src/cmd/dist/util.go - go - Git at Google

 // 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 main

 import (
 	"bytes"
 	"flag"
 	"fmt"
 	"io"
 	"io/ioutil"
 	"os"
 	"os/exec"
 	"path/filepath"
 	"sort"
 	"strconv"
 	"strings"
 	"sync"
 	"time"
 )

 // pathf is fmt.Sprintf for generating paths
 // (on windows it turns / into \ after the printf).
 func pathf(format string, args ...interface{}) string {
 	return filepath.Clean(fmt.Sprintf(format, args...))
 }

 // filter returns a slice containing the elements x from list for which f(x) == true.
 func filter(list []string, f func(string) bool) []string {
 	var out []string
 	for _, x := range list {
 		if f(x) {
 			out = append(out, x)
 		}
 	}
 	return out
 }

 // uniq returns a sorted slice containing the unique elements of list.
 func uniq(list []string) []string {
 	out := make([]string, len(list))
 	copy(out, list)
 	sort.Strings(out)
 	keep := out[:0]
 	for _, x := range out {
 		if len(keep) == 0 || keep[len(keep)-1] != x {
 			keep = append(keep, x)
 		}
 	}
 	return keep
 }

 const (
 	CheckExit = 1 << iota
 	ShowOutput
 	Background
 )

 var outputLock sync.Mutex

 // run runs the command line cmd in dir.
 // If mode has ShowOutput set and Background unset, run passes cmd's output to
 // stdout/stderr directly. Otherwise, run returns cmd's output as a string.
 // If mode has CheckExit set and the command fails, run calls fatalf.
 // If mode has Background set, this command is being run as a
 // Background job. Only bgrun should use the Background mode,
 // not other callers.
 func run(dir string, mode int, cmd ...string) string {
 	if vflag > 1 {
 		errprintf("run: %s\n", strings.Join(cmd, " "))
 	}

 	xcmd := exec.Command(cmd[0], cmd[1:]...)
 	xcmd.Dir = dir
 	var data []byte
 	var err error

 	// If we want to show command output and this is not
 	// a background command, assume it's the only thing
 	// running, so we can just let it write directly stdout/stderr
 	// as it runs without fear of mixing the output with some
 	// other command's output. Not buffering lets the output
 	// appear as it is printed instead of once the command exits.
 	// This is most important for the invocation of 'go1.4 build -v bootstrap/...'.
 	if mode&(Background|ShowOutput) == ShowOutput {
 		xcmd.Stdout = os.Stdout
 		xcmd.Stderr = os.Stderr
 		err = xcmd.Run()
 	} else {
 		data, err = xcmd.CombinedOutput()
 	}
 	if err != nil && mode&CheckExit != 0 {
 		outputLock.Lock()
 		if len(data) > 0 {
 			xprintf("%s\n", data)
 		}
 		outputLock.Unlock()
 		if mode&Background != 0 {
 			// Prevent fatalf from waiting on our own goroutine's
 			// bghelper to exit:
 			bghelpers.Done()
 		}
 		fatalf("FAILED: %v: %v", strings.Join(cmd, " "), err)
 	}
 	if mode&ShowOutput != 0 {
 		outputLock.Lock()
 		os.Stdout.Write(data)
 		outputLock.Unlock()
 	}
 	if vflag > 2 {
 		errprintf("run: %s DONE\n", strings.Join(cmd, " "))
 	}
 	return string(data)
 }

 var maxbg = 4 /* maximum number of jobs to run at once */

 var (
 	bgwork = make(chan func(), 1e5)

 	bghelpers sync.WaitGroup

 	dieOnce sync.Once // guards close of dying
 	dying   = make(chan struct{})
 )

 func bginit() {
 	bghelpers.Add(maxbg)
 	for i := 0; i < maxbg; i++ {
 		go bghelper()
 	}
 }

 func bghelper() {
 	defer bghelpers.Done()
 	for {
 		select {
 		case <-dying:
 			return
 		case w := <-bgwork:
 			// Dying takes precedence over doing more work.
 			select {
 			case <-dying:
 				return
 			default:
 				w()
 			}
 		}
 	}
 }

 // bgrun is like run but runs the command in the background.
 // CheckExit|ShowOutput mode is implied (since output cannot be returned).
 // bgrun adds 1 to wg immediately, and calls Done when the work completes.
 func bgrun(wg *sync.WaitGroup, dir string, cmd ...string) {
 	wg.Add(1)
 	bgwork <- func() {
 		defer wg.Done()
 		run(dir, CheckExit|ShowOutput|Background, cmd...)
 	}
 }

