Adding upstream version 1.16.10.upstream/1.16.10upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

19 files changed, 3439 insertions, 0 deletions

diff --git a/src/os/exec/bench_test.go b/src/os/exec/bench_test.go
new file mode 100644
index 0000000..9a94001
--- /dev/null
+++ b/src/os/exec/bench_test.go
@@ -0,0 +1,23 @@
+// Copyright 2019 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.
+
+package exec
+
+import (
+	"testing"
+)
+
+func BenchmarkExecHostname(b *testing.B) {
+	b.ReportAllocs()
+	path, err := LookPath("hostname")
+	if err != nil {
+		b.Fatalf("could not find hostname: %v", err)
+	}
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		if err := Command(path).Run(); err != nil {
+			b.Fatalf("hostname: %v", err)
+		}
+	}
+}
diff --git a/src/os/exec/env_test.go b/src/os/exec/env_test.go
new file mode 100644
index 0000000..b5ac398
--- /dev/null
+++ b/src/os/exec/env_test.go
@@ -0,0 +1,39 @@
+// Copyright 2017 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.
+
+package exec
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestDedupEnv(t *testing.T) {
+	tests := []struct {
+		noCase bool
+		in     []string
+		want   []string
+	}{
+		{
+			noCase: true,
+			in:     []string{"k1=v1", "k2=v2", "K1=v3"},
+			want:   []string{"K1=v3", "k2=v2"},
+		},
+		{
+			noCase: false,
+			in:     []string{"k1=v1", "K1=V2", "k1=v3"},
+			want:   []string{"k1=v3", "K1=V2"},
+		},
+		{
+			in:   []string{"=a", "=b", "foo", "bar"},
+			want: []string{"=b", "foo", "bar"},
+		},
+	}
+	for _, tt := range tests {
+		got := dedupEnvCase(tt.noCase, tt.in)
+		if !reflect.DeepEqual(got, tt.want) {
+			t.Errorf("Dedup(%v, %q) = %q; want %q", tt.noCase, tt.in, got, tt.want)
+		}
+	}
+}
diff --git a/src/os/exec/example_test.go b/src/os/exec/example_test.go
new file mode 100644
index 0000000..a66890b
--- /dev/null
+++ b/src/os/exec/example_test.go
@@ -0,0 +1,155 @@
+// 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.
+
+package exec_test
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"fmt"
+	"io"
+	"log"
+	"os"
+	"os/exec"
+	"strings"
+	"time"
+)
+
+func ExampleLookPath() {
+	path, err := exec.LookPath("fortune")
+	if err != nil {
+		log.Fatal("installing fortune is in your future")
+	}
+	fmt.Printf("fortune is available at %s\n", path)
+}
+
+func ExampleCommand() {
+	cmd := exec.Command("tr", "a-z", "A-Z")
+	cmd.Stdin = strings.NewReader("some input")
+	var out bytes.Buffer
+	cmd.Stdout = &out
+	err := cmd.Run()
+	if err != nil {
+		log.Fatal(err)
+	}
+	fmt.Printf("in all caps: %q\n", out.String())
+}
+
+func ExampleCommand_environment() {
+	cmd := exec.Command("prog")
+	cmd.Env = append(os.Environ(),
+		"FOO=duplicate_value", // ignored
+		"FOO=actual_value",    // this value is used
+	)
+	if err := cmd.Run(); err != nil {
+		log.Fatal(err)
+	}
+}
+
+func ExampleCmd_Output() {
+	out, err := exec.Command("date").Output()
+	if err != nil {
+		log.Fatal(err)
+	}
+	fmt.Printf("The date is %s\n", out)
+}
+
+func ExampleCmd_Run() {
+	cmd := exec.Command("sleep", "1")
+	log.Printf("Running command and waiting for it to finish...")
+	err := cmd.Run()
+	log.Printf("Command finished with error: %v", err)
+}
+
+func ExampleCmd_Start() {
+	cmd := exec.Command("sleep", "5")
+	err := cmd.Start()
+	if err != nil {
+		log.Fatal(err)
+	}
+	log.Printf("Waiting for command to finish...")
+	err = cmd.Wait()
+	log.Printf("Command finished with error: %v", err)
+}
+
+func ExampleCmd_StdoutPipe() {
+	cmd := exec.Command("echo", "-n", `{"Name": "Bob", "Age": 32}`)
+	stdout, err := cmd.StdoutPipe()
+	if err != nil {
+		log.Fatal(err)
+	}
+	if err := cmd.Start(); err != nil {
+		log.Fatal(err)
+	}
+	var person struct {
+		Name string
+		Age  int
+	}
+	if err := json.NewDecoder(stdout).Decode(&person); err != nil {
+		log.Fatal(err)
+	}
+	if err := cmd.Wait(); err != nil {
+		log.Fatal(err)
+	}
+	fmt.Printf("%s is %d years old\n", person.Name, person.Age)
+}
+
+func ExampleCmd_StdinPipe() {
+	cmd := exec.Command("cat")
+	stdin, err := cmd.StdinPipe()
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	go func() {
+		defer stdin.Close()
+		io.WriteString(stdin, "values written to stdin are passed to cmd's standard input")
+	}()
+
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	fmt.Printf("%s\n", out)
+}
+
+func ExampleCmd_StderrPipe() {
+	cmd := exec.Command("sh", "-c", "echo stdout; echo 1>&2 stderr")
+	stderr, err := cmd.StderrPipe()
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	if err := cmd.Start(); err != nil {
+		log.Fatal(err)
+	}
+
+	slurp, _ := io.ReadAll(stderr)
+	fmt.Printf("%s\n", slurp)
+
+	if err := cmd.Wait(); err != nil {
+		log.Fatal(err)
+	}
+}
+
+func ExampleCmd_CombinedOutput() {
+	cmd := exec.Command("sh", "-c", "echo stdout; echo 1>&2 stderr")
+	stdoutStderr, err := cmd.CombinedOutput()
+	if err != nil {
+		log.Fatal(err)
+	}
+	fmt.Printf("%s\n", stdoutStderr)
+}
+
+func ExampleCommandContext() {
+	ctx, cancel := context.WithTimeout(context.Background(), 100*time.Millisecond)
+	defer cancel()
+
+	if err := exec.CommandContext(ctx, "sleep", "5").Run(); err != nil {
+		// This will fail after 100 milliseconds. The 5 second sleep
+		// will be interrupted.
+	}
+}
diff --git a/src/os/exec/exec.go b/src/os/exec/exec.go
new file mode 100644
index 0000000..0c49575
--- /dev/null
+++ b/src/os/exec/exec.go
@@ -0,0 +1,789 @@
+// 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.
+
+// Package exec runs external commands. It wraps os.StartProcess to make it
+// easier to remap stdin and stdout, connect I/O with pipes, and do other
+// adjustments.
+//
+// Unlike the "system" library call from C and other languages, the
+// os/exec package intentionally does not invoke the system shell and
+// does not expand any glob patterns or handle other expansions,
+// pipelines, or redirections typically done by shells. The package
+// behaves more like C's "exec" family of functions. To expand glob
+// patterns, either call the shell directly, taking care to escape any
+// dangerous input, or use the path/filepath package's Glob function.
+// To expand environment variables, use package os's ExpandEnv.
+//
+// Note that the examples in this package assume a Unix system.
+// They may not run on Windows, and they do not run in the Go Playground
+// used by golang.org and godoc.org.
+package exec
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"internal/syscall/execenv"
+	"io"
+	"os"
+	"path/filepath"
+	"runtime"
+	"strconv"
+	"strings"
+	"sync"
+	"syscall"
+)
+
+// Error is returned by LookPath when it fails to classify a file as an
+// executable.
+type Error struct {
+	// Name is the file name for which the error occurred.
+	Name string
+	// Err is the underlying error.
+	Err error
+}
+
+func (e *Error) Error() string {
+	return "exec: " + strconv.Quote(e.Name) + ": " + e.Err.Error()
+}
+
+func (e *Error) Unwrap() error { return e.Err }
+
+// Cmd represents an external command being prepared or run.
+//
+// A Cmd cannot be reused after calling its Run, Output or CombinedOutput
+// methods.
+type Cmd struct {
+	// Path is the path of the command to run.
+	//
+	// This is the only field that must be set to a non-zero
+	// value. If Path is relative, it is evaluated relative
+	// to Dir.
+	Path string
+
+	// Args holds command line arguments, including the command as Args[0].
+	// If the Args field is empty or nil, Run uses {Path}.
+	//
+	// In typical use, both Path and Args are set by calling Command.
+	Args []string
+
+	// Env specifies the environment of the process.
+	// Each entry is of the form "key=value".
+	// If Env is nil, the new process uses the current process's
+	// environment.
+	// If Env contains duplicate environment keys, only the last
+	// value in the slice for each duplicate key is used.
+	// As a special case on Windows, SYSTEMROOT is always added if
+	// missing and not explicitly set to the empty string.
+	Env []string
+
+	// Dir specifies the working directory of the command.
+	// If Dir is the empty string, Run runs the command in the
+	// calling process's current directory.
+	Dir string
+
+	// Stdin specifies the process's standard input.
+	//
+	// If Stdin is nil, the process reads from the null device (os.DevNull).
+	//
+	// If Stdin is an *os.File, the process's standard input is connected
+	// directly to that file.
+	//
+	// Otherwise, during the execution of the command a separate
+	// goroutine reads from Stdin and delivers that data to the command
+	// over a pipe. In this case, Wait does not complete until the goroutine
+	// stops copying, either because it has reached the end of Stdin
+	// (EOF or a read error) or because writing to the pipe returned an error.
+	Stdin io.Reader
+
+	// Stdout and Stderr specify the process's standard output and error.
+	//
+	// If either is nil, Run connects the corresponding file descriptor
+	// to the null device (os.DevNull).
+	//
+	// If either is an *os.File, the corresponding output from the process
+	// is connected directly to that file.
+	//
+	// Otherwise, during the execution of the command a separate goroutine
+	// reads from the process over a pipe and delivers that data to the
+	// corresponding Writer. In this case, Wait does not complete until the
+	// goroutine reaches EOF or encounters an error.
+	//
+	// If Stdout and Stderr are the same writer, and have a type that can
+	// be compared with ==, at most one goroutine at a time will call Write.
