completed successfully")
  		}
  		c <- 1
  	}()
  	select {
  	case <-c:
  		// ok
  	case <-time.After(5 * time.Second):
  		t.Fatalf("yes got stuck writing to bad writer")
  	}
  }
  
  func TestOutputStderrCapture(t *testing.T) {
  	testenv.MustHaveExec(t)
  
  	cmd := helperCommand(t, "stderrfail")
  	_, err := cmd.Output()
  	ee, ok := err.(*exec.ExitError)
  	if !ok {
  		t.Fatalf("Output error type = %T; want ExitError", err)
  	}
  	got := string(ee.Stderr)
  	want := "some stderr text\n"
  	if got != want {
  		t.Errorf("ExitError.Stderr = %q; want %q", got, want)
  	}
  }
  
  func TestContext(t *testing.T) {
  	ctx, cancel := context.WithCancel(context.Background())
  	c := helperCommandContext(t, ctx, "pipetest")
  	stdin, err := c.StdinPipe()
  	if err != nil {
  		t.Fatal(err)
  	}
  	stdout, err := c.StdoutPipe()
  	if err != nil {
  		t.Fatal(err)
  	}
  	if err := c.Start(); err != nil {
  		t.Fatal(err)
  	}
  
  	if _, err := stdin.Write([]byte("O:hi\n")); err != nil {
  		t.Fatal(err)
  	}
  	buf := make([]byte, 5)
  	n, err := io.ReadFull(stdout, buf)
  	if n != len(buf) || err != nil || string(buf) != "O:hi\n" {
  		t.Fatalf("ReadFull = %d, %v, %q", n, err, buf[:n])
  	}
  	waitErr := make(chan error, 1)
  	go func() {
  		waitErr <- c.Wait()
  	}()
  	cancel()
  	select {
  	case err := <-waitErr:
  		if err == nil {
  			t.Fatal("expected Wait failure")
  		}
  	case <-time.After(3 * time.Second):
  		t.Fatal("timeout waiting for child process death")
  	}
  }
  
  func TestContextCancel(t *testing.T) {
  	if testenv.Builder() == "windows-386-xp" {
  		t.Skipf("known to fail on Windows XP. Issue 17245")
  	}
  	ctx, cancel := context.WithCancel(context.Background())
  	defer cancel()
  	c := helperCommandContext(t, ctx, "cat")
  
  	r, w, err := os.Pipe()
  	if err != nil {
  		t.Fatal(err)
  	}
  	c.Stdin = r
  
  	stdout, err := c.StdoutPipe()
  	if err != nil {
  		t.Fatal(err)
  	}
  	readDone := make(chan struct{})
  	go func() {
  		defer close(readDone)
  		var a [1024]byte
  		for {
  			n, err := stdout.Read(a[:])
  			if err != nil {
  				if err != io.EOF {
  					t.Errorf("unexpected read error: %v", err)
  				}
  				return
  			}
  			t.Logf("%s", a[:n])
  		}
  	}()
  
  	if err := c.Start(); err != nil {
  		t.Fatal(err)
  	}
  
  	if err := r.Close(); err != nil {
  		t.Fatal(err)
  	}
  
  	if _, err := io.WriteString(w, "echo"); err != nil {
  		t.Fatal(err)
  	}
  
  	cancel()
  
  	// Calling cancel should have killed the process, so writes
  	// should now fail.  Give the process a little while to die.
  	start := time.Now()
  	for {
  		if _, err := io.WriteString(w, "echo"); err != nil {
  			break
  		}
  		if time.Since(start) > time.Second {
  			t.Fatal("canceling context did not stop program")
  		}
  		time.Sleep(time.Millisecond)
  	}
  
  	if err := w.Close(); err != nil {
  		t.Errorf("error closing write end of pipe: %v", err)
  	}
  	<-readDone
  
  	if err := c.Wait(); err == nil {
  		t.Error("program unexpectedly exited successfully")
  	} else {
  		t.Logf("exit status: %v", err)
  	}
  }
  
  // test that environment variables are de-duped.
  func TestDedupEnvEcho(t *testing.T) {
  	testenv.MustHaveExec(t)
  
  	cmd := helperCommand(t, "echoenv", "FOO")
  	cmd.Env = append(cmd.Env, "FOO=bad", "FOO=good")
  	out, err := cmd.CombinedOutput()
  	if err != nil {
  		t.Fatal(err)
  	}
  	if got, want := strings.TrimSpace(string(out)), "good"; got != want {
  		t.Errorf("output = %q; want %q", got, want)
  	}
  }
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