// 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"
	"encoding/json"
	"flag"
	"fmt"
	"io/ioutil"
	"log"
	"os"
	"os/exec"
	"path/filepath"
	"sort"
	"strings"
	"sync"
	"time"
)

// Initialization for any invocation.

// The usual variables.
var (
	goarch           string
	gobin            string
	gohostarch       string
	gohostos         string
	goos             string
	goarm            string
	go386            string
	gomips           string
	gomips64         string
	goppc64          string
	goroot           string
	goroot_final     string
	goextlinkenabled string
	gogcflags        string // For running built compiler
	goldflags        string
	workdir          string
	tooldir          string
	oldgoos          string
	oldgoarch        string
	exe              string
	defaultcc        map[string]string
	defaultcxx       map[string]string
	defaultcflags    string
	defaultldflags   string
	defaultpkgconfig string
	defaultldso      string

	rebuildall   bool
	defaultclang bool

	vflag int // verbosity
)

// The known architectures.
var okgoarch = []string{
	"386",
	"amd64",
	"arm",
	"arm64",
	"mips",
	"mipsle",
	"mips64",
	"mips64le",
	"ppc64",
	"ppc64le",
	"riscv64",
	"s390x",
	"sparc64",
	"wasm",
}

// The known operating systems.
var okgoos = []string{
	"darwin",
	"dragonfly",
	"illumos",
	"js",
	"linux",
	"android",
	"solaris",
	"freebsd",
	"nacl", // keep;
	"netbsd",
	"openbsd",
	"plan9",
	"windows",
	"aix",
}

// find reports the first index of p in l[0:n], or else -1.
func find(p string, l []string) int {
	for i, s := range l {
		if p == s {
			return i
		}
	}
	return -1
}

// xinit handles initialization of the various global state, like goroot and goarch.
func xinit() {
	b := os.Getenv("GOROOT")
	if b == "" {
		fatalf("$GOROOT must be set")
	}
	goroot = filepath.Clean(b)

	b = os.Getenv("GOROOT_FINAL")
	if b == "" {
		b = goroot
	}
	goroot_final = b

	b = os.Getenv("GOBIN")
	if b == "" {
		b = pathf("%s/bin", goroot)
	}
	gobin = b

	b = os.Getenv("GOOS")
	if b == "" {
		b = gohostos
	}
	goos = b
	if find(goos, okgoos) < 0 {
		fatalf("unknown $GOOS %s", goos)
	}

	b = os.Getenv("GOARM")
	if b == "" {
		b = xgetgoarm()
	}
	goarm = b

	b = os.Getenv("GO386")
	if b == "" {
		if cansse2() {
			b = "sse2"
		} else {
			b = "387"
		}
	}
	go386 = b

	b = os.Getenv("GOMIPS")
	if b == "" {
		b = "hardfloat"
	}
	gomips = b

	b = os.Getenv("GOMIPS64")
	if b == "" {
		b = "hardfloat"
	}
	gomips64 = b

	b = os.Getenv("GOPPC64")
	if b == "" {
		b = "power8"
	}
	goppc64 = b

	if p := pathf("%s/src/all.bash", goroot); !isfile(p) {
		fatalf("$GOROOT is not set correctly or not exported\n"+
			"\tGOROOT=%s\n"+
			"\t%s does not exist", goroot, p)
	}

	b = os.Getenv("GOHOSTARCH")
	if b != "" {
		gohostarch = b
	}
	if find(gohostarch, okgoarch) < 0 {
		fatalf("unknown $GOHOSTARCH %s", gohostarch)
	}

	b = os.Getenv("GOARCH")
	if b == "" {
		b = gohostarch
	}
	goarch = b
	if find(goarch, okgoarch) < 0 {
		fatalf("unknown $GOARCH %s", goarch)
	}

	b = os.Getenv("GO_EXTLINK_ENABLED")
	if b != "" {
		if b != "0" && b != "1" {
			fatalf("unknown $GO_EXTLINK_ENABLED %s", b)
		}
		goextlinkenabled = b
	}

	gogcflags = os.Getenv("BOOT_GO_GCFLAGS")
	goldflags = os.Getenv("BOOT_GO_LDFLAGS")

	cc, cxx := "gcc", "g++"
	if defaultclang {
		cc, cxx = "clang", "clang++"
	}
	defaultcc = compilerEnv("CC", cc)
	defaultcxx = compilerEnv("CXX", cxx)

	defaultcflags = os.Getenv("CFLAGS")
	defaultldflags = os.Getenv("LDFLAGS")

	b = os.Getenv("PKG_CONFIG")
	if b == "" {
		b = "pkg-config"
	}
	defaultpkgconfig = b

	defaultldso = os.Getenv("GO_LDSO")

	// For tools being invoked but also for os.ExpandEnv.
	os.Setenv("GO386", go386)
	os.Setenv("GOARCH", goarch)
	os.Setenv("GOARM", goarm)
	os.Setenv("GOHOSTARCH", gohostarch)
	os.Setenv("GOHOSTOS", gohostos)
	os.Setenv("GOOS", goos)
	os.Setenv("GOMIPS", gomips)
	os.Setenv("GOMIPS64", gomips64)
	os.Setenv("GOPPC64", goppc64)
	os.Setenv("GOROOT", goroot)
	os.Setenv("GOROOT_FINAL", goroot_final)

	// Use a build cache separate from the default user one.
	// Also one that will be wiped out during startup, so that
	// make.bash really does start from a clean slate.
	os.Setenv("GOCACHE", pathf("%s/pkg/obj/go-build", goroot))