+	Stdout io.Writer
+	Stderr io.Writer
+
+	// ExtraFiles specifies additional open files to be inherited by the
+	// new process. It does not include standard input, standard output, or
+	// standard error. If non-nil, entry i becomes file descriptor 3+i.
+	//
+	// ExtraFiles is not supported on Windows.
+	ExtraFiles []*os.File
+
+	// SysProcAttr holds optional, operating system-specific attributes.
+	// Run passes it to os.StartProcess as the os.ProcAttr's Sys field.
+	SysProcAttr *syscall.SysProcAttr
+
+	// Process is the underlying process, once started.
+	Process *os.Process
+
+	// ProcessState contains information about an exited process,
+	// available after a call to Wait or Run.
+	ProcessState *os.ProcessState
+
+	ctx             context.Context // nil means none
+	lookPathErr     error           // LookPath error, if any.
+	finished        bool            // when Wait was called
+	childFiles      []*os.File
+	closeAfterStart []io.Closer
+	closeAfterWait  []io.Closer
+	goroutine       []func() error
+	errch           chan error // one send per goroutine
+	waitDone        chan struct{}
+}
+
+// Command returns the Cmd struct to execute the named program with
+// the given arguments.
+//
+// It sets only the Path and Args in the returned structure.
+//
+// If name contains no path separators, Command uses LookPath to
+// resolve name to a complete path if possible. Otherwise it uses name
+// directly as Path.
+//
+// The returned Cmd's Args field is constructed from the command name
+// followed by the elements of arg, so arg should not include the
+// command name itself. For example, Command("echo", "hello").
+// Args[0] is always name, not the possibly resolved Path.
+//
+// On Windows, processes receive the whole command line as a single string
+// and do their own parsing. Command combines and quotes Args into a command
+// line string with an algorithm compatible with applications using
+// CommandLineToArgvW (which is the most common way). Notable exceptions are
+// msiexec.exe and cmd.exe (and thus, all batch files), which have a different
+// unquoting algorithm. In these or other similar cases, you can do the
+// quoting yourself and provide the full command line in SysProcAttr.CmdLine,
+// leaving Args empty.
+func Command(name string, arg ...string) *Cmd {
+	cmd := &Cmd{
+		Path: name,
+		Args: append([]string{name}, arg...),
+	}
+	if filepath.Base(name) == name {
+		if lp, err := LookPath(name); err != nil {
+			cmd.lookPathErr = err
+		} else {
+			cmd.Path = lp
+		}
+	}
+	return cmd
+}
+
+// CommandContext is like Command but includes a context.
+//
+// The provided context is used to kill the process (by calling
+// os.Process.Kill) if the context becomes done before the command
+// completes on its own.
+func CommandContext(ctx context.Context, name string, arg ...string) *Cmd {
+	if ctx == nil {
+		panic("nil Context")
+	}
+	cmd := Command(name, arg...)
+	cmd.ctx = ctx
+	return cmd
+}
+
+// String returns a human-readable description of c.
+// It is intended only for debugging.
+// In particular, it is not suitable for use as input to a shell.
+// The output of String may vary across Go releases.
+func (c *Cmd) String() string {
+	if c.lookPathErr != nil {
+		// failed to resolve path; report the original requested path (plus args)
+		return strings.Join(c.Args, " ")
+	}
+	// report the exact executable path (plus args)
+	b := new(strings.Builder)
+	b.WriteString(c.Path)
+	for _, a := range c.Args[1:] {
+		b.WriteByte(' ')
+		b.WriteString(a)
+	}
+	return b.String()
+}
+
+// interfaceEqual protects against panics from doing equality tests on
+// two interfaces with non-comparable underlying types.
+func interfaceEqual(a, b interface{}) bool {
+	defer func() {
+		recover()
+	}()
+	return a == b
+}
+
+func (c *Cmd) envv() ([]string, error) {
+	if c.Env != nil {
+		return c.Env, nil
+	}
+	return execenv.Default(c.SysProcAttr)
+}
+
+func (c *Cmd) argv() []string {
+	if len(c.Args) > 0 {
+		return c.Args
+	}
+	return []string{c.Path}
+}
+
+// skipStdinCopyError optionally specifies a function which reports
+// whether the provided stdin copy error should be ignored.
+var skipStdinCopyError func(error) bool
+
+func (c *Cmd) stdin() (f *os.File, err error) {
+	if c.Stdin == nil {
+		f, err = os.Open(os.DevNull)
+		if err != nil {
+			return
+		}
+		c.closeAfterStart = append(c.closeAfterStart, f)
+		return
+	}
+
+	if f, ok := c.Stdin.(*os.File); ok {
+		return f, nil
+	}
+
+	pr, pw, err := os.Pipe()
+	if err != nil {
+		return
+	}
+
+	c.closeAfterStart = append(c.closeAfterStart, pr)
+	c.closeAfterWait = append(c.closeAfterWait, pw)
+	c.goroutine = append(c.goroutine, func() error {
+		_, err := io.Copy(pw, c.Stdin)
+		if skip := skipStdinCopyError; skip != nil && skip(err) {
+			err = nil
+		}
+		if err1 := pw.Close(); err == nil {
+			err = err1
+		}
+		return err
+	})
+	return pr, nil
+}
+
+func (c *Cmd) stdout() (f *os.File, err error) {
+	return c.writerDescriptor(c.Stdout)
+}
+
+func (c *Cmd) stderr() (f *os.File, err error) {
+	if c.Stderr != nil && interfaceEqual(c.Stderr, c.Stdout) {
+		return c.childFiles[1], nil
+	}
+	return c.writerDescriptor(c.Stderr)
+}
+
+func (c *Cmd) writerDescriptor(w io.Writer) (f *os.File, err error) {
+	if w == nil {
+		f, err = os.OpenFile(os.DevNull, os.O_WRONLY, 0)
+		if err != nil {
+			return
+		}
+		c.closeAfterStart = append(c.closeAfterStart, f)
+		return
+	}
+
+	if f, ok := w.(*os.File); ok {
+		return f, nil
+	}
+
+	pr, pw, err := os.Pipe()
+	if err != nil {
+		return
+	}
+
+	c.closeAfterStart = append(c.closeAfterStart, pw)
+	c.closeAfterWait = append(c.closeAfterWait, pr)
+	c.goroutine = append(c.goroutine, func() error {
+		_, err := io.Copy(w, pr)
+		pr.Close() // in case io.Copy stopped due to write error
+		return err
+	})
+	return pw, nil
+}
+
+func (c *Cmd) closeDescriptors(closers []io.Closer) {
+	for _, fd := range closers {
+		fd.Close()
+	}
+}
+
+// Run starts the specified command and waits for it to complete.
+//
+// The returned error is nil if the command runs, has no problems
+// copying stdin, stdout, and stderr, and exits with a zero exit
+// status.
+//
+// If the command starts but does not complete successfully, the error is of
+// type *ExitError. Other error types may be returned for other situations.
+//
+// If the calling goroutine has locked the operating system thread
+// with runtime.LockOSThread and modified any inheritable OS-level
+// thread state (for example, Linux or Plan 9 name spaces), the new
+// process will inherit the caller's thread state.
+func (c *Cmd) Run() error {
+	if err := c.Start(); err != nil {
+		return err
+	}
+	return c.Wait()
+}
+
+// lookExtensions finds windows executable by its dir and path.
+// It uses LookPath to try appropriate extensions.
+// lookExtensions does not search PATH, instead it converts `prog` into `.\prog`.
+func lookExtensions(path, dir string) (string, error) {
+	if filepath.Base(path) == path {
+		path = filepath.Join(".", path)
+	}
+	if dir == "" {
+		return LookPath(path)
+	}
+	if filepath.VolumeName(path) != "" {
+		return LookPath(path)
+	}
+	if len(path) > 1 && os.IsPathSeparator(path[0]) {
+		return LookPath(path)
+	}
+	dirandpath := filepath.Join(dir, path)
+	// We assume that LookPath will only add file extension.
+	lp, err := LookPath(dirandpath)
+	if err != nil {
+		return "", err
+	}
+	ext := strings.TrimPrefix(lp, dirandpath)
+	return path + ext, nil
+}
+
+// Start starts the specified command but does not wait for it to complete.
+//
+// If Start returns successfully, the c.Process field will be set.
+//
+// The Wait method will return the exit code and release associated resources
+// once the command exits.
+func (c *Cmd) Start() error {
+	if c.lookPathErr != nil {
+		c.closeDescriptors(c.closeAfterStart)
+		c.closeDescriptors(c.closeAfterWait)
+		return c.lookPathErr
+	}
+	if runtime.GOOS == "windows" {
+		lp, err := lookExtensions(c.Path, c.Dir)
+		if err != nil {
+			c.closeDescriptors(c.closeAfterStart)
+			c.closeDescriptors(c.closeAfterWait)
+			return err
+		}
+		c.Path = lp
+	}
+	if c.Process != nil {
+		return errors.New("exec: already started")
+	}
+	if c.ctx != nil {
+		select {
+		case <-c.ctx.Done():
+			c.closeDescriptors(c.closeAfterStart)
+			c.closeDescriptors(c.closeAfterWait)
+			return c.ctx.Err()
+		default:
+		}
+	}
+
+	c.childFiles = make([]*os.File, 0, 3+len(c.ExtraFiles))
+	type F func(*Cmd) (*os.File, error)
+	for _, setupFd := range []F{(*Cmd).stdin, (*Cmd).stdout, (*Cmd).stderr} {
+		fd, err := setupFd(c)
+		if err != nil {
+			c.closeDescriptors(c.closeAfterStart)
+			c.closeDescriptors(c.closeAfterWait)
+			return err
+		}
+		c.childFiles = append(c.childFiles, fd)
+	}
+	c.childFiles = append(c.childFiles, c.ExtraFiles...)
+
+	envv, err := c.envv()
+	if err != nil {
+		return err
+	}
+
+	c.Process, err = os.StartProcess(c.Path, c.argv(), &os.ProcAttr{
+		Dir:   c.Dir,
+		Files: c.childFiles,
+		Env:   addCriticalEnv(dedupEnv(envv)),
+		Sys:   c.SysProcAttr,
+	})
+	if err != nil {
+		c.closeDescriptors(c.closeAfterStart)
+		c.closeDescriptors(c.closeAfterWait)
+		return err
+	}
+
+	c.closeDescriptors(c.closeAfterStart)
+
+	// Don't allocate the channel unless there are goroutines to fire.