 // bgwait waits for pending bgruns to finish.
 // bgwait must be called from only a single goroutine at a time.
 func bgwait(wg *sync.WaitGroup) {
 	done := make(chan struct{})
 	go func() {
 		wg.Wait()
 		close(done)
 	}()
 	select {
 	case <-done:
 	case <-dying:
 	}
 }

 // xgetwd returns the current directory.
 func xgetwd() string {
 	wd, err := os.Getwd()
 	if err != nil {
 		fatalf("%s", err)
 	}
 	return wd
 }

 // xrealwd returns the 'real' name for the given path.
 // real is defined as what xgetwd returns in that directory.
 func xrealwd(path string) string {
 	old := xgetwd()
 	if err := os.Chdir(path); err != nil {
 		fatalf("chdir %s: %v", path, err)
 	}
 	real := xgetwd()
 	if err := os.Chdir(old); err != nil {
 		fatalf("chdir %s: %v", old, err)
 	}
 	return real
 }

 // isdir reports whether p names an existing directory.
 func isdir(p string) bool {
 	fi, err := os.Stat(p)
 	return err == nil && fi.IsDir()
 }

 // isfile reports whether p names an existing file.
 func isfile(p string) bool {
 	fi, err := os.Stat(p)
 	return err == nil && fi.Mode().IsRegular()
 }

 // mtime returns the modification time of the file p.
 func mtime(p string) time.Time {
 	fi, err := os.Stat(p)
 	if err != nil {
 		return time.Time{}
 	}
 	return fi.ModTime()
 }

 // readfile returns the content of the named file.
 func readfile(file string) string {
 	data, err := ioutil.ReadFile(file)
 	if err != nil {
 		fatalf("%v", err)
 	}
 	return string(data)
 }

 const (
 	writeExec = 1 << iota
 	writeSkipSame
 )

 // writefile writes text to the named file, creating it if needed.
 // if exec is non-zero, marks the file as executable.
 // If the file already exists and has the expected content,
 // it is not rewritten, to avoid changing the time stamp.
 func writefile(text, file string, flag int) {
 	new := []byte(text)
 	if flag&writeSkipSame != 0 {
 		old, err := ioutil.ReadFile(file)
 		if err == nil && bytes.Equal(old, new) {
 			return
 		}
 	}
 	mode := os.FileMode(0666)
 	if flag&writeExec != 0 {
 		mode = 0777
 	}
 	err := ioutil.WriteFile(file, new, mode)
 	if err != nil {
 		fatalf("%v", err)
 	}
 }

 // xmkdir creates the directory p.
 func xmkdir(p string) {
 	err := os.Mkdir(p, 0777)
 	if err != nil {
 		fatalf("%v", err)
 	}
 }

 // xmkdirall creates the directory p and its parents, as needed.
 func xmkdirall(p string) {
 	err := os.MkdirAll(p, 0777)
 	if err != nil {
 		fatalf("%v", err)
 	}
 }

 // xremove removes the file p.
 func xremove(p string) {
 	if vflag > 2 {
 		errprintf("rm %s\n", p)
 	}
 	os.Remove(p)
 }

 // xremoveall removes the file or directory tree rooted at p.
 func xremoveall(p string) {
 	if vflag > 2 {
 		errprintf("rm -r %s\n", p)
 	}
 	os.RemoveAll(p)
 }

 // xreaddir replaces dst with a list of the names of the files and subdirectories in dir.
 // The names are relative to dir; they are not full paths.
 func xreaddir(dir string) []string {
 	f, err := os.Open(dir)
 	if err != nil {
 		fatalf("%v", err)
 	}
 	defer f.Close()
 	names, err := f.Readdirnames(-1)
 	if err != nil {
 		fatalf("reading %s: %v", dir, err)
 	}
 	return names
 }

 // xreaddir replaces dst with a list of the names of the files in dir.
 // The names are relative to dir; they are not full paths.
 func xreaddirfiles(dir string) []string {
 	f, err := os.Open(dir)
 	if err != nil {
 		fatalf("%v", err)
 	}
 	defer f.Close()
 	infos, err := f.Readdir(-1)
 	if err != nil {
 		fatalf("reading %s: %v", dir, err)
 	}
 	var names []string
 	for _, fi := range infos {
 		if !fi.IsDir() {
 			names = append(names, fi.Name())
 		}
 	}
 	return names
 }