	// Make the environment more predictable.
	os.Setenv("LANG", "C")
	os.Setenv("LANGUAGE", "en_US.UTF8")

	workdir = xworkdir()
	xatexit(rmworkdir)

	tooldir = pathf("%s/pkg/tool/%s_%s", goroot, gohostos, gohostarch)
}

// compilerEnv returns a map from "goos/goarch" to the
// compiler setting to use for that platform.
// The entry for key "" covers any goos/goarch not explicitly set in the map.
// For example, compilerEnv("CC", "gcc") returns the C compiler settings
// read from $CC, defaulting to gcc.
//
// The result is a map because additional environment variables
// can be set to change the compiler based on goos/goarch settings.
// The following applies to all envNames but CC is assumed to simplify
// the presentation.
//
// If no environment variables are set, we use def for all goos/goarch.
// $CC, if set, applies to all goos/goarch but is overridden by the following.
// $CC_FOR_TARGET, if set, applies to all goos/goarch except gohostos/gohostarch,
// but is overridden by the following.
// If gohostos=goos and gohostarch=goarch, then $CC_FOR_TARGET applies even for gohostos/gohostarch.
// $CC_FOR_goos_goarch, if set, applies only to goos/goarch.
func compilerEnv(envName, def string) map[string]string {
	m := map[string]string{"": def}

	if env := os.Getenv(envName); env != "" {
		m[""] = env
	}
	if env := os.Getenv(envName + "_FOR_TARGET"); env != "" {
		if gohostos != goos || gohostarch != goarch {
			m[gohostos+"/"+gohostarch] = m[""]
		}
		m[""] = env
	}

	for _, goos := range okgoos {
		for _, goarch := range okgoarch {
			if env := os.Getenv(envName + "_FOR_" + goos + "_" + goarch); env != "" {
				m[goos+"/"+goarch] = env
			}
		}
	}

	return m
}

// compilerEnvLookup returns the compiler settings for goos/goarch in map m.
func compilerEnvLookup(m map[string]string, goos, goarch string) string {
	if cc := m[goos+"/"+goarch]; cc != "" {
		return cc
	}
	return m[""]
}

// rmworkdir deletes the work directory.
func rmworkdir() {
	if vflag > 1 {
		errprintf("rm -rf %s\n", workdir)
	}
	xremoveall(workdir)
}

// Remove trailing spaces.
func chomp(s string) string {
	return strings.TrimRight(s, " \t\r\n")
}

func branchtag(branch string) (tag string, precise bool) {
	log := run(goroot, CheckExit, "git", "log", "--decorate=full", "--format=format:%d", "master.."+branch)
	tag = branch
	for row, line := range strings.Split(log, "\n") {
		// Each line is either blank, or looks like
		//	  (tag: refs/tags/go1.4rc2, refs/remotes/origin/release-branch.go1.4, refs/heads/release-branch.go1.4)
		// We need to find an element starting with refs/tags/.
		const s = " refs/tags/"
		i := strings.Index(line, s)
		if i < 0 {
			continue
		}
		// Trim off known prefix.
		line = line[i+len(s):]
		// The tag name ends at a comma or paren.
		j := strings.IndexAny(line, ",)")
		if j < 0 {
			continue // malformed line; ignore it
		}
		tag = line[:j]
		if row == 0 {
			precise = true // tag denotes HEAD
		}
		break
	}
	return
}

// findgoversion determines the Go version to use in the version string.
func findgoversion() string {
	// The $GOROOT/VERSION file takes priority, for distributions
	// without the source repo.
	path := pathf("%s/VERSION", goroot)
	if isfile(path) {
		b := chomp(readfile(path))
		// Commands such as "dist version > VERSION" will cause
		// the shell to create an empty VERSION file and set dist's
		// stdout to its fd. dist in turn looks at VERSION and uses
		// its content if available, which is empty at this point.
		// Only use the VERSION file if it is non-empty.
		if b != "" {
			// Some builders cross-compile the toolchain on linux-amd64
			// and then copy the toolchain to the target builder (say, linux-arm)
			// for use there. But on non-release (devel) branches, the compiler
			// used on linux-amd64 will be an amd64 binary, and the compiler
			// shipped to linux-arm will be an arm binary, so they will have different
			// content IDs (they are binaries for different architectures) and so the
			// packages compiled by the running-on-amd64 compiler will appear
			// stale relative to the running-on-arm compiler. Avoid this by setting
			// the version string to something that doesn't begin with devel.
			// Then the version string will be used in place of the content ID,
			// and the packages will look up-to-date.
			// TODO(rsc): Really the builders could be writing out a better VERSION file instead,
			// but it is easier to change cmd/dist than to try to make changes to
			// the builder while Brad is away.
			if strings.HasPrefix(b, "devel") {
				if hostType := os.Getenv("META_BUILDLET_HOST_TYPE"); strings.Contains(hostType, "-cross") {
					fmt.Fprintf(os.Stderr, "warning: changing VERSION from %q to %q\n", b, "builder "+hostType)
					b = "builder " + hostType
				}
			}
			return b
		}
	}

	// The $GOROOT/VERSION.cache file is a cache to avoid invoking
	// git every time we run this command. Unlike VERSION, it gets
	// deleted by the clean command.
	path = pathf("%s/VERSION.cache", goroot)
	if isfile(path) {
		return chomp(readfile(path))
	}

	// Show a nicer error message if this isn't a Git repo.
	if !isGitRepo() {
		fatalf("FAILED: not a Git repo; must put a VERSION file in $GOROOT")
	}

	// Otherwise, use Git.
	// What is the current branch?
	branch := chomp(run(goroot, CheckExit, "git", "rev-parse", "--abbrev-ref", "HEAD"))

	// What are the tags along the current branch?
	tag := "devel"
	precise := false

	// If we're on a release branch, use the closest matching tag
	// that is on the release branch (and not on the master branch).
	if strings.HasPrefix(branch, "release-branch.") {
		tag, precise = branchtag(branch)
	}

	if !precise {
		// Tag does not point at HEAD; add hash and date to version.
		tag += chomp(run(goroot, CheckExit, "git", "log", "-n", "1", "--format=format: +%h %cd", "HEAD"))
	}

	// Cache version.
	writefile(tag, path, 0)

	return tag
}

// isGitRepo reports whether the working directory is inside a Git repository.
func isGitRepo() bool {
	// NB: simply checking the exit code of `git rev-parse --git-dir` would
	// suffice here, but that requires deviating from the infrastructure
	// provided by `run`.
	gitDir := chomp(run(goroot, 0, "git", "rev-parse", "--git-dir"))
	if !filepath.IsAbs(gitDir) {
		gitDir = filepath.Join(goroot, gitDir)
	}
	return isdir(gitDir)
}