+	if len(c.goroutine) > 0 {
+		c.errch = make(chan error, len(c.goroutine))
+		for _, fn := range c.goroutine {
+			go func(fn func() error) {
+				c.errch <- fn()
+			}(fn)
+		}
+	}
+
+	if c.ctx != nil {
+		c.waitDone = make(chan struct{})
+		go func() {
+			select {
+			case <-c.ctx.Done():
+				c.Process.Kill()
+			case <-c.waitDone:
+			}
+		}()
+	}
+
+	return nil
+}
+
+// An ExitError reports an unsuccessful exit by a command.
+type ExitError struct {
+	*os.ProcessState
+
+	// Stderr holds a subset of the standard error output from the
+	// Cmd.Output method if standard error was not otherwise being
+	// collected.
+	//
+	// If the error output is long, Stderr may contain only a prefix
+	// and suffix of the output, with the middle replaced with
+	// text about the number of omitted bytes.
+	//
+	// Stderr is provided for debugging, for inclusion in error messages.
+	// Users with other needs should redirect Cmd.Stderr as needed.
+	Stderr []byte
+}
+
+func (e *ExitError) Error() string {
+	return e.ProcessState.String()
+}
+
+// Wait waits for the command to exit and waits for any copying to
+// stdin or copying from stdout or stderr to complete.
+//
+// The command must have been started by Start.
+//
+// The returned error is nil if the command runs, has no problems
+// copying stdin, stdout, and stderr, and exits with a zero exit
+// status.
+//
+// If the command fails to run or doesn't complete successfully, the
+// error is of type *ExitError. Other error types may be
+// returned for I/O problems.
+//
+// If any of c.Stdin, c.Stdout or c.Stderr are not an *os.File, Wait also waits
+// for the respective I/O loop copying to or from the process to complete.
+//
+// Wait releases any resources associated with the Cmd.
+func (c *Cmd) Wait() error {
+	if c.Process == nil {
+		return errors.New("exec: not started")
+	}
+	if c.finished {
+		return errors.New("exec: Wait was already called")
+	}
+	c.finished = true
+
+	state, err := c.Process.Wait()
+	if c.waitDone != nil {
+		close(c.waitDone)
+	}
+	c.ProcessState = state
+
+	var copyError error
+	for range c.goroutine {
+		if err := <-c.errch; err != nil && copyError == nil {
+			copyError = err
+		}
+	}
+
+	c.closeDescriptors(c.closeAfterWait)
+
+	if err != nil {
+		return err
+	} else if !state.Success() {
+		return &ExitError{ProcessState: state}
+	}
+
+	return copyError
+}
+
+// Output runs the command and returns its standard output.
+// Any returned error will usually be of type *ExitError.
+// If c.Stderr was nil, Output populates ExitError.Stderr.
+func (c *Cmd) Output() ([]byte, error) {
+	if c.Stdout != nil {
+		return nil, errors.New("exec: Stdout already set")
+	}
+	var stdout bytes.Buffer
+	c.Stdout = &stdout
+
+	captureErr := c.Stderr == nil
+	if captureErr {
+		c.Stderr = &prefixSuffixSaver{N: 32 << 10}
+	}
+
+	err := c.Run()
+	if err != nil && captureErr {
+		if ee, ok := err.(*ExitError); ok {
+			ee.Stderr = c.Stderr.(*prefixSuffixSaver).Bytes()
+		}
+	}
+	return stdout.Bytes(), err
+}
+
+// CombinedOutput runs the command and returns its combined standard
+// output and standard error.
+func (c *Cmd) CombinedOutput() ([]byte, error) {
+	if c.Stdout != nil {
+		return nil, errors.New("exec: Stdout already set")
+	}
+	if c.Stderr != nil {
+		return nil, errors.New("exec: Stderr already set")
+	}
+	var b bytes.Buffer
+	c.Stdout = &b
+	c.Stderr = &b
+	err := c.Run()
+	return b.Bytes(), err
+}
+
+// StdinPipe returns a pipe that will be connected to the command's
+// standard input when the command starts.
+// The pipe will be closed automatically after Wait sees the command exit.
+// A caller need only call Close to force the pipe to close sooner.
+// For example, if the command being run will not exit until standard input
+// is closed, the caller must close the pipe.
+func (c *Cmd) StdinPipe() (io.WriteCloser, error) {
+	if c.Stdin != nil {
+		return nil, errors.New("exec: Stdin already set")
+	}
+	if c.Process != nil {
+		return nil, errors.New("exec: StdinPipe after process started")
+	}
+	pr, pw, err := os.Pipe()
+	if err != nil {
+		return nil, err
+	}
+	c.Stdin = pr
+	c.closeAfterStart = append(c.closeAfterStart, pr)
+	wc := &closeOnce{File: pw}
+	c.closeAfterWait = append(c.closeAfterWait, wc)
+	return wc, nil
+}
+
+type closeOnce struct {
+	*os.File
+
+	once sync.Once
+	err  error
+}
+
+func (c *closeOnce) Close() error {
+	c.once.Do(c.close)
+	return c.err
+}
+
+func (c *closeOnce) close() {
+	c.err = c.File.Close()
+}
+
+// StdoutPipe returns a pipe that will be connected to the command's
+// standard output when the command starts.
+//
+// Wait will close the pipe after seeing the command exit, so most callers
+// need not close the pipe themselves. It is thus incorrect to call Wait
+// before all reads from the pipe have completed.
+// For the same reason, it is incorrect to call Run when using StdoutPipe.
+// See the example for idiomatic usage.
+func (c *Cmd) StdoutPipe() (io.ReadCloser, error) {
+	if c.Stdout != nil {
+		return nil, errors.New("exec: Stdout already set")
+	}
+	if c.Process != nil {
+		return nil, errors.New("exec: StdoutPipe after process started")
+	}
+	pr, pw, err := os.Pipe()
+	if err != nil {
+		return nil, err
+	}
+	c.Stdout = pw
+	c.closeAfterStart = append(c.closeAfterStart, pw)
+	c.closeAfterWait = append(c.closeAfterWait, pr)
+	return pr, nil
+}
+
+// StderrPipe returns a pipe that will be connected to the command's
+// standard error when the command starts.
+//
+// Wait will close the pipe after seeing the command exit, so most callers
+// need not close the pipe themselves. It is thus incorrect to call Wait
+// before all reads from the pipe have completed.
+// For the same reason, it is incorrect to use Run when using StderrPipe.
+// See the StdoutPipe example for idiomatic usage.
+func (c *Cmd) StderrPipe() (io.ReadCloser, error) {
+	if c.Stderr != nil {
+		return nil, errors.New("exec: Stderr already set")
+	}
+	if c.Process != nil {
+		return nil, errors.New("exec: StderrPipe after process started")
+	}
+	pr, pw, err := os.Pipe()
+	if err != nil {
+		return nil, err
+	}
+	c.Stderr = pw
+	c.closeAfterStart = append(c.closeAfterStart, pw)
+	c.closeAfterWait = append(c.closeAfterWait, pr)
+	return pr, nil
+}
+
+// prefixSuffixSaver is an io.Writer which retains the first N bytes
+// and the last N bytes written to it. The Bytes() methods reconstructs
+// it with a pretty error message.
+type prefixSuffixSaver struct {
+	N         int // max size of prefix or suffix
+	prefix    []byte
+	suffix    []byte // ring buffer once len(suffix) == N
+	suffixOff int    // offset to write into suffix
+	skipped   int64
+
+	// TODO(bradfitz): we could keep one large []byte and use part of it for
+	// the prefix, reserve space for the '... Omitting N bytes ...' message,
+	// then the ring buffer suffix, and just rearrange the ring buffer
+	// suffix when Bytes() is called, but it doesn't seem worth it for
+	// now just for error messages. It's only ~64KB anyway.
+}
+
+func (w *prefixSuffixSaver) Write(p []byte) (n int, err error) {
+	lenp := len(p)
+	p = w.fill(&w.prefix, p)
+
+	// Only keep the last w.N bytes of suffix data.
+	if overage := len(p) - w.N; overage > 0 {
+		p = p[overage:]
+		w.skipped += int64(overage)
+	}
+	p = w.fill(&w.suffix, p)
+
+	// w.suffix is full now if p is non-empty. Overwrite it in a circle.
+	for len(p) > 0 { // 0, 1, or 2 iterations.
+		n := copy(w.suffix[w.suffixOff:], p)
+		p = p[n:]
+		w.skipped += int64(n)
+		w.suffixOff += n
+		if w.suffixOff == w.N {
+			w.suffixOff = 0
+		}
+	}
+	return lenp, nil
+}
+
+// fill appends up to len(p) bytes of p to *dst, such that *dst does not
+// grow larger than w.N. It returns the un-appended suffix of p.
+func (w *prefixSuffixSaver) fill(dst *[]byte, p []byte) (pRemain []byte) {
+	if remain := w.N - len(*dst); remain > 0 {
+		add := minInt(len(p), remain)
+		*dst = append(*dst, p[:add]...)
+		p = p[add:]
+	}
+	return p
+}
+
+func (w *prefixSuffixSaver) Bytes() []byte {
+	if w.suffix == nil {
+		return w.prefix
+	}
+	if w.skipped == 0 {
+		return append(w.prefix, w.suffix...)
+	}
+	var buf bytes.Buffer
+	buf.Grow(len(w.prefix) + len(w.suffix) + 50)
+	buf.Write(w.prefix)
+	buf.WriteString("\n... omitting ")
+	buf.WriteString(strconv.FormatInt(w.skipped, 10))
+	buf.WriteString(" bytes ...\n")
+	buf.Write(w.suffix[w.suffixOff:])
+	buf.Write(w.suffix[:w.suffixOff])
+	return buf.Bytes()
+}
+
+func minInt(a, b int) int {
+	if a < b {
+		return a
+	}
+	return b
+}
+
+// dedupEnv returns a copy of env with any duplicates removed, in favor of
+// later values.
+// Items not of the normal environment "key=value" form are preserved unchanged.
+func dedupEnv(env []string) []string {
+	return dedupEnvCase(runtime.GOOS == "windows", env)
+}
+
+// dedupEnvCase is dedupEnv with a case option for testing.