 // xworkdir creates a new temporary directory to hold object files
 // and returns the name of that directory.
 func xworkdir() string {
 	name, err := ioutil.TempDir(os.Getenv("GOTMPDIR"), "go-tool-dist-")
 	if err != nil {
 		fatalf("%v", err)
 	}
 	return name
 }

 // fatalf prints an error message to standard error and exits.
 func fatalf(format string, args ...interface{}) {
 	fmt.Fprintf(os.Stderr, "go tool dist: %s\n", fmt.Sprintf(format, args...))

 	dieOnce.Do(func() { close(dying) })

 	// Wait for background goroutines to finish,
 	// so that exit handler that removes the work directory
 	// is not fighting with active writes or open files.
 	bghelpers.Wait()

 	xexit(2)
 }

 var atexits []func()

 // xexit exits the process with return code n.
 func xexit(n int) {
 	for i := len(atexits) - 1; i >= 0; i-- {
 		atexits[i]()
 	}
 	os.Exit(n)
 }

 // xatexit schedules the exit-handler f to be run when the program exits.
 func xatexit(f func()) {
 	atexits = append(atexits, f)
 }

 // xprintf prints a message to standard output.
 func xprintf(format string, args ...interface{}) {
 	fmt.Printf(format, args...)
 }

 // errprintf prints a message to standard output.
 func errprintf(format string, args ...interface{}) {
 	fmt.Fprintf(os.Stderr, format, args...)
 }

 // xsamefile reports whether f1 and f2 are the same file (or dir)
 func xsamefile(f1, f2 string) bool {
 	fi1, err1 := os.Stat(f1)
 	fi2, err2 := os.Stat(f2)
 	if err1 != nil || err2 != nil {
 		return f1 == f2
 	}
 	return os.SameFile(fi1, fi2)
 }

 func xgetgoarm() string {
 	if goos == "nacl" {
 		// NaCl guarantees VFPv3 and is always cross-compiled.
 		return "7"
 	}
 	if goos == "darwin" || goos == "android" {
 		// Assume all darwin/arm and android devices have VFPv3.
 		// These ports are also mostly cross-compiled, so it makes little
 		// sense to auto-detect the setting.
 		return "7"
 	}
 	if gohostarch != "arm" || goos != gohostos {
 		// Conservative default for cross-compilation.
 		return "5"
 	}
 	if goos == "freebsd" || goos == "openbsd" {
 		// FreeBSD has broken VFP support.
 		// OpenBSD currently only supports softfloat.
 		return "5"
 	}

 	// Try to exec ourselves in a mode to detect VFP support.
 	// Seeing how far it gets determines which instructions failed.
 	// The test is OS-agnostic.
 	out := run("", 0, os.Args[0], "-check-goarm")
 	v1ok := strings.Contains(out, "VFPv1 OK.")
 	v3ok := strings.Contains(out, "VFPv3 OK.")

 	if v1ok && v3ok {
 		return "7"
 	}
 	if v1ok {
 		return "6"
 	}
 	return "5"
 }

 func min(a, b int) int {
 	if a < b {
 		return a
 	}
 	return b
 }

 // elfIsLittleEndian detects if the ELF file is little endian.
 func elfIsLittleEndian(fn string) bool {
 	// read the ELF file header to determine the endianness without using the
 	// debug/elf package.
 	file, err := os.Open(fn)
 	if err != nil {
 		fatalf("failed to open file to determine endianness: %v", err)
 	}
 	defer file.Close()
 	var hdr [16]byte
 	if _, err := io.ReadFull(file, hdr[:]); err != nil {
 		fatalf("failed to read ELF header to determine endianness: %v", err)
 	}
 	// hdr[5] is EI_DATA byte, 1 is ELFDATA2LSB and 2 is ELFDATA2MSB
 	switch hdr[5] {
 	default:
 		fatalf("unknown ELF endianness of %s: EI_DATA = %d", fn, hdr[5])
 	case 1:
 		return true
 	case 2:
 		return false
 	}
 	panic("unreachable")
 }