/*
 * Initial tree setup.
 */

// The old tools that no longer live in $GOBIN or $GOROOT/bin.
var oldtool = []string{
	"5a", "5c", "5g", "5l",
	"6a", "6c", "6g", "6l",
	"8a", "8c", "8g", "8l",
	"9a", "9c", "9g", "9l",
	"6cov",
	"6nm",
	"6prof",
	"cgo",
	"ebnflint",
	"goapi",
	"gofix",
	"goinstall",
	"gomake",
	"gopack",
	"gopprof",
	"gotest",
	"gotype",
	"govet",
	"goyacc",
	"quietgcc",
}

// Unreleased directories (relative to $GOROOT) that should
// not be in release branches.
var unreleased = []string{
	"src/cmd/newlink",
	"src/cmd/objwriter",
	"src/debug/goobj",
	"src/old",
}

// setup sets up the tree for the initial build.
func setup() {
	// Create bin directory.
	if p := pathf("%s/bin", goroot); !isdir(p) {
		xmkdir(p)
	}

	// Create package directory.
	if p := pathf("%s/pkg", goroot); !isdir(p) {
		xmkdir(p)
	}

	p := pathf("%s/pkg/%s_%s", goroot, gohostos, gohostarch)
	if rebuildall {
		xremoveall(p)
	}
	xmkdirall(p)

	if goos != gohostos || goarch != gohostarch {
		p := pathf("%s/pkg/%s_%s", goroot, goos, goarch)
		if rebuildall {
			xremoveall(p)
		}
		xmkdirall(p)
	}

	// Create object directory.
	// We used to use it for C objects.
	// Now we use it for the build cache, to separate dist's cache
	// from any other cache the user might have.
	p = pathf("%s/pkg/obj/go-build", goroot)
	if rebuildall {
		xremoveall(p)
	}
	xmkdirall(p)
	xatexit(func() { xremoveall(p) })

	// Create tool directory.
	// We keep it in pkg/, just like the object directory above.
	if rebuildall {
		xremoveall(tooldir)
	}
	xmkdirall(tooldir)

	// Remove tool binaries from before the tool/gohostos_gohostarch
	xremoveall(pathf("%s/bin/tool", goroot))

	// Remove old pre-tool binaries.
	for _, old := range oldtool {
		xremove(pathf("%s/bin/%s", goroot, old))
	}

	// If $GOBIN is set and has a Go compiler, it must be cleaned.
	for _, char := range "56789" {
		if isfile(pathf("%s/%c%s", gobin, char, "g")) {
			for _, old := range oldtool {
				xremove(pathf("%s/%s", gobin, old))
			}
			break
		}
	}

	// For release, make sure excluded things are excluded.
	goversion := findgoversion()
	if strings.HasPrefix(goversion, "release.") || (strings.HasPrefix(goversion, "go") && !strings.Contains(goversion, "beta")) {
		for _, dir := range unreleased {
			if p := pathf("%s/%s", goroot, dir); isdir(p) {
				fatalf("%s should not exist in release build", p)
			}
		}
	}
}

/*
 * Tool building
 */

// deptab lists changes to the default dependencies for a given prefix.
// deps ending in /* read the whole directory; deps beginning with -
// exclude files with that prefix.
// Note that this table applies only to the build of cmd/go,
// after the main compiler bootstrap.
var deptab = []struct {
	prefix string   // prefix of target
	dep    []string // dependency tweaks for targets with that prefix
}{
	{"cmd/go/internal/cfg", []string{
		"zdefaultcc.go",
		"zosarch.go",
	}},
	{"runtime/internal/sys", []string{
		"zversion.go",
	}},
	{"go/build", []string{
		"zcgo.go",
	}},
}

// depsuffix records the allowed suffixes for source files.
var depsuffix = []string{
	".s",
	".go",
}

// gentab records how to generate some trivial files.
var gentab = []struct {
	nameprefix string
	gen        func(string, string)
}{
	{"zdefaultcc.go", mkzdefaultcc},
	{"zosarch.go", mkzosarch},
	{"zversion.go", mkzversion},
	{"zcgo.go", mkzcgo},

	// not generated anymore, but delete the file if we see it
	{"enam.c", nil},
	{"anames5.c", nil},
	{"anames6.c", nil},
	{"anames8.c", nil},
	{"anames9.c", nil},
}

// installed maps from a dir name (as given to install) to a chan
// closed when the dir's package is installed.
var installed = make(map[string]chan struct{})
var installedMu sync.Mutex

func install(dir string) {
	<-startInstall(dir)
}

func startInstall(dir string) chan struct{} {
	installedMu.Lock()
	ch := installed[dir]
	if ch == nil {
		ch = make(chan struct{})
		installed[dir] = ch
		go runInstall(dir, ch)
	}
	installedMu.Unlock()
	return ch
}

// runInstall installs the library, package, or binary associated with dir,
// which is relative to $GOROOT/src.
func runInstall(dir string, ch chan struct{}) {
	if dir == "net" || dir == "os/user" || dir == "crypto/x509" {
		fatalf("go_bootstrap cannot depend on cgo package %s", dir)
	}

	defer close(ch)

	if dir == "unsafe" {
		return
	}

	if vflag > 0 {
		if goos != gohostos || goarch != gohostarch {
			errprintf("%s (%s/%s)\n", dir, goos, goarch)
		} else {
			errprintf("%s\n", dir)
		}
	}

	workdir := pathf("%s/%s", workdir, dir)
	xmkdirall(workdir)

	var clean []string
	defer func() {
		for _, name := range clean {
			xremove(name)
		}
	}()

	// path = full path to dir.
	path := pathf("%s/src/%s", goroot, dir)
	name := filepath.Base(dir)

	ispkg := !strings.HasPrefix(dir, "cmd/") || strings.Contains(dir, "/internal/")