+// If caseInsensitive is true, the case of keys is ignored.
+func dedupEnvCase(caseInsensitive bool, env []string) []string {
+	out := make([]string, 0, len(env))
+	saw := make(map[string]int, len(env)) // key => index into out
+	for _, kv := range env {
+		eq := strings.Index(kv, "=")
+		if eq < 0 {
+			out = append(out, kv)
+			continue
+		}
+		k := kv[:eq]
+		if caseInsensitive {
+			k = strings.ToLower(k)
+		}
+		if dupIdx, isDup := saw[k]; isDup {
+			out[dupIdx] = kv
+			continue
+		}
+		saw[k] = len(out)
+		out = append(out, kv)
+	}
+	return out
+}
+
+// addCriticalEnv adds any critical environment variables that are required
+// (or at least almost always required) on the operating system.
+// Currently this is only used for Windows.
+func addCriticalEnv(env []string) []string {
+	if runtime.GOOS != "windows" {
+		return env
+	}
+	for _, kv := range env {
+		eq := strings.Index(kv, "=")
+		if eq < 0 {
+			continue
+		}
+		k := kv[:eq]
+		if strings.EqualFold(k, "SYSTEMROOT") {
+			// We already have it.
+			return env
+		}
+	}
+	return append(env, "SYSTEMROOT="+os.Getenv("SYSTEMROOT"))
+}
diff --git a/src/os/exec/exec_linux_test.go b/src/os/exec/exec_linux_test.go
new file mode 100644
index 0000000..6f85020
--- /dev/null
+++ b/src/os/exec/exec_linux_test.go
@@ -0,0 +1,45 @@
+// Copyright 2020 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 linux,cgo
+
+// On systems that use glibc, calling malloc can create a new arena,
+// and creating a new arena can read /sys/devices/system/cpu/online.
+// If we are using cgo, we will call malloc when creating a new thread.
+// That can break TestExtraFiles if we create a new thread that creates
+// a new arena and opens the /sys file while we are checking for open
+// file descriptors. Work around the problem by creating threads up front.
+// See issue 25628.
+
+package exec_test
+
+import (
+	"os"
+	"sync"
+	"syscall"
+	"time"
+)
+
+func init() {
+	if os.Getenv("GO_WANT_HELPER_PROCESS") != "1" {
+		return
+	}
+
+	// Start some threads. 10 is arbitrary but intended to be enough
+	// to ensure that the code won't have to create any threads itself.
+	// In particular this should be more than the number of threads
+	// the garbage collector might create.
+	const threads = 10
+
+	var wg sync.WaitGroup
+	wg.Add(threads)
+	ts := syscall.NsecToTimespec((100 * time.Microsecond).Nanoseconds())
+	for i := 0; i < threads; i++ {
+		go func() {
+			defer wg.Done()
+			syscall.Nanosleep(&ts, nil)
+		}()
+	}
+	wg.Wait()
+}
diff --git a/src/os/exec/exec_plan9.go b/src/os/exec/exec_plan9.go
new file mode 100644
index 0000000..21ac7b7
--- /dev/null
+++ b/src/os/exec/exec_plan9.go
@@ -0,0 +1,19 @@
+// Copyright 2019 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.
+
+package exec
+
+import "io/fs"
+
+func init() {
+	skipStdinCopyError = func(err error) bool {
+		// Ignore hungup errors copying to stdin if the program
+		// completed successfully otherwise.
+		// See Issue 35753.
+		pe, ok := err.(*fs.PathError)
+		return ok &&
+			pe.Op == "write" && pe.Path == "|1" &&
+			pe.Err.Error() == "i/o on hungup channel"
+	}
+}
diff --git a/src/os/exec/exec_posix_test.go b/src/os/exec/exec_posix_test.go
new file mode 100644
index 0000000..d4d67ac
--- /dev/null
+++ b/src/os/exec/exec_posix_test.go
@@ -0,0 +1,88 @@
+// Copyright 2017 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 aix darwin dragonfly freebsd linux netbsd openbsd solaris
+
+package exec_test
+
+import (
+	"os/user"
+	"runtime"
+	"strconv"
+	"syscall"
+	"testing"
+	"time"
+)
+
+func TestCredentialNoSetGroups(t *testing.T) {
+	if runtime.GOOS == "android" {
+		t.Skip("unsupported on Android")
+	}
+
+	u, err := user.Current()
+	if err != nil {
+		t.Fatalf("error getting current user: %v", err)
+	}
+
+	uid, err := strconv.Atoi(u.Uid)
+	if err != nil {
+		t.Fatalf("error converting Uid=%s to integer: %v", u.Uid, err)
+	}
+
+	gid, err := strconv.Atoi(u.Gid)
+	if err != nil {
+		t.Fatalf("error converting Gid=%s to integer: %v", u.Gid, err)
+	}
+
+	// If NoSetGroups is true, setgroups isn't called and cmd.Run should succeed
+	cmd := helperCommand(t, "echo", "foo")
+	cmd.SysProcAttr = &syscall.SysProcAttr{
+		Credential: &syscall.Credential{
+			Uid:         uint32(uid),
+			Gid:         uint32(gid),
+			NoSetGroups: true,
+		},
+	}
+
+	if err = cmd.Run(); err != nil {
+		t.Errorf("Failed to run command: %v", err)
+	}
+}
+
+// For issue #19314: make sure that SIGSTOP does not cause the process
+// to appear done.
+func TestWaitid(t *testing.T) {
+	t.Parallel()
+
+	cmd := helperCommand(t, "sleep")
+	if err := cmd.Start(); err != nil {
+		t.Fatal(err)
+	}
+
+	// The sleeps here are unnecessary in the sense that the test
+	// should still pass, but they are useful to make it more
+	// likely that we are testing the expected state of the child.
+	time.Sleep(100 * time.Millisecond)
+
+	if err := cmd.Process.Signal(syscall.SIGSTOP); err != nil {
+		cmd.Process.Kill()
+		t.Fatal(err)
+	}
+
+	ch := make(chan error)
+	go func() {
+		ch <- cmd.Wait()
+	}()
+
+	time.Sleep(100 * time.Millisecond)
+
+	if err := cmd.Process.Signal(syscall.SIGCONT); err != nil {
+		t.Error(err)
+		syscall.Kill(cmd.Process.Pid, syscall.SIGCONT)
+	}
+
+	cmd.Process.Kill()
+
+	<-ch
+}
diff --git a/src/os/exec/exec_test.go b/src/os/exec/exec_test.go
new file mode 100644
index 0000000..8b0c93f
--- /dev/null
+++ b/src/os/exec/exec_test.go
@@ -0,0 +1,1176 @@
+// 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"
+	"log"
+	"net"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"runtime"
+	"strconv"
+	"strings"
+	"testing"
+	"time"
+)
+
+// haveUnexpectedFDs is set at init time to report whether any
+// file descriptors were open at program start.
+var haveUnexpectedFDs bool
+
+// unfinalizedFiles holds files that should not be finalized,
+// because that would close the associated file descriptor,
+// which we don't want to do.
+var unfinalizedFiles []*os.File
+
+func init() {
+	if os.Getenv("GO_WANT_HELPER_PROCESS") == "1" {
+		return
+	}
+	if runtime.GOOS == "windows" {
+		return
+	}
+	for fd := uintptr(3); fd <= 100; fd++ {
+		if poll.IsPollDescriptor(fd) {
+			continue
+		}
+		// We have no good portable way to check whether an FD is open.
+		// We use NewFile to create a *os.File, which lets us
+		// know whether it is open, but then we have to cope with
+		// the finalizer on the *os.File.
+		f := os.NewFile(fd, "")
+		if _, err := f.Stat(); err != nil {
+			// Close the file to clear the finalizer.
+			// We expect the Close to fail.
+			f.Close()
+		} else {
+			fmt.Printf("fd %d open at test start\n", fd)
+			haveUnexpectedFDs = true
+			// Use a global variable to avoid running
+			// the finalizer, which would close the FD.
+			unfinalizedFiles = append(unfinalizedFiles, f)
+		}
+	}
+}
+
+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 = append(os.Environ(), "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 TestNoExistExecutable(t *testing.T) {
+	// Can't run a non-existent executable
+	err := exec.Command("/no-exist-executable").Run()
+	if err == nil {
+		t.Error("expected error from /no-exist-executable")
+	}
+}
+
+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 TestExitCode(t *testing.T) {
+	// Test that exit code are returned correctly
+	cmd := helperCommand(t, "exit", "42")
+	cmd.Run()
+	want := 42
+	if runtime.GOOS == "plan9" {
+		want = 1
+	}
+	got := cmd.ProcessState.ExitCode()
+	if want != got {
+		t.Errorf("ExitCode got %d, want %d", got, want)
+	}
+
+	cmd = helperCommand(t, "/no-exist-executable")
+	cmd.Run()
+	want = 2
+	if runtime.GOOS == "plan9" {
+		want = 1
+	}
+	got = cmd.ProcessState.ExitCode()
+	if want != got {
+		t.Errorf("ExitCode got %d, want %d", got, want)
+	}
+
+	cmd = helperCommand(t, "exit", "255")
+	cmd.Run()
+	want = 255
+	if runtime.GOOS == "plan9" {
+		want = 1
+	}
+	got = cmd.ProcessState.ExitCode()
+	if want != got {
+		t.Errorf("ExitCode got %d, want %d", got, want)
+	}
+
+	cmd = helperCommand(t, "cat")
+	cmd.Run()
+	want = 0
+	got = cmd.ProcessState.ExitCode()
+	if want != got {
+		t.Errorf("ExitCode got %d, want %d", got, want)
+	}
+
+	// Test when command does not call Run().
+	cmd = helperCommand(t, "cat")
+	want = -1
+	got = cmd.ProcessState.ExitCode()
+	if want != got {
+		t.Errorf("ExitCode got %d, want %d", got, want)
+	}
+}
+
+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 := os.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-executable")
+		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
+}
+
+func TestExtraFilesFDShuffle(t *testing.T) {
+	t.Skip("flaky test; see https://golang.org/issue/5780")
+	switch runtime.GOOS {
+	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) {
+	if haveUnexpectedFDs {
+		// The point of this test is to make sure that any
+		// descriptors we open are marked close-on-exec.