 // count is a flag.Value that is like a flag.Bool and a flag.Int.
 // If used as -name, it increments the count, but -name=x sets the count.
 // Used for verbose flag -v.
 type count int

 func (c *count) String() string {
 	return fmt.Sprint(int(*c))
 }

 func (c *count) Set(s string) error {
 	switch s {
 	case "true":
 		*c++
 	case "false":
 		*c = 0
 	default:
 		n, err := strconv.Atoi(s)
 		if err != nil {
 			return fmt.Errorf("invalid count %q", s)
 		}
 		*c = count(n)
 	}
 	return nil
 }

 func (c *count) IsBoolFlag() bool {
 	return true
 }

 func xflagparse(maxargs int) {
 	flag.Var((*count)(&vflag), "v", "verbosity")
 	flag.Parse()
 	if maxargs >= 0 && flag.NArg() > maxargs {
 		flag.Usage()
 	}
 }
	// 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 main

	import (
	"bytes"
	"flag"
	"fmt"
	"io"
	"io/ioutil"
	"os"
	"os/exec"
	"path/filepath"
	"sort"
	"strconv"
	"strings"
	"sync"
	"time"
	)

	// pathf is fmt.Sprintf for generating paths
	// (on windows it turns / into \ after the printf).
	func pathf(format string, args ...interface{}) string {
	return filepath.Clean(fmt.Sprintf(format, args...))
	}

	// filter returns a slice containing the elements x from list for which f(x) == true.
	func filter(list []string, f func(string) bool) []string {
	var out []string
	for _, x := range list {
	if f(x) {
	out = append(out, x)
	}
	}
	return out
	}

	// uniq returns a sorted slice containing the unique elements of list.
	func uniq(list []string) []string {
	out := make([]string, len(list))
	copy(out, list)
	sort.Strings(out)
	keep := out[:0]
	for _, x := range out {
	if len(keep) == 0 \|\| keep[len(keep)-1] != x {
	keep = append(keep, x)
	}
	}
	return keep
	}

	const (
	CheckExit = 1 << iota
	ShowOutput
	Background
	)

	var outputLock sync.Mutex

	// run runs the command line cmd in dir.
	// If mode has ShowOutput set and Background unset, run passes cmd's output to
	// stdout/stderr directly. Otherwise, run returns cmd's output as a string.
	// If mode has CheckExit set and the command fails, run calls fatalf.
	// If mode has Background set, this command is being run as a
	// Background job. Only bgrun should use the Background mode,
	// not other callers.
	func run(dir string, mode int, cmd ...string) string {
	if vflag > 1 {
	errprintf("run: %s\n", strings.Join(cmd, " "))
	}

	xcmd := exec.Command(cmd[0], cmd[1:]...)
	xcmd.Dir = dir
	var data []byte
	var err error

	// If we want to show command output and this is not
	// a background command, assume it's the only thing
	// running, so we can just let it write directly stdout/stderr
	// as it runs without fear of mixing the output with some
	// other command's output. Not buffering lets the output
	// appear as it is printed instead of once the command exits.
	// This is most important for the invocation of 'go1.4 build -v bootstrap/...'.
	if mode&(Background\|ShowOutput) == ShowOutput {
	xcmd.Stdout = os.Stdout
	xcmd.Stderr = os.Stderr
	err = xcmd.Run()
	} else {
	data, err = xcmd.CombinedOutput()
	}
	if err != nil && mode&CheckExit != 0 {
	outputLock.Lock()
	if len(data) > 0 {
	xprintf("%s\n", data)
	}
	outputLock.Unlock()
	if mode&Background != 0 {
	// Prevent fatalf from waiting on our own goroutine's
	// bghelper to exit:
	bghelpers.Done()
	}
	fatalf("FAILED: %v: %v", strings.Join(cmd, " "), err)
	}
	if mode&ShowOutput != 0 {
	outputLock.Lock()
	os.Stdout.Write(data)
	outputLock.Unlock()
	}
	if vflag > 2 {
	errprintf("run: %s DONE\n", strings.Join(cmd, " "))
	}
	return string(data)
	}

	var maxbg = 4 /* maximum number of jobs to run at once */

	var (
	bgwork = make(chan func(), 1e5)

	bghelpers sync.WaitGroup

	dieOnce sync.Once // guards close of dying
	dying = make(chan struct{})
	)

	func bginit() {
	bghelpers.Add(maxbg)
	for i := 0; i < maxbg; i++ {
	go bghelper()
	}
	}

	func bghelper() {
	defer bghelpers.Done()
	for {
	select {
	case <-dying:
	return
	case w := <-bgwork:
	// Dying takes precedence over doing more work.
	select {
	case <-dying:
	return
	default:
	w()
	}
	}
	}
	}

	// bgrun is like run but runs the command in the background.
	// CheckExit\|ShowOutput mode is implied (since output cannot be returned).
	// bgrun adds 1 to wg immediately, and calls Done when the work completes.
	func bgrun(wg *sync.WaitGroup, dir string, cmd ...string) {
	wg.Add(1)
	bgwork <- func() {
	defer wg.Done()
	run(dir, CheckExit\|ShowOutput\|Background, cmd...)
	}
	}