	// Start final link command line.
	// Note: code below knows that link.p[targ] is the target.
	var (
		link      []string
		targ      int
		ispackcmd bool
	)
	if ispkg {
		// Go library (package).
		ispackcmd = true
		link = []string{"pack", pathf("%s/pkg/%s_%s/%s.a", goroot, goos, goarch, dir)}
		targ = len(link) - 1
		xmkdirall(filepath.Dir(link[targ]))
	} else {
		// Go command.
		elem := name
		if elem == "go" {
			elem = "go_bootstrap"
		}
		link = []string{pathf("%s/link", tooldir)}
		if goos == "android" {
			link = append(link, "-buildmode=pie")
		}
		if goldflags != "" {
			link = append(link, goldflags)
		}
		link = append(link, "-extld="+compilerEnvLookup(defaultcc, goos, goarch))
		link = append(link, "-o", pathf("%s/%s%s", tooldir, elem, exe))
		targ = len(link) - 1
	}
	ttarg := mtime(link[targ])

	// Gather files that are sources for this target.
	// Everything in that directory, and any target-specific
	// additions.
	files := xreaddir(path)

	// Remove files beginning with . or _,
	// which are likely to be editor temporary files.
	// This is the same heuristic build.ScanDir uses.
	// There do exist real C files beginning with _,
	// so limit that check to just Go files.
	files = filter(files, func(p string) bool {
		return !strings.HasPrefix(p, ".") && (!strings.HasPrefix(p, "_") || !strings.HasSuffix(p, ".go"))
	})

	for _, dt := range deptab {
		if dir == dt.prefix || strings.HasSuffix(dt.prefix, "/") && strings.HasPrefix(dir, dt.prefix) {
			for _, p := range dt.dep {
				p = os.ExpandEnv(p)
				files = append(files, p)
			}
		}
	}
	files = uniq(files)

	// Convert to absolute paths.
	for i, p := range files {
		if !filepath.IsAbs(p) {
			files[i] = pathf("%s/%s", path, p)
		}
	}

	// Is the target up-to-date?
	var gofiles, sfiles, missing []string
	stale := rebuildall
	files = filter(files, func(p string) bool {
		for _, suf := range depsuffix {
			if strings.HasSuffix(p, suf) {
				goto ok
			}
		}
		return false
	ok:
		t := mtime(p)
		if !t.IsZero() && !strings.HasSuffix(p, ".a") && !shouldbuild(p, dir) {
			return false
		}
		if strings.HasSuffix(p, ".go") {
			gofiles = append(gofiles, p)
		} else if strings.HasSuffix(p, ".s") {
			sfiles = append(sfiles, p)
		}
		if t.After(ttarg) {
			stale = true
		}
		if t.IsZero() {
			missing = append(missing, p)
		}
		return true
	})

	// If there are no files to compile, we're done.
	if len(files) == 0 {
		return
	}

	if !stale {
		return
	}

	// For package runtime, copy some files into the work space.
	if dir == "runtime" {
		xmkdirall(pathf("%s/pkg/include", goroot))
		// For use by assembly and C files.
		copyfile(pathf("%s/pkg/include/textflag.h", goroot),
			pathf("%s/src/runtime/textflag.h", goroot), 0)
		copyfile(pathf("%s/pkg/include/funcdata.h", goroot),
			pathf("%s/src/runtime/funcdata.h", goroot), 0)
		copyfile(pathf("%s/pkg/include/asm_ppc64x.h", goroot),
			pathf("%s/src/runtime/asm_ppc64x.h", goroot), 0)
	}

	// Generate any missing files; regenerate existing ones.
	for _, p := range files {
		elem := filepath.Base(p)
		for _, gt := range gentab {
			if gt.gen == nil {
				continue
			}
			if strings.HasPrefix(elem, gt.nameprefix) {
				if vflag > 1 {
					errprintf("generate %s\n", p)
				}
				gt.gen(path, p)
				// Do not add generated file to clean list.
				// In runtime, we want to be able to
				// build the package with the go tool,
				// and it assumes these generated files already
				// exist (it does not know how to build them).
				// The 'clean' command can remove
				// the generated files.
				goto built
			}
		}
		// Did not rebuild p.
		if find(p, missing) >= 0 {
			fatalf("missing file %s", p)
		}
	built:
	}

	// Make sure dependencies are installed.
	var deps []string
	for _, p := range gofiles {
		deps = append(deps, readimports(p)...)
	}
	for _, dir1 := range deps {
		startInstall(dir1)
	}
	for _, dir1 := range deps {
		install(dir1)
	}

	if goos != gohostos || goarch != gohostarch {
		// We've generated the right files; the go command can do the build.
		if vflag > 1 {
			errprintf("skip build for cross-compile %s\n", dir)
		}
		return
	}

	asmArgs := []string{
		pathf("%s/asm", tooldir),
		"-I", workdir,
		"-I", pathf("%s/pkg/include", goroot),
		"-D", "GOOS_" + goos,
		"-D", "GOARCH_" + goarch,
		"-D", "GOOS_GOARCH_" + goos + "_" + goarch,
	}
	if goarch == "mips" || goarch == "mipsle" {
		// Define GOMIPS_value from gomips.
		asmArgs = append(asmArgs, "-D", "GOMIPS_"+gomips)
	}
	if goarch == "mips64" || goarch == "mipsle64" {
		// Define GOMIPS64_value from gomips64.
		asmArgs = append(asmArgs, "-D", "GOMIPS64_"+gomips64)
	}
	goasmh := pathf("%s/go_asm.h", workdir)

	// Collect symabis from assembly code.
	var symabis string
	if len(sfiles) > 0 {
		symabis = pathf("%s/symabis", workdir)
		var wg sync.WaitGroup
		asmabis := append(asmArgs[:len(asmArgs):len(asmArgs)], "-gensymabis", "-o", symabis)
		asmabis = append(asmabis, sfiles...)
		if err := ioutil.WriteFile(goasmh, nil, 0666); err != nil {
			fatalf("cannot write empty go_asm.h: %s", err)
		}
		bgrun(&wg, path, asmabis...)
		bgwait(&wg)
	}

	var archive string
	// The next loop will compile individual non-Go files.
	// Hand the Go files to the compiler en masse.
	// For packages containing assembly, this writes go_asm.h, which
	// the assembly files will need.
	pkg := dir
	if strings.HasPrefix(dir, "cmd/") && strings.Count(dir, "/") == 1 {
		pkg = "main"
	}
	b := pathf("%s/_go_.a", workdir)
	clean = append(clean, b)
	if !ispackcmd {
		link = append(link, b)
	} else {
		archive = b
	}