+		// If haveUnexpectedFDs is true then there were other
+		// descriptors open when we started the test,
+		// so those descriptors are clearly not close-on-exec,
+		// and they will confuse the test. We could modify
+		// the test to expect those descriptors to remain open,
+		// but since we don't know where they came from or what
+		// they are doing, that seems fragile. For example,
+		// perhaps they are from the startup code on this
+		// system for some reason. Also, this test is not
+		// system-specific; as long as most systems do not skip
+		// the test, we will still be testing what we care about.
+		t.Skip("skipping test because test was run with FDs open")
+	}
+
+	testenv.MustHaveExec(t)
+	testenv.MustHaveGoBuild(t)
+
+	// This test runs with cgo disabled. External linking needs cgo, so
+	// it doesn't work if external linking is required.
+	testenv.MustInternalLink(t)
+
+	if runtime.GOOS == "windows" {
+		t.Skipf("skipping test on %q", runtime.GOOS)
+	}
+
+	// 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(io.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 := os.CreateTemp("", "")
+	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)
+	}
+
+	tempdir := t.TempDir()
+	exe := filepath.Join(tempdir, "read3.exe")
+
+	c := exec.Command(testenv.GoToolPath(t), "build", "-o", exe, "read3.go")
+	// Build the test without cgo, so that C library functions don't
+	// open descriptors unexpectedly. See issue 25628.
+	c.Env = append(os.Environ(), "CGO_ENABLED=0")
+	if output, err := c.CombinedOutput(); err != nil {
+		t.Logf("go build -o %s read3.go\n%s", exe, output)
+		t.Fatalf("go build failed: %v", err)
+	}
+
+	// Use a deadline to try to get some output even if the program hangs.
+	ctx := context.Background()
+	if deadline, ok := t.Deadline(); ok {
+		// Leave a 20% grace period to flush output, which may be large on the
+		// linux/386 builders because we're running the subprocess under strace.
+		deadline = deadline.Add(-time.Until(deadline) / 5)
+
+		var cancel context.CancelFunc
+		ctx, cancel = context.WithDeadline(ctx, deadline)
+		defer cancel()
+	}
+
+	c = exec.CommandContext(ctx, exe)
+	var stdout, stderr bytes.Buffer
+	c.Stdout = &stdout
+	c.Stderr = &stderr
+	c.ExtraFiles = []*os.File{tf}
+	if runtime.GOOS == "illumos" {
+		// Some facilities in illumos are implemented via access
+		// to /proc by libc; such accesses can briefly occupy a
+		// low-numbered fd.  If this occurs concurrently with the
+		// test that checks for leaked descriptors, the check can
+		// become confused and report a spurious leaked descriptor.
+		// (See issue #42431 for more detailed analysis.)
+		//
+		// Attempt to constrain the use of additional threads in the
+		// child process to make this test less flaky:
+		c.Env = append(os.Environ(), "GOMAXPROCS=1")
+	}
+	err = c.Run()
+	if err != nil {
+		t.Fatalf("Run: %v\n--- stdout:\n%s--- stderr:\n%s", 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++ {
+		if testing.Short() && i >= 3 {
+			break
+		}
+		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)
+
+	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 := io.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 "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)
+
+	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) {
+	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)
+	}
+}
+
+func TestString(t *testing.T) {
+	echoPath, err := exec.LookPath("echo")
+	if err != nil {
+		t.Skip(err)
+	}
+	tests := [...]struct {
+		path string
+		args []string
+		want string
+	}{
+		{"echo", nil, echoPath},
+		{"echo", []string{"a"}, echoPath + " a"},
+		{"echo", []string{"a", "b"}, echoPath + " a b"},
+	}
+	for _, test := range tests {
+		cmd := exec.Command(test.path, test.args...)
+		if got := cmd.String(); got != test.want {
+			t.Errorf("String(%q, %q) = %q, want %q", test.path, test.args, got, test.want)
+		}
+	}
+}
+
+func TestStringPathNotResolved(t *testing.T) {
+	_, err := exec.LookPath("makemeasandwich")
+	if err == nil {
+		t.Skip("wow, thanks")
+	}
+	cmd := exec.Command("makemeasandwich", "-lettuce")
+	want := "makemeasandwich -lettuce"
+	if got := cmd.String(); got != want {
+		t.Errorf("String(%q, %q) = %q, want %q", "makemeasandwich", "-lettuce", got, want)
+	}
+}
+
+// start a child process without the user code explicitly starting
+// with a copy of the parent's. (The Windows SYSTEMROOT issue: Issue
+// 25210)
+func TestChildCriticalEnv(t *testing.T) {
+	testenv.MustHaveExec(t)
+	if runtime.GOOS != "windows" {
+		t.Skip("only testing on Windows")
+	}
+	cmd := helperCommand(t, "echoenv", "SYSTEMROOT")
+	cmd.Env = []string{"GO_WANT_HELPER_PROCESS=1"}
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if strings.TrimSpace(string(out)) == "" {
+		t.Error("no SYSTEMROOT found")
+	}
+}
diff --git a/src/os/exec/exec_unix.go b/src/os/exec/exec_unix.go
new file mode 100644
index 0000000..51c5242
--- /dev/null
+++ b/src/os/exec/exec_unix.go
@@ -0,0 +1,24 @@
+// Copyright 2015 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 !plan9,!windows
+
+package exec
+
+import (
+	"io/fs"
+	"syscall"
+)
+
+func init() {
+	skipStdinCopyError = func(err error) bool {
+		// Ignore EPIPE errors copying to stdin if the program
+		// completed successfully otherwise.
+		// See Issue 9173.
+		pe, ok := err.(*fs.PathError)
+		return ok &&
+			pe.Op == "write" && pe.Path == "|1" &&
+			pe.Err == syscall.EPIPE
+	}
+}
diff --git a/src/os/exec/exec_windows.go b/src/os/exec/exec_windows.go
new file mode 100644
index 0000000..bb937f8
--- /dev/null
+++ b/src/os/exec/exec_windows.go
@@ -0,0 +1,23 @@
+// Copyright 2017 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.
+
+package exec
+
+import (
+	"io/fs"
+	"syscall"
+)
+
+func init() {
+	skipStdinCopyError = func(err error) bool {
+		// Ignore ERROR_BROKEN_PIPE and ERROR_NO_DATA errors copying
+		// to stdin if the program completed successfully otherwise.
+		// See Issue 20445.
+		const _ERROR_NO_DATA = syscall.Errno(0xe8)
+		pe, ok := err.(*fs.PathError)
+		return ok &&
+			pe.Op == "write" && pe.Path == "|1" &&
+			(pe.Err == syscall.ERROR_BROKEN_PIPE || pe.Err == _ERROR_NO_DATA)
+	}
+}
diff --git a/src/os/exec/internal_test.go b/src/os/exec/internal_test.go
new file mode 100644
index 0000000..68d517f
--- /dev/null
+++ b/src/os/exec/internal_test.go
@@ -0,0 +1,61 @@
+// Copyright 2015 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.
+
+package exec
+
+import (
+	"io"
+	"testing"
+)
+
+func TestPrefixSuffixSaver(t *testing.T) {
+	tests := []struct {
+		N      int
+		writes []string
+		want   string
+	}{
+		{
+			N:      2,
+			writes: nil,
+			want:   "",
+		},
+		{
+			N:      2,
+			writes: []string{"a"},
+			want:   "a",
+		},
+		{
+			N:      2,
+			writes: []string{"abc", "d"},
+			want:   "abcd",
+		},
+		{
+			N:      2,
+			writes: []string{"abc", "d", "e"},
+			want:   "ab\n... omitting 1 bytes ...\nde",
+		},
+		{
+			N:      2,
+			writes: []string{"ab______________________yz"},
+			want:   "ab\n... omitting 22 bytes ...\nyz",
+		},
+		{
+			N:      2,
+			writes: []string{"ab_______________________y", "z"},
+			want:   "ab\n... omitting 23 bytes ...\nyz",
+		},
+	}
+	for i, tt := range tests {
+		w := &prefixSuffixSaver{N: tt.N}
+		for _, s := range tt.writes {
+			n, err := io.WriteString(w, s)
+			if err != nil || n != len(s) {
+				t.Errorf("%d. WriteString(%q) = %v, %v; want %v, %v", i, s, n, err, len(s), nil)
+			}
+		}
+		if got := string(w.Bytes()); got != tt.want {
+			t.Errorf("%d. Bytes = %q; want %q", i, got, tt.want)
+		}
+	}
+}
diff --git a/src/os/exec/lp_js.go b/src/os/exec/lp_js.go
new file mode 100644
index 0000000..6750fb9
--- /dev/null
+++ b/src/os/exec/lp_js.go
@@ -0,0 +1,23 @@
+// Copyright 2018 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 js,wasm
+
+package exec
+
+import (
+	"errors"
+)
+
+// ErrNotFound is the error resulting if a path search failed to find an executable file.
+var ErrNotFound = errors.New("executable file not found in $PATH")
+
+// LookPath searches for an executable named file in the
+// directories named by the PATH environment variable.
+// If file contains a slash, it is tried directly and the PATH is not consulted.
+// The result may be an absolute path or a path relative to the current directory.
+func LookPath(file string) (string, error) {
+	// Wasm can not execute processes, so act as if there are no executables at all.
+	return "", &Error{file, ErrNotFound}
+}
diff --git a/src/os/exec/lp_plan9.go b/src/os/exec/lp_plan9.go
new file mode 100644
index 0000000..e8826a5
--- /dev/null
+++ b/src/os/exec/lp_plan9.go
@@ -0,0 +1,56 @@
+// Copyright 2011 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.
+
+package exec
+
+import (
+	"errors"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// ErrNotFound is the error resulting if a path search failed to find an executable file.
+var ErrNotFound = errors.New("executable file not found in $path")
+
+func findExecutable(file string) error {
+	d, err := os.Stat(file)
+	if err != nil {
+		return err
+	}
+	if m := d.Mode(); !m.IsDir() && m&0111 != 0 {
+		return nil
+	}
+	return fs.ErrPermission
+}
+
+// LookPath searches for an executable named file in the
+// directories named by the path environment variable.
+// If file begins with "/", "#", "./", or "../", it is tried
+// directly and the path is not consulted.
+// The result may be an absolute path or a path relative to the current directory.