	// bgwait waits for pending bgruns to finish.
	// bgwait must be called from only a single goroutine at a time.
	func bgwait(wg *sync.WaitGroup) {
	done := make(chan struct{})
	go func() {
	wg.Wait()
	close(done)
	}()
	select {
	case <-done:
	case <-dying:
	}
	}

	// xgetwd returns the current directory.
	func xgetwd() string {
	wd, err := os.Getwd()
	if err != nil {
	fatalf("%s", err)
	}
	return wd
	}

	// xrealwd returns the 'real' name for the given path.
	// real is defined as what xgetwd returns in that directory.
	func xrealwd(path string) string {
	old := xgetwd()
	if err := os.Chdir(path); err != nil {
	fatalf("chdir %s: %v", path, err)
	}
	real := xgetwd()
	if err := os.Chdir(old); err != nil {
	fatalf("chdir %s: %v", old, err)
	}
	return real
	}

	// isdir reports whether p names an existing directory.
	func isdir(p string) bool {
	fi, err := os.Stat(p)
	return err == nil && fi.IsDir()
	}

	// isfile reports whether p names an existing file.
	func isfile(p string) bool {
	fi, err := os.Stat(p)
	return err == nil && fi.Mode().IsRegular()
	}

	// mtime returns the modification time of the file p.
	func mtime(p string) time.Time {
	fi, err := os.Stat(p)
	if err != nil {
	return time.Time{}
	}
	return fi.ModTime()
	}

	// readfile returns the content of the named file.
	func readfile(file string) string {
	data, err := ioutil.ReadFile(file)
	if err != nil {
	fatalf("%v", err)
	}
	return string(data)
	}

	const (
	writeExec = 1 << iota
	writeSkipSame
	)

	// writefile writes text to the named file, creating it if needed.
	// if exec is non-zero, marks the file as executable.
	// If the file already exists and has the expected content,
	// it is not rewritten, to avoid changing the time stamp.
	func writefile(text, file string, flag int) {
	new := []byte(text)
	if flag&writeSkipSame != 0 {
	old, err := ioutil.ReadFile(file)
	if err == nil && bytes.Equal(old, new) {
	return
	}
	}
	mode := os.FileMode(0666)
	if flag&writeExec != 0 {
	mode = 0777
	}
	err := ioutil.WriteFile(file, new, mode)
	if err != nil {
	fatalf("%v", err)
	}
	}

	// xmkdir creates the directory p.
	func xmkdir(p string) {
	err := os.Mkdir(p, 0777)
	if err != nil {
	fatalf("%v", err)
	}
	}

	// xmkdirall creates the directory p and its parents, as needed.
	func xmkdirall(p string) {
	err := os.MkdirAll(p, 0777)
	if err != nil {
	fatalf("%v", err)
	}
	}

	// xremove removes the file p.
	func xremove(p string) {
	if vflag > 2 {
	errprintf("rm %s\n", p)
	}
	os.Remove(p)
	}

	// xremoveall removes the file or directory tree rooted at p.
	func xremoveall(p string) {
	if vflag > 2 {
	errprintf("rm -r %s\n", p)
	}
	os.RemoveAll(p)
	}

	// xreaddir replaces dst with a list of the names of the files and subdirectories in dir.
	// The names are relative to dir; they are not full paths.
	func xreaddir(dir string) []string {
	f, err := os.Open(dir)
	if err != nil {
	fatalf("%v", err)
	}
	defer f.Close()
	names, err := f.Readdirnames(-1)
	if err != nil {
	fatalf("reading %s: %v", dir, err)
	}
	return names
	}

	// xreaddir replaces dst with a list of the names of the files in dir.
	// The names are relative to dir; they are not full paths.
	func xreaddirfiles(dir string) []string {
	f, err := os.Open(dir)
	if err != nil {
	fatalf("%v", err)
	}
	defer f.Close()
	infos, err := f.Readdir(-1)
	if err != nil {
	fatalf("reading %s: %v", dir, err)
	}
	var names []string
	for _, fi := range infos {
	if !fi.IsDir() {
	names = append(names, fi.Name())
	}
	}
	return names
	}

	// xworkdir creates a new temporary directory to hold object files
	// and returns the name of that directory.
	func xworkdir() string {
	name, err := ioutil.TempDir(os.Getenv("GOTMPDIR"), "go-tool-dist-")
	if err != nil {
	fatalf("%v", err)
	}
	return name
	}