	// Compile Go code.
	compile := []string{pathf("%s/compile", tooldir), "-std", "-pack", "-o", b, "-p", pkg}
	if gogcflags != "" {
		compile = append(compile, strings.Fields(gogcflags)...)
	}
	if dir == "runtime" {
		compile = append(compile, "-+")
	}
	if len(sfiles) > 0 {
		compile = append(compile, "-asmhdr", goasmh)
	}
	if symabis != "" {
		compile = append(compile, "-symabis", symabis)
	}
	if goos == "android" {
		compile = append(compile, "-shared")
	}

	compile = append(compile, gofiles...)
	var wg sync.WaitGroup
	// We use bgrun and immediately wait for it instead of calling run() synchronously.
	// This executes all jobs through the bgwork channel and allows the process
	// to exit cleanly in case an error occurs.
	bgrun(&wg, path, compile...)
	bgwait(&wg)

	// Compile the files.
	for _, p := range sfiles {
		// Assembly file for a Go package.
		compile := asmArgs[:len(asmArgs):len(asmArgs)]

		doclean := true
		b := pathf("%s/%s", workdir, filepath.Base(p))

		// Change the last character of the output file (which was c or s).
		b = b[:len(b)-1] + "o"
		compile = append(compile, "-o", b, p)
		bgrun(&wg, path, compile...)

		link = append(link, b)
		if doclean {
			clean = append(clean, b)
		}
	}
	bgwait(&wg)

	if ispackcmd {
		xremove(link[targ])
		dopack(link[targ], archive, link[targ+1:])
		return
	}

	// Remove target before writing it.
	xremove(link[targ])
	bgrun(&wg, "", link...)
	bgwait(&wg)
}

// matchfield reports whether the field (x,y,z) matches this build.
// all the elements in the field must be satisfied.
func matchfield(f string) bool {
	for _, tag := range strings.Split(f, ",") {
		if !matchtag(tag) {
			return false
		}
	}
	return true
}

// matchtag reports whether the tag (x or !x) matches this build.
func matchtag(tag string) bool {
	if tag == "" {
		return false
	}
	if tag[0] == '!' {
		if len(tag) == 1 || tag[1] == '!' {
			return false
		}
		return !matchtag(tag[1:])
	}
	return tag == "gc" || tag == goos || tag == goarch || tag == "cmd_go_bootstrap" || tag == "go1.1" || (goos == "android" && tag == "linux") || (goos == "illumos" && tag == "solaris")
}

// shouldbuild reports whether we should build this file.
// It applies the same rules that are used with context tags
// in package go/build, except it's less picky about the order
// of GOOS and GOARCH.
// We also allow the special tag cmd_go_bootstrap.
// See ../go/bootstrap.go and package go/build.
func shouldbuild(file, dir string) bool {
	// Check file name for GOOS or GOARCH.
	name := filepath.Base(file)
	excluded := func(list []string, ok string) bool {
		for _, x := range list {
			if x == ok || (ok == "android" && x == "linux") || (ok == "illumos" && x == "solaris") {
				continue
			}
			i := strings.Index(name, x)
			if i <= 0 || name[i-1] != '_' {
				continue
			}
			i += len(x)
			if i == len(name) || name[i] == '.' || name[i] == '_' {
				return true
			}
		}
		return false
	}
	if excluded(okgoos, goos) || excluded(okgoarch, goarch) {
		return false
	}

	// Omit test files.
	if strings.Contains(name, "_test") {
		return false
	}

	// Check file contents for // +build lines.
	for _, p := range strings.Split(readfile(file), "\n") {
		p = strings.TrimSpace(p)
		if p == "" {
			continue
		}
		code := p
		i := strings.Index(code, "//")
		if i > 0 {
			code = strings.TrimSpace(code[:i])
		}
		if code == "package documentation" {
			return false
		}
		if code == "package main" && dir != "cmd/go" && dir != "cmd/cgo" {
			return false
		}
		if !strings.HasPrefix(p, "//") {
			break
		}
		if !strings.Contains(p, "+build") {
			continue
		}
		fields := strings.Fields(p[2:])
		if len(fields) < 1 || fields[0] != "+build" {
			continue
		}
		for _, p := range fields[1:] {
			if matchfield(p) {
				goto fieldmatch
			}
		}
		return false
	fieldmatch:
	}

	return true
}

// copy copies the file src to dst, via memory (so only good for small files).
func copyfile(dst, src string, flag int) {
	if vflag > 1 {
		errprintf("cp %s %s\n", src, dst)
	}
	writefile(readfile(src), dst, flag)
}

// dopack copies the package src to dst,
// appending the files listed in extra.
// The archive format is the traditional Unix ar format.
func dopack(dst, src string, extra []string) {
	bdst := bytes.NewBufferString(readfile(src))
	for _, file := range extra {
		b := readfile(file)
		// find last path element for archive member name
		i := strings.LastIndex(file, "/") + 1
		j := strings.LastIndex(file, `\`) + 1
		if i < j {
			i = j
		}
		fmt.Fprintf(bdst, "%-16.16s%-12d%-6d%-6d%-8o%-10d`\n", file[i:], 0, 0, 0, 0644, len(b))
		bdst.WriteString(b)
		if len(b)&1 != 0 {
			bdst.WriteByte(0)
		}
	}
	writefile(bdst.String(), dst, 0)
}

var runtimegen = []string{
	"zaexperiment.h",
	"zversion.go",
}

// cleanlist is a list of packages with generated files and commands.
var cleanlist = []string{
	"runtime/internal/sys",
	"cmd/cgo",
	"cmd/go/internal/cfg",
	"go/build",
}

func clean() {
	for _, name := range cleanlist {
		path := pathf("%s/src/%s", goroot, name)
		// Remove generated files.
		for _, elem := range xreaddir(path) {
			for _, gt := range gentab {
				if strings.HasPrefix(elem, gt.nameprefix) {
					xremove(pathf("%s/%s", path, elem))
				}
			}
		}
		// Remove generated binary named for directory.
		if strings.HasPrefix(name, "cmd/") {
			xremove(pathf("%s/%s", path, name[4:]))
		}
	}