+func LookPath(file string) (string, error) {
+	// skip the path lookup for these prefixes
+	skip := []string{"/", "#", "./", "../"}
+
+	for _, p := range skip {
+		if strings.HasPrefix(file, p) {
+			err := findExecutable(file)
+			if err == nil {
+				return file, nil
+			}
+			return "", &Error{file, err}
+		}
+	}
+
+	path := os.Getenv("path")
+	for _, dir := range filepath.SplitList(path) {
+		path := filepath.Join(dir, file)
+		if err := findExecutable(path); err == nil {
+			return path, nil
+		}
+	}
+	return "", &Error{file, ErrNotFound}
+}
diff --git a/src/os/exec/lp_test.go b/src/os/exec/lp_test.go
new file mode 100644
index 0000000..77d8e84
--- /dev/null
+++ b/src/os/exec/lp_test.go
@@ -0,0 +1,33 @@
+// Copyright 2011 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.
+
+package exec
+
+import (
+	"testing"
+)
+
+var nonExistentPaths = []string{
+	"some-non-existent-path",
+	"non-existent-path/slashed",
+}
+
+func TestLookPathNotFound(t *testing.T) {
+	for _, name := range nonExistentPaths {
+		path, err := LookPath(name)
+		if err == nil {
+			t.Fatalf("LookPath found %q in $PATH", name)
+		}
+		if path != "" {
+			t.Fatalf("LookPath path == %q when err != nil", path)
+		}
+		perr, ok := err.(*Error)
+		if !ok {
+			t.Fatal("LookPath error is not an exec.Error")
+		}
+		if perr.Name != name {
+			t.Fatalf("want Error name %q, got %q", name, perr.Name)
+		}
+	}
+}
diff --git a/src/os/exec/lp_unix.go b/src/os/exec/lp_unix.go
new file mode 100644
index 0000000..66c1df7
--- /dev/null
+++ b/src/os/exec/lp_unix.go
@@ -0,0 +1,59 @@
+// Copyright 2010 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 aix darwin dragonfly freebsd linux netbsd openbsd solaris
+
+package exec
+
+import (
+	"errors"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// ErrNotFound is the error resulting if a path search failed to find an executable file.
+var ErrNotFound = errors.New("executable file not found in $PATH")
+
+func findExecutable(file string) error {
+	d, err := os.Stat(file)
+	if err != nil {
+		return err
+	}
+	if m := d.Mode(); !m.IsDir() && m&0111 != 0 {
+		return nil
+	}
+	return fs.ErrPermission
+}
+
+// LookPath searches for an executable named file in the
+// directories named by the PATH environment variable.
+// If file contains a slash, it is tried directly and the PATH is not consulted.
+// The result may be an absolute path or a path relative to the current directory.
+func LookPath(file string) (string, error) {
+	// NOTE(rsc): I wish we could use the Plan 9 behavior here
+	// (only bypass the path if file begins with / or ./ or ../)
+	// but that would not match all the Unix shells.
+
+	if strings.Contains(file, "/") {
+		err := findExecutable(file)
+		if err == nil {
+			return file, nil
+		}
+		return "", &Error{file, err}
+	}
+	path := os.Getenv("PATH")
+	for _, dir := range filepath.SplitList(path) {
+		if dir == "" {
+			// Unix shell semantics: path element "" means "."
+			dir = "."
+		}
+		path := filepath.Join(dir, file)
+		if err := findExecutable(path); err == nil {
+			return path, nil
+		}
+	}
+	return "", &Error{file, ErrNotFound}
+}
diff --git a/src/os/exec/lp_unix_test.go b/src/os/exec/lp_unix_test.go
new file mode 100644
index 0000000..296480f
--- /dev/null
+++ b/src/os/exec/lp_unix_test.go
@@ -0,0 +1,54 @@
+// Copyright 2013 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 aix darwin dragonfly freebsd linux netbsd openbsd solaris
+
+package exec
+
+import (
+	"os"
+	"testing"
+)
+
+func TestLookPathUnixEmptyPath(t *testing.T) {
+	tmp, err := os.MkdirTemp("", "TestLookPathUnixEmptyPath")
+	if err != nil {
+		t.Fatal("TempDir failed: ", err)
+	}
+	defer os.RemoveAll(tmp)
+	wd, err := os.Getwd()
+	if err != nil {
+		t.Fatal("Getwd failed: ", err)
+	}
+	err = os.Chdir(tmp)
+	if err != nil {
+		t.Fatal("Chdir failed: ", err)
+	}
+	defer os.Chdir(wd)
+
+	f, err := os.OpenFile("exec_me", os.O_CREATE|os.O_EXCL, 0700)
+	if err != nil {
+		t.Fatal("OpenFile failed: ", err)
+	}
+	err = f.Close()
+	if err != nil {
+		t.Fatal("Close failed: ", err)
+	}
+
+	pathenv := os.Getenv("PATH")
+	defer os.Setenv("PATH", pathenv)
+
+	err = os.Setenv("PATH", "")
+	if err != nil {
+		t.Fatal("Setenv failed: ", err)
+	}
+
+	path, err := LookPath("exec_me")
+	if err == nil {
+		t.Fatal("LookPath found exec_me in empty $PATH")
+	}
+	if path != "" {
+		t.Fatalf("LookPath path == %q when err != nil", path)
+	}
+}
diff --git a/src/os/exec/lp_windows.go b/src/os/exec/lp_windows.go
new file mode 100644
index 0000000..e7a2cdf
--- /dev/null
+++ b/src/os/exec/lp_windows.go
@@ -0,0 +1,94 @@
+// Copyright 2010 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.
+
+package exec
+
+import (
+	"errors"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// ErrNotFound is the error resulting if a path search failed to find an executable file.
+var ErrNotFound = errors.New("executable file not found in %PATH%")
+
+func chkStat(file string) error {
+	d, err := os.Stat(file)
+	if err != nil {
+		return err
+	}
+	if d.IsDir() {
+		return fs.ErrPermission
+	}
+	return nil
+}
+
+func hasExt(file string) bool {
+	i := strings.LastIndex(file, ".")
+	if i < 0 {
+		return false
+	}
+	return strings.LastIndexAny(file, `:\/`) < i
+}
+
+func findExecutable(file string, exts []string) (string, error) {
+	if len(exts) == 0 {
+		return file, chkStat(file)
+	}
+	if hasExt(file) {
+		if chkStat(file) == nil {
+			return file, nil
+		}
+	}
+	for _, e := range exts {
+		if f := file + e; chkStat(f) == nil {
+			return f, nil
+		}
+	}
+	return "", fs.ErrNotExist
+}
+
+// LookPath searches for an executable named file in the
+// directories named by the PATH environment variable.
+// If file contains a slash, it is tried directly and the PATH is not consulted.
+// LookPath also uses PATHEXT environment variable to match
+// a suitable candidate.
+// The result may be an absolute path or a path relative to the current directory.
+func LookPath(file string) (string, error) {
+	var exts []string
+	x := os.Getenv(`PATHEXT`)
+	if x != "" {
+		for _, e := range strings.Split(strings.ToLower(x), `;`) {
+			if e == "" {
+				continue
+			}
+			if e[0] != '.' {
+				e = "." + e
+			}
+			exts = append(exts, e)
+		}
+	} else {
+		exts = []string{".com", ".exe", ".bat", ".cmd"}
+	}
+
+	if strings.ContainsAny(file, `:\/`) {
+		if f, err := findExecutable(file, exts); err == nil {
+			return f, nil
+		} else {
+			return "", &Error{file, err}
+		}
+	}
+	if f, err := findExecutable(filepath.Join(".", file), exts); err == nil {
+		return f, nil
+	}
+	path := os.Getenv("path")
+	for _, dir := range filepath.SplitList(path) {
+		if f, err := findExecutable(filepath.Join(dir, file), exts); err == nil {
+			return f, nil
+		}
+	}
+	return "", &Error{file, ErrNotFound}
+}
diff --git a/src/os/exec/lp_windows_test.go b/src/os/exec/lp_windows_test.go
new file mode 100644
index 0000000..c6f3d5d
--- /dev/null
+++ b/src/os/exec/lp_windows_test.go
@@ -0,0 +1,577 @@
+// Copyright 2013 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 -> internal/testenv -> os/exec
+// circular dependency.
+
+package exec_test
+
+import (
+	"fmt"
+	"internal/testenv"
+	"io"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strconv"
+	"strings"
+	"testing"
+)
+
+func installExe(t *testing.T, dest, src string) {
+	fsrc, err := os.Open(src)
+	if err != nil {
+		t.Fatal("os.Open failed: ", err)
+	}
+	defer fsrc.Close()
+	fdest, err := os.Create(dest)
+	if err != nil {
+		t.Fatal("os.Create failed: ", err)
+	}
+	defer fdest.Close()
+	_, err = io.Copy(fdest, fsrc)
+	if err != nil {
+		t.Fatal("io.Copy failed: ", err)
+	}
+}
+
+func installBat(t *testing.T, dest string) {
+	f, err := os.Create(dest)
+	if err != nil {
+		t.Fatalf("failed to create batch file: %v", err)
+	}
+	defer f.Close()
+	fmt.Fprintf(f, "@echo %s\n", dest)
+}
+
+func installProg(t *testing.T, dest, srcExe string) {
+	err := os.MkdirAll(filepath.Dir(dest), 0700)
+	if err != nil {
+		t.Fatal("os.MkdirAll failed: ", err)
+	}
+	if strings.ToLower(filepath.Ext(dest)) == ".bat" {
+		installBat(t, dest)
+		return
+	}
+	installExe(t, dest, srcExe)
+}
+
+type lookPathTest struct {
+	rootDir   string
+	PATH      string
+	PATHEXT   string
+	files     []string
+	searchFor string
+	fails     bool // test is expected to fail
+}
+
+func (test lookPathTest) runProg(t *testing.T, env []string, args ...string) (string, error) {
+	cmd := exec.Command(args[0], args[1:]...)