	// fatalf prints an error message to standard error and exits.
	func fatalf(format string, args ...interface{}) {
	fmt.Fprintf(os.Stderr, "go tool dist: %s\n", fmt.Sprintf(format, args...))

	dieOnce.Do(func() { close(dying) })

	// Wait for background goroutines to finish,
	// so that exit handler that removes the work directory
	// is not fighting with active writes or open files.
	bghelpers.Wait()

	xexit(2)
	}

	var atexits []func()

	// xexit exits the process with return code n.
	func xexit(n int) {
	for i := len(atexits) - 1; i >= 0; i-- {
	atexits[i]()
	}
	os.Exit(n)
	}

	// xatexit schedules the exit-handler f to be run when the program exits.
	func xatexit(f func()) {
	atexits = append(atexits, f)
	}

	// xprintf prints a message to standard output.
	func xprintf(format string, args ...interface{}) {
	fmt.Printf(format, args...)
	}

	// errprintf prints a message to standard output.
	func errprintf(format string, args ...interface{}) {
	fmt.Fprintf(os.Stderr, format, args...)
	}

	// xsamefile reports whether f1 and f2 are the same file (or dir)
	func xsamefile(f1, f2 string) bool {
	fi1, err1 := os.Stat(f1)
	fi2, err2 := os.Stat(f2)
	if err1 != nil \|\| err2 != nil {
	return f1 == f2
	}
	return os.SameFile(fi1, fi2)
	}

	func xgetgoarm() string {
	if goos == "nacl" {
	// NaCl guarantees VFPv3 and is always cross-compiled.
	return "7"
	}
	if goos == "darwin" \|\| goos == "android" {
	// Assume all darwin/arm and android devices have VFPv3.
	// These ports are also mostly cross-compiled, so it makes little
	// sense to auto-detect the setting.
	return "7"
	}
	if gohostarch != "arm" \|\| goos != gohostos {
	// Conservative default for cross-compilation.
	return "5"
	}
	if goos == "freebsd" \|\| goos == "openbsd" {
	// FreeBSD has broken VFP support.
	// OpenBSD currently only supports softfloat.
	return "5"
	}

	// Try to exec ourselves in a mode to detect VFP support.
	// Seeing how far it gets determines which instructions failed.
	// The test is OS-agnostic.
	out := run("", 0, os.Args[0], "-check-goarm")
	v1ok := strings.Contains(out, "VFPv1 OK.")
	v3ok := strings.Contains(out, "VFPv3 OK.")

	if v1ok && v3ok {
	return "7"
	}
	if v1ok {
	return "6"
	}
	return "5"
	}

	func min(a, b int) int {
	if a < b {
	return a
	}
	return b
	}

	// elfIsLittleEndian detects if the ELF file is little endian.
	func elfIsLittleEndian(fn string) bool {
	// read the ELF file header to determine the endianness without using the
	// debug/elf package.
	file, err := os.Open(fn)
	if err != nil {
	fatalf("failed to open file to determine endianness: %v", err)
	}
	defer file.Close()
	var hdr [16]byte
	if _, err := io.ReadFull(file, hdr[:]); err != nil {
	fatalf("failed to read ELF header to determine endianness: %v", err)
	}
	// hdr[5] is EI_DATA byte, 1 is ELFDATA2LSB and 2 is ELFDATA2MSB
	switch hdr[5] {
	default:
	fatalf("unknown ELF endianness of %s: EI_DATA = %d", fn, hdr[5])
	case 1:
	return true
	case 2:
	return false
	}
	panic("unreachable")
	}

	// count is a flag.Value that is like a flag.Bool and a flag.Int.
	// If used as -name, it increments the count, but -name=x sets the count.
	// Used for verbose flag -v.
	type count int

	func (c *count) String() string {
	return fmt.Sprint(int(*c))
	}

	func (c *count) Set(s string) error {
	switch s {
	case "true":
	*c++
	case "false":
	*c = 0
	default:
	n, err := strconv.Atoi(s)
	if err != nil {
	return fmt.Errorf("invalid count %q", s)
	}
	*c = count(n)
	}
	return nil
	}

	func (c *count) IsBoolFlag() bool {
	return true
	}

	func xflagparse(maxargs int) {
	flag.Var((*count)(&vflag), "v", "verbosity")
	flag.Parse()
	if maxargs >= 0 && flag.NArg() > maxargs {
	flag.Usage()
	}
	}