	// remove runtimegen files.
	path := pathf("%s/src/runtime", goroot)
	for _, elem := range runtimegen {
		xremove(pathf("%s/%s", path, elem))
	}

	if rebuildall {
		// Remove object tree.
		xremoveall(pathf("%s/pkg/obj/%s_%s", goroot, gohostos, gohostarch))

		// Remove installed packages and tools.
		xremoveall(pathf("%s/pkg/%s_%s", goroot, gohostos, gohostarch))
		xremoveall(pathf("%s/pkg/%s_%s", goroot, goos, goarch))
		xremoveall(pathf("%s/pkg/%s_%s_race", goroot, gohostos, gohostarch))
		xremoveall(pathf("%s/pkg/%s_%s_race", goroot, goos, goarch))
		xremoveall(tooldir)

		// Remove cached version info.
		xremove(pathf("%s/VERSION.cache", goroot))
	}
}

/*
 * command implementations
 */

// The env command prints the default environment.
func cmdenv() {
	path := flag.Bool("p", false, "emit updated PATH")
	plan9 := flag.Bool("9", false, "emit plan 9 syntax")
	windows := flag.Bool("w", false, "emit windows syntax")
	xflagparse(0)

	format := "%s=\"%s\"\n"
	switch {
	case *plan9:
		format = "%s='%s'\n"
	case *windows:
		format = "set %s=%s\r\n"
	}

	xprintf(format, "GOARCH", goarch)
	xprintf(format, "GOBIN", gobin)
	xprintf(format, "GOCACHE", os.Getenv("GOCACHE"))
	xprintf(format, "GODEBUG", os.Getenv("GODEBUG"))
	xprintf(format, "GOHOSTARCH", gohostarch)
	xprintf(format, "GOHOSTOS", gohostos)
	xprintf(format, "GOOS", goos)
	xprintf(format, "GOPROXY", os.Getenv("GOPROXY"))
	xprintf(format, "GOROOT", goroot)
	xprintf(format, "GOTMPDIR", os.Getenv("GOTMPDIR"))
	xprintf(format, "GOTOOLDIR", tooldir)
	if goarch == "arm" {
		xprintf(format, "GOARM", goarm)
	}
	if goarch == "386" {
		xprintf(format, "GO386", go386)
	}
	if goarch == "mips" || goarch == "mipsle" {
		xprintf(format, "GOMIPS", gomips)
	}
	if goarch == "mips64" || goarch == "mips64le" {
		xprintf(format, "GOMIPS64", gomips64)
	}
	if goarch == "ppc64" || goarch == "ppc64le" {
		xprintf(format, "GOPPC64", goppc64)
	}

	if *path {
		sep := ":"
		if gohostos == "windows" {
			sep = ";"
		}
		xprintf(format, "PATH", fmt.Sprintf("%s%s%s", gobin, sep, os.Getenv("PATH")))
	}
}

var (
	timeLogEnabled = os.Getenv("GOBUILDTIMELOGFILE") != ""
	timeLogMu      sync.Mutex
	timeLogFile    *os.File
	timeLogStart   time.Time
)

func timelog(op, name string) {
	if !timeLogEnabled {
		return
	}
	timeLogMu.Lock()
	defer timeLogMu.Unlock()
	if timeLogFile == nil {
		f, err := os.OpenFile(os.Getenv("GOBUILDTIMELOGFILE"), os.O_RDWR|os.O_APPEND, 0666)
		if err != nil {
			log.Fatal(err)
		}
		buf := make([]byte, 100)
		n, _ := f.Read(buf)
		s := string(buf[:n])
		if i := strings.Index(s, "\n"); i >= 0 {
			s = s[:i]
		}
		i := strings.Index(s, " start")
		if i < 0 {
			log.Fatalf("time log %s does not begin with start line", os.Getenv("GOBULDTIMELOGFILE"))
		}
		t, err := time.Parse(time.UnixDate, s[:i])
		if err != nil {
			log.Fatalf("cannot parse time log line %q: %v", s, err)
		}
		timeLogStart = t
		timeLogFile = f
	}
	t := time.Now()
	fmt.Fprintf(timeLogFile, "%s %+.1fs %s %s\n", t.Format(time.UnixDate), t.Sub(timeLogStart).Seconds(), op, name)
}

var toolchain = []string{"cmd/asm", "cmd/cgo", "cmd/compile", "cmd/link"}

// The bootstrap command runs a build from scratch,
// stopping at having installed the go_bootstrap command.
//
// WARNING: This command runs after cmd/dist is built with Go 1.4.
// It rebuilds and installs cmd/dist with the new toolchain, so other
// commands (like "go tool dist test" in run.bash) can rely on bug fixes
// made since Go 1.4, but this function cannot. In particular, the uses
// of os/exec in this function cannot assume that
//	cmd.Env = append(os.Environ(), "X=Y")
// sets $X to Y in the command's environment. That guarantee was
// added after Go 1.4, and in fact in Go 1.4 it was typically the opposite:
// if $X was already present in os.Environ(), most systems preferred
// that setting, not the new one.
func cmdbootstrap() {
	timelog("start", "dist bootstrap")
	defer timelog("end", "dist bootstrap")

	var noBanner bool
	var debug bool
	flag.BoolVar(&rebuildall, "a", rebuildall, "rebuild all")
	flag.BoolVar(&debug, "d", debug, "enable debugging of bootstrap process")
	flag.BoolVar(&noBanner, "no-banner", noBanner, "do not print banner")

	xflagparse(0)