+	cmd.Env = env
+	cmd.Dir = test.rootDir
+	args[0] = filepath.Base(args[0])
+	cmdText := fmt.Sprintf("%q command", strings.Join(args, " "))
+	out, err := cmd.CombinedOutput()
+	if (err != nil) != test.fails {
+		if test.fails {
+			t.Fatalf("test=%+v: %s succeeded, but expected to fail", test, cmdText)
+		}
+		t.Fatalf("test=%+v: %s failed, but expected to succeed: %v - %v", test, cmdText, err, string(out))
+	}
+	if err != nil {
+		return "", fmt.Errorf("test=%+v: %s failed: %v - %v", test, cmdText, err, string(out))
+	}
+	// normalise program output
+	p := string(out)
+	// trim terminating \r and \n that batch file outputs
+	for len(p) > 0 && (p[len(p)-1] == '\n' || p[len(p)-1] == '\r') {
+		p = p[:len(p)-1]
+	}
+	if !filepath.IsAbs(p) {
+		return p, nil
+	}
+	if p[:len(test.rootDir)] != test.rootDir {
+		t.Fatalf("test=%+v: %s output is wrong: %q must have %q prefix", test, cmdText, p, test.rootDir)
+	}
+	return p[len(test.rootDir)+1:], nil
+}
+
+func updateEnv(env []string, name, value string) []string {
+	for i, e := range env {
+		if strings.HasPrefix(strings.ToUpper(e), name+"=") {
+			env[i] = name + "=" + value
+			return env
+		}
+	}
+	return append(env, name+"="+value)
+}
+
+func createEnv(dir, PATH, PATHEXT string) []string {
+	env := os.Environ()
+	env = updateEnv(env, "PATHEXT", PATHEXT)
+	// Add dir in front of every directory in the PATH.
+	dirs := filepath.SplitList(PATH)
+	for i := range dirs {
+		dirs[i] = filepath.Join(dir, dirs[i])
+	}
+	path := strings.Join(dirs, ";")
+	env = updateEnv(env, "PATH", os.Getenv("SystemRoot")+"/System32;"+path)
+	return env
+}
+
+// createFiles copies srcPath file into multiply files.
+// It uses dir as prefix for all destination files.
+func createFiles(t *testing.T, dir string, files []string, srcPath string) {
+	for _, f := range files {
+		installProg(t, filepath.Join(dir, f), srcPath)
+	}
+}
+
+func (test lookPathTest) run(t *testing.T, tmpdir, printpathExe string) {
+	test.rootDir = tmpdir
+	createFiles(t, test.rootDir, test.files, printpathExe)
+	env := createEnv(test.rootDir, test.PATH, test.PATHEXT)
+	// Run "cmd.exe /c test.searchFor" with new environment and
+	// work directory set. All candidates are copies of printpath.exe.
+	// These will output their program paths when run.
+	should, errCmd := test.runProg(t, env, "cmd", "/c", test.searchFor)
+	// Run the lookpath program with new environment and work directory set.
+	env = append(env, "GO_WANT_HELPER_PROCESS=1")
+	have, errLP := test.runProg(t, env, os.Args[0], "-test.run=TestHelperProcess", "--", "lookpath", test.searchFor)
+	// Compare results.
+	if errCmd == nil && errLP == nil {
+		// both succeeded
+		if should != have {
+			t.Fatalf("test=%+v failed: expected to find %q, but found %q", test, should, have)
+		}
+		return
+	}
+	if errCmd != nil && errLP != nil {
+		// both failed -> continue
+		return
+	}
+	if errCmd != nil {
+		t.Fatal(errCmd)
+	}
+	if errLP != nil {
+		t.Fatal(errLP)
+	}
+}
+
+var lookPathTests = []lookPathTest{
+	{
+		PATHEXT:   `.COM;.EXE;.BAT`,
+		PATH:      `p1;p2`,
+		files:     []string{`p1\a.exe`, `p2\a.exe`, `p2\a`},
+		searchFor: `a`,
+	},
+	{
+		PATHEXT:   `.COM;.EXE;.BAT`,
+		PATH:      `p1.dir;p2.dir`,
+		files:     []string{`p1.dir\a`, `p2.dir\a.exe`},
+		searchFor: `a`,
+	},
+	{
+		PATHEXT:   `.COM;.EXE;.BAT`,
+		PATH:      `p1;p2`,
+		files:     []string{`p1\a.exe`, `p2\a.exe`},
+		searchFor: `a.exe`,
+	},
+	{
+		PATHEXT:   `.COM;.EXE;.BAT`,
+		PATH:      `p1;p2`,
+		files:     []string{`p1\a.exe`, `p2\b.exe`},
+		searchFor: `b`,
+	},
+	{
+		PATHEXT:   `.COM;.EXE;.BAT`,
+		PATH:      `p1;p2`,
+		files:     []string{`p1\b`, `p2\a`},
+		searchFor: `a`,
+		fails:     true, // TODO(brainman): do not know why this fails
+	},
+	// If the command name specifies a path, the shell searches
+	// the specified path for an executable file matching
+	// the command name. If a match is found, the external
+	// command (the executable file) executes.
+	{
+		PATHEXT:   `.COM;.EXE;.BAT`,
+		PATH:      `p1;p2`,
+		files:     []string{`p1\a.exe`, `p2\a.exe`},
+		searchFor: `p2\a`,
+	},
+	// If the command name specifies a path, the shell searches
+	// the specified path for an executable file matching the command
+	// name. ... If no match is found, the shell reports an error
+	// and command processing completes.
+	{
+		PATHEXT:   `.COM;.EXE;.BAT`,
+		PATH:      `p1;p2`,
+		files:     []string{`p1\b.exe`, `p2\a.exe`},
+		searchFor: `p2\b`,
+		fails:     true,
+	},
+	// If the command name does not specify a path, the shell
+	// searches the current directory for an executable file
+	// matching the command name. If a match is found, the external
+	// command (the executable file) executes.
+	{
+		PATHEXT:   `.COM;.EXE;.BAT`,
+		PATH:      `p1;p2`,
+		files:     []string{`a`, `p1\a.exe`, `p2\a.exe`},
+		searchFor: `a`,
+	},
+	// The shell now searches each directory specified by the
+	// PATH environment variable, in the order listed, for an
+	// executable file matching the command name. If a match
+	// is found, the external command (the executable file) executes.
+	{
+		PATHEXT:   `.COM;.EXE;.BAT`,
+		PATH:      `p1;p2`,
+		files:     []string{`p1\a.exe`, `p2\a.exe`},
+		searchFor: `a`,
+	},
+	// The shell now searches each directory specified by the
+	// PATH environment variable, in the order listed, for an
+	// executable file matching the command name. If no match
+	// is found, the shell reports an error and command processing
+	// completes.
+	{
+		PATHEXT:   `.COM;.EXE;.BAT`,
+		PATH:      `p1;p2`,
+		files:     []string{`p1\a.exe`, `p2\a.exe`},
+		searchFor: `b`,
+		fails:     true,
+	},
+	// If the command name includes a file extension, the shell
+	// searches each directory for the exact file name specified
+	// by the command name.
+	{
+		PATHEXT:   `.COM;.EXE;.BAT`,
+		PATH:      `p1;p2`,
+		files:     []string{`p1\a.exe`, `p2\a.exe`},
+		searchFor: `a.exe`,
+	},
+	{
+		PATHEXT:   `.COM;.EXE;.BAT`,
+		PATH:      `p1;p2`,
+		files:     []string{`p1\a.exe`, `p2\a.exe`},
+		searchFor: `a.com`,
+		fails:     true, // includes extension and not exact file name match
+	},
+	{
+		PATHEXT:   `.COM;.EXE;.BAT`,
+		PATH:      `p1`,
+		files:     []string{`p1\a.exe.exe`},
+		searchFor: `a.exe`,
+	},
+	{
+		PATHEXT:   `.COM;.BAT`,
+		PATH:      `p1;p2`,
+		files:     []string{`p1\a.exe`, `p2\a.exe`},
+		searchFor: `a.exe`,
+	},
+	// If the command name does not include a file extension, the shell
+	// adds the extensions listed in the PATHEXT environment variable,
+	// one by one, and searches the directory for that file name. Note
+	// that the shell tries all possible file extensions in a specific
+	// directory before moving on to search the next directory
+	// (if there is one).
+	{
+		PATHEXT:   `.COM;.EXE`,
+		PATH:      `p1;p2`,
+		files:     []string{`p1\a.bat`, `p2\a.exe`},
+		searchFor: `a`,
+	},
+	{
+		PATHEXT:   `.COM;.EXE;.BAT`,
+		PATH:      `p1;p2`,
+		files:     []string{`p1\a.bat`, `p2\a.exe`},
+		searchFor: `a`,
+	},
+	{
+		PATHEXT:   `.COM;.EXE;.BAT`,
+		PATH:      `p1;p2`,
+		files:     []string{`p1\a.bat`, `p1\a.exe`, `p2\a.bat`, `p2\a.exe`},
+		searchFor: `a`,
+	},
+	{
+		PATHEXT:   `.COM`,
+		PATH:      `p1;p2`,
+		files:     []string{`p1\a.bat`, `p2\a.exe`},
+		searchFor: `a`,
+		fails:     true, // tried all extensions in PATHEXT, but none matches
+	},
+}
+
+func TestLookPath(t *testing.T) {
+	tmp, err := os.MkdirTemp("", "TestLookPath")
+	if err != nil {
+		t.Fatal("TempDir failed: ", err)
+	}
+	defer os.RemoveAll(tmp)
+
+	printpathExe := buildPrintPathExe(t, tmp)
+
+	// Run all tests.