	// Set GOPATH to an internal directory. We shouldn't actually
	// need to store files here, since the toolchain won't
	// depend on modules outside of vendor directories, but if
	// GOPATH points somewhere else (e.g., to GOROOT), the
	// go tool may complain.
	os.Setenv("GOPATH", pathf("%s/pkg/obj/gopath", goroot))

	if debug {
		// cmd/buildid is used in debug mode.
		toolchain = append(toolchain, "cmd/buildid")
	}

	if isdir(pathf("%s/src/pkg", goroot)) {
		fatalf("\n\n"+
			"The Go package sources have moved to $GOROOT/src.\n"+
			"*** %s still exists. ***\n"+
			"It probably contains stale files that may confuse the build.\n"+
			"Please (check what's there and) remove it and try again.\n"+
			"See https://golang.org/s/go14nopkg\n",
			pathf("%s/src/pkg", goroot))
	}

	if rebuildall {
		clean()
	}

	setup()

	timelog("build", "toolchain1")
	checkCC()
	bootstrapBuildTools()

	// Remember old content of $GOROOT/bin for comparison below.
	oldBinFiles, _ := filepath.Glob(pathf("%s/bin/*", goroot))

	// For the main bootstrap, building for host os/arch.
	oldgoos = goos
	oldgoarch = goarch
	goos = gohostos
	goarch = gohostarch
	os.Setenv("GOHOSTARCH", gohostarch)
	os.Setenv("GOHOSTOS", gohostos)
	os.Setenv("GOARCH", goarch)
	os.Setenv("GOOS", goos)

	timelog("build", "go_bootstrap")
	xprintf("Building Go bootstrap cmd/go (go_bootstrap) using Go toolchain1.\n")
	install("runtime") // dependency not visible in sources; also sets up textflag.h
	install("cmd/go")
	if vflag > 0 {
		xprintf("\n")
	}

	gogcflags = os.Getenv("GO_GCFLAGS") // we were using $BOOT_GO_GCFLAGS until now
	goldflags = os.Getenv("GO_LDFLAGS") // we were using $BOOT_GO_LDFLAGS until now
	goBootstrap := pathf("%s/go_bootstrap", tooldir)
	cmdGo := pathf("%s/go", gobin)
	if debug {
		run("", ShowOutput|CheckExit, pathf("%s/compile", tooldir), "-V=full")
		copyfile(pathf("%s/compile1", tooldir), pathf("%s/compile", tooldir), writeExec)
	}

	// To recap, so far we have built the new toolchain
	// (cmd/asm, cmd/cgo, cmd/compile, cmd/link)
	// using Go 1.4's toolchain and go command.
	// Then we built the new go command (as go_bootstrap)
	// using the new toolchain and our own build logic (above).
	//
	//	toolchain1 = mk(new toolchain, go1.4 toolchain, go1.4 cmd/go)
	//	go_bootstrap = mk(new cmd/go, toolchain1, cmd/dist)
	//
	// The toolchain1 we built earlier is built from the new sources,
	// but because it was built using cmd/go it has no build IDs.
	// The eventually installed toolchain needs build IDs, so we need
	// to do another round:
	//
	//	toolchain2 = mk(new toolchain, toolchain1, go_bootstrap)
	//
	timelog("build", "toolchain2")
	if vflag > 0 {
		xprintf("\n")
	}
	xprintf("Building Go toolchain2 using go_bootstrap and Go toolchain1.\n")
	os.Setenv("CC", compilerEnvLookup(defaultcc, goos, goarch))
	goInstall(goBootstrap, append([]string{"-i"}, toolchain...)...)
	if debug {
		run("", ShowOutput|CheckExit, pathf("%s/compile", tooldir), "-V=full")
		run("", ShowOutput|CheckExit, pathf("%s/buildid", tooldir), pathf("%s/pkg/%s_%s/runtime/internal/sys.a", goroot, goos, goarch))
		copyfile(pathf("%s/compile2", tooldir), pathf("%s/compile", tooldir), writeExec)
	}

	// Toolchain2 should be semantically equivalent to toolchain1,
	// but it was built using the new compilers instead of the Go 1.4 compilers,
	// so it should at the least run faster. Also, toolchain1 had no build IDs
	// in the binaries, while toolchain2 does. In non-release builds, the
	// toolchain's build IDs feed into constructing the build IDs of built targets,
	// so in non-release builds, everything now looks out-of-date due to
	// toolchain2 having build IDs - that is, due to the go command seeing
	// that there are new compilers. In release builds, the toolchain's reported
	// version is used in place of the build ID, and the go command does not
	// see that change from toolchain1 to toolchain2, so in release builds,
	// nothing looks out of date.
	// To keep the behavior the same in both non-release and release builds,
	// we force-install everything here.
	//
	//	toolchain3 = mk(new toolchain, toolchain2, go_bootstrap)
	//
	timelog("build", "toolchain3")
	if vflag > 0 {
		xprintf("\n")
	}
	xprintf("Building Go toolchain3 using go_bootstrap and Go toolchain2.\n")
	goInstall(goBootstrap, append([]string{"-a", "-i"}, toolchain...)...)
	if debug {
		run("", ShowOutput|CheckExit, pathf("%s/compile", tooldir), "-V=full")
		run("", ShowOutput|CheckExit, pathf("%s/buildid", tooldir), pathf("%s/pkg/%s_%s/runtime/internal/sys.a", goroot, goos, goarch))
		copyfile(pathf("%s/compile3", tooldir), pathf("%s/compile", tooldir), writeExec)
	}
	checkNotStale(goBootstrap, append(toolchain, "runtime/internal/sys")...)