+	for i, test := range lookPathTests {
+		dir := filepath.Join(tmp, "d"+strconv.Itoa(i))
+		err := os.Mkdir(dir, 0700)
+		if err != nil {
+			t.Fatal("Mkdir failed: ", err)
+		}
+		test.run(t, dir, printpathExe)
+	}
+}
+
+type commandTest struct {
+	PATH  string
+	files []string
+	dir   string
+	arg0  string
+	want  string
+	fails bool // test is expected to fail
+}
+
+func (test commandTest) isSuccess(rootDir, output string, err error) error {
+	if err != nil {
+		return fmt.Errorf("test=%+v: exec: %v %v", test, err, output)
+	}
+	path := output
+	if path[:len(rootDir)] != rootDir {
+		return fmt.Errorf("test=%+v: %q must have %q prefix", test, path, rootDir)
+	}
+	path = path[len(rootDir)+1:]
+	if path != test.want {
+		return fmt.Errorf("test=%+v: want %q, got %q", test, test.want, path)
+	}
+	return nil
+}
+
+func (test commandTest) runOne(rootDir string, env []string, dir, arg0 string) error {
+	cmd := exec.Command(os.Args[0], "-test.run=TestHelperProcess", "--", "exec", dir, arg0)
+	cmd.Dir = rootDir
+	cmd.Env = env
+	output, err := cmd.CombinedOutput()
+	err = test.isSuccess(rootDir, string(output), err)
+	if (err != nil) != test.fails {
+		if test.fails {
+			return fmt.Errorf("test=%+v: succeeded, but expected to fail", test)
+		}
+		return err
+	}
+	return nil
+}
+
+func (test commandTest) run(t *testing.T, rootDir, printpathExe string) {
+	createFiles(t, rootDir, test.files, printpathExe)
+	PATHEXT := `.COM;.EXE;.BAT`
+	env := createEnv(rootDir, test.PATH, PATHEXT)
+	env = append(env, "GO_WANT_HELPER_PROCESS=1")
+	err := test.runOne(rootDir, env, test.dir, test.arg0)
+	if err != nil {
+		t.Error(err)
+	}
+}
+
+var commandTests = []commandTest{
+	// testing commands with no slash, like `a.exe`
+	{
+		// should find a.exe in current directory
+		files: []string{`a.exe`},
+		arg0:  `a.exe`,
+		want:  `a.exe`,
+	},
+	{
+		// like above, but add PATH in attempt to break the test
+		PATH:  `p2;p`,
+		files: []string{`a.exe`, `p\a.exe`, `p2\a.exe`},
+		arg0:  `a.exe`,
+		want:  `a.exe`,
+	},
+	{
+		// like above, but use "a" instead of "a.exe" for command
+		PATH:  `p2;p`,
+		files: []string{`a.exe`, `p\a.exe`, `p2\a.exe`},
+		arg0:  `a`,
+		want:  `a.exe`,
+	},
+	// testing commands with slash, like `.\a.exe`
+	{
+		// should find p\a.exe
+		files: []string{`p\a.exe`},
+		arg0:  `p\a.exe`,
+		want:  `p\a.exe`,
+	},
+	{
+		// like above, but adding `.` in front of executable should still be OK
+		files: []string{`p\a.exe`},
+		arg0:  `.\p\a.exe`,
+		want:  `p\a.exe`,
+	},
+	{
+		// like above, but with PATH added in attempt to break it
+		PATH:  `p2`,
+		files: []string{`p\a.exe`, `p2\a.exe`},
+		arg0:  `p\a.exe`,
+		want:  `p\a.exe`,
+	},
+	{
+		// like above, but make sure .exe is tried even for commands with slash
+		PATH:  `p2`,
+		files: []string{`p\a.exe`, `p2\a.exe`},
+		arg0:  `p\a`,
+		want:  `p\a.exe`,
+	},
+	// tests commands, like `a.exe`, with c.Dir set
+	{
+		// should not find a.exe in p, because LookPath(`a.exe`) will fail
+		files: []string{`p\a.exe`},
+		dir:   `p`,
+		arg0:  `a.exe`,
+		want:  `p\a.exe`,
+		fails: true,
+	},
+	{
+		// LookPath(`a.exe`) will find `.\a.exe`, but prefixing that with
+		// dir `p\a.exe` will refer to a non-existent file
+		files: []string{`a.exe`, `p\not_important_file`},
+		dir:   `p`,
+		arg0:  `a.exe`,
+		want:  `a.exe`,
+		fails: true,
+	},
+	{
+		// like above, but making test succeed by installing file
+		// in referred destination (so LookPath(`a.exe`) will still
+		// find `.\a.exe`, but we successfully execute `p\a.exe`)
+		files: []string{`a.exe`, `p\a.exe`},
+		dir:   `p`,
+		arg0:  `a.exe`,
+		want:  `p\a.exe`,
+	},
+	{
+		// like above, but add PATH in attempt to break the test
+		PATH:  `p2;p`,
+		files: []string{`a.exe`, `p\a.exe`, `p2\a.exe`},
+		dir:   `p`,
+		arg0:  `a.exe`,
+		want:  `p\a.exe`,
+	},
+	{
+		// like above, but use "a" instead of "a.exe" for command
+		PATH:  `p2;p`,
+		files: []string{`a.exe`, `p\a.exe`, `p2\a.exe`},
+		dir:   `p`,
+		arg0:  `a`,
+		want:  `p\a.exe`,
+	},
+	{
+		// finds `a.exe` in the PATH regardless of dir set
+		// because LookPath returns full path in that case
+		PATH:  `p2;p`,
+		files: []string{`p\a.exe`, `p2\a.exe`},
+		dir:   `p`,
+		arg0:  `a.exe`,
+		want:  `p2\a.exe`,
+	},
+	// tests commands, like `.\a.exe`, with c.Dir set
+	{
+		// should use dir when command is path, like ".\a.exe"
+		files: []string{`p\a.exe`},
+		dir:   `p`,
+		arg0:  `.\a.exe`,
+		want:  `p\a.exe`,
+	},
+	{
+		// like above, but with PATH added in attempt to break it
+		PATH:  `p2`,
+		files: []string{`p\a.exe`, `p2\a.exe`},
+		dir:   `p`,
+		arg0:  `.\a.exe`,
+		want:  `p\a.exe`,
+	},
+	{
+		// like above, but make sure .exe is tried even for commands with slash
+		PATH:  `p2`,
+		files: []string{`p\a.exe`, `p2\a.exe`},
+		dir:   `p`,
+		arg0:  `.\a`,
+		want:  `p\a.exe`,
+	},
+}
+
+func TestCommand(t *testing.T) {
+	tmp, err := os.MkdirTemp("", "TestCommand")
+	if err != nil {
+		t.Fatal("TempDir failed: ", err)
+	}
+	defer os.RemoveAll(tmp)
+
+	printpathExe := buildPrintPathExe(t, tmp)
+
+	// Run all tests.
+	for i, test := range commandTests {
+		dir := filepath.Join(tmp, "d"+strconv.Itoa(i))
+		err := os.Mkdir(dir, 0700)
+		if err != nil {
+			t.Fatal("Mkdir failed: ", err)
+		}
+		test.run(t, dir, printpathExe)
+	}
+}
+
+// buildPrintPathExe creates a Go program that prints its own path.
+// dir is a temp directory where executable will be created.
+// The function returns full path to the created program.
+func buildPrintPathExe(t *testing.T, dir string) string {
+	const name = "printpath"
+	srcname := name + ".go"
+	err := os.WriteFile(filepath.Join(dir, srcname), []byte(printpathSrc), 0644)
+	if err != nil {
+		t.Fatalf("failed to create source: %v", err)
+	}
+	if err != nil {
+		t.Fatalf("failed to execute template: %v", err)
+	}
+	outname := name + ".exe"
+	cmd := exec.Command(testenv.GoToolPath(t), "build", "-o", outname, srcname)
+	cmd.Dir = dir
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("failed to build executable: %v - %v", err, string(out))
+	}
+	return filepath.Join(dir, outname)
+}
+
+const printpathSrc = `
+package main
+
+import (
+	"os"
+	"syscall"
+	"unicode/utf16"
+	"unsafe"
+)
+
+func getMyName() (string, error) {
+	var sysproc = syscall.MustLoadDLL("kernel32.dll").MustFindProc("GetModuleFileNameW")
+	b := make([]uint16, syscall.MAX_PATH)
+	r, _, err := sysproc.Call(0, uintptr(unsafe.Pointer(&b[0])), uintptr(len(b)))
+	n := uint32(r)
+	if n == 0 {
+		return "", err
+	}
+	return string(utf16.Decode(b[0:n])), nil
+}
+
+func main() {
+	path, err := getMyName()
+	if err != nil {
+		os.Stderr.Write([]byte("getMyName failed: " + err.Error() + "\n"))
+		os.Exit(1)
+	}
+	os.Stdout.Write([]byte(path))
+}
+`
diff --git a/src/os/exec/read3.go b/src/os/exec/read3.go
new file mode 100644
index 0000000..8cc24da
--- /dev/null
+++ b/src/os/exec/read3.go
@@ -0,0 +1,101 @@
+// Copyright 2020 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 ignore
+
+// This is a test program that verifies that it can read from
+// descriptor 3 and that no other descriptors are open.
+// This is not done via TestHelperProcess and GO_WANT_HELPER_PROCESS
+// because we want to ensure that this program does not use cgo,
+// because C libraries can open file descriptors behind our backs
+// and confuse the test. See issue 25628.
+package main
+
+import (
+	"fmt"
+	"internal/poll"
+	"io"
+	"os"
+	"os/exec"
+	"runtime"
+	"strings"
+)
+
+func main() {
+	fd3 := os.NewFile(3, "fd3")
+	bs, err := io.ReadAll(fd3)
+	if err != nil {
+		fmt.Printf("ReadAll from fd 3: %v\n", err)
+		os.Exit(1)
+	}
+
+	// Now verify that there are no other open fds.
+	// stdin == 0
+	// stdout == 1
+	// stderr == 2
+	// descriptor from parent == 3
+	// All descriptors 4 and up should be available,
+	// except for any used by the network poller.
+	var files []*os.File
+	for wantfd := uintptr(4); wantfd <= 100; wantfd++ {
+		if poll.IsPollDescriptor(wantfd) {
+			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)
+			fdfile := fmt.Sprintf("/proc/self/fd/%d", wantfd)
+			link, err := os.Readlink(fdfile)
+			fmt.Printf("readlink(%q) = %q, %v\n", fdfile, link, err)
+			var args []string
+			switch runtime.GOOS {
+			case "plan9":
+				args = []string{fmt.Sprintf("/proc/%d/fd", os.Getpid())}
+			case "aix", "solaris", "illumos":
+				args = []string{fmt.Sprint(os.Getpid())}
+			default:
+				args = []string{"-p", fmt.Sprint(os.Getpid())}
+			}
+
+			// 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"
+			case "aix":
+				ofcmd = "procfiles"
+			case "solaris", "illumos":
+				ofcmd = "pfiles"
+			}
+
+			cmd := exec.Command(ofcmd, args...)
+			out, err := cmd.CombinedOutput()
+			if err != nil {
+				fmt.Fprintf(os.Stderr, "%s failed: %v\n", strings.Join(cmd.Args, " "), err)
+			}
+			fmt.Printf("%s", 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)
+}