	if goos == oldgoos && goarch == oldgoarch {
		// Common case - not setting up for cross-compilation.
		timelog("build", "toolchain")
		if vflag > 0 {
			xprintf("\n")
		}
		xprintf("Building packages and commands for %s/%s.\n", goos, goarch)
	} else {
		// GOOS/GOARCH does not match GOHOSTOS/GOHOSTARCH.
		// Finish GOHOSTOS/GOHOSTARCH installation and then
		// run GOOS/GOARCH installation.
		timelog("build", "host toolchain")
		if vflag > 0 {
			xprintf("\n")
		}
		xprintf("Building packages and commands for host, %s/%s.\n", goos, goarch)
		goInstall(goBootstrap, "std", "cmd")
		checkNotStale(goBootstrap, "std", "cmd")
		checkNotStale(cmdGo, "std", "cmd")

		timelog("build", "target toolchain")
		if vflag > 0 {
			xprintf("\n")
		}
		goos = oldgoos
		goarch = oldgoarch
		os.Setenv("GOOS", goos)
		os.Setenv("GOARCH", goarch)
		os.Setenv("CC", compilerEnvLookup(defaultcc, goos, goarch))
		xprintf("Building packages and commands for target, %s/%s.\n", goos, goarch)
	}
	targets := []string{"std", "cmd"}
	if goos == "js" && goarch == "wasm" {
		// Skip the cmd tools for js/wasm. They're not usable.
		targets = targets[:1]
	}
	goInstall(goBootstrap, targets...)
	checkNotStale(goBootstrap, targets...)
	checkNotStale(cmdGo, targets...)
	if debug {
		run("", ShowOutput|CheckExit, pathf("%s/compile", tooldir), "-V=full")
		run("", ShowOutput|CheckExit, pathf("%s/buildid", tooldir), pathf("%s/pkg/%s_%s/runtime/internal/sys.a", goroot, goos, goarch))
		checkNotStale(goBootstrap, append(toolchain, "runtime/internal/sys")...)
		copyfile(pathf("%s/compile4", tooldir), pathf("%s/compile", tooldir), writeExec)
	}

	// Check that there are no new files in $GOROOT/bin other than
	// go and gofmt and $GOOS_$GOARCH (target bin when cross-compiling).
	binFiles, _ := filepath.Glob(pathf("%s/bin/*", goroot))
	ok := map[string]bool{}
	for _, f := range oldBinFiles {
		ok[f] = true
	}
	for _, f := range binFiles {
		elem := strings.TrimSuffix(filepath.Base(f), ".exe")
		if !ok[f] && elem != "go" && elem != "gofmt" && elem != goos+"_"+goarch {
			fatalf("unexpected new file in $GOROOT/bin: %s", elem)
		}
	}

	// Remove go_bootstrap now that we're done.
	xremove(pathf("%s/go_bootstrap", tooldir))

	if goos == "android" {
		// Make sure the exec wrapper will sync a fresh $GOROOT to the device.
		xremove(pathf("%s/go_android_exec-adb-sync-status", os.TempDir()))
	}

	if wrapperPath := wrapperPathFor(goos, goarch); wrapperPath != "" {
		oldcc := os.Getenv("CC")
		os.Setenv("GOOS", gohostos)
		os.Setenv("GOARCH", gohostarch)
		os.Setenv("CC", compilerEnvLookup(defaultcc, gohostos, gohostarch))
		goCmd(cmdGo, "build", "-o", pathf("%s/go_%s_%s_exec%s", gobin, goos, goarch, exe), wrapperPath)
		// Restore environment.
		// TODO(elias.naur): support environment variables in goCmd?
		os.Setenv("GOOS", goos)
		os.Setenv("GOARCH", goarch)
		os.Setenv("CC", oldcc)
	}

	// Print trailing banner unless instructed otherwise.
	if !noBanner {
		banner()
	}
}

func wrapperPathFor(goos, goarch string) string {
	switch {
	case goos == "android":
		if gohostos != "android" {
			return pathf("%s/misc/android/go_android_exec.go", goroot)
		}
	case goos == "darwin" && (goarch == "arm" || goarch == "arm64"):
		if gohostos != "darwin" || (gohostarch != "arm" && gohostarch != "arm64") {
			return pathf("%s/misc/ios/go_darwin_arm_exec.go", goroot)
		}
	}
	return ""
}

func goInstall(goBinary string, args ...string) {
	goCmd(goBinary, "install", args...)
}

func goCmd(goBinary string, cmd string, args ...string) {
	goCmd := []string{goBinary, cmd, "-gcflags=all=" + gogcflags, "-ldflags=all=" + goldflags}
	if vflag > 0 {
		goCmd = append(goCmd, "-v")
	}

	// Force only one process at a time on vx32 emulation.
	if gohostos == "plan9" && os.Getenv("sysname") == "vx32" {
		goCmd = append(goCmd, "-p=1")
	}

	run(goroot, ShowOutput|CheckExit, append(goCmd, args...)...)
}

func checkNotStale(goBinary string, targets ...string) {
	out := run(goroot, CheckExit,
		append([]string{
			goBinary,
			"list", "-gcflags=all=" + gogcflags, "-ldflags=all=" + goldflags,
			"-f={{if .Stale}}\tSTALE {{.ImportPath}}: {{.StaleReason}}{{end}}",
		}, targets...)...)
	if strings.Contains(out, "\tSTALE ") {
		os.Setenv("GODEBUG", "gocachehash=1")
		for _, target := range []string{"runtime/internal/sys", "cmd/dist", "cmd/link"} {
			if strings.Contains(out, "STALE "+target) {
				run(goroot, ShowOutput|CheckExit, goBinary, "list", "-f={{.ImportPath}} {{.Stale}}", target)
				break
			}
		}
		fatalf("unexpected stale targets reported by %s list -gcflags=\"%s\" -ldflags=\"%s\" for %v:\n%s", goBinary, gogcflags, goldflags, targets, out)
	}
}

// Cannot use go/build directly because cmd/dist for a new release
// builds against an old release's go/build, which may be out of sync.
// To reduce duplication, we generate the list for go/build from this.
//
// We list all supported platforms in this list, so that this is the
// single point of truth for supported platforms. This list is used
// by 'go tool dist list'.
var cgoEnabled = map[string]bool{
	"aix/ppc64":       true,
	"darwin/386":      false, // Issue 31751
	"darwin/amd64":    true,
	"darwin/arm":      true,
	"darwin/arm64":    true,
	"dragonfly/amd64": true,
	"freebsd/386":     true,
	"freebsd/amd64":   true,
	"freebsd/arm":     true,
	"illumos/amd64":   true,
	"linux/386":       true,
	"linux/amd64":     true,
	"linux/arm":       true,
	"linux/arm64":     true,
	"linux/ppc64":     false,
	"linux/ppc64le":   true,
	"linux/mips":      true,
	"linux/mipsle":    true,
	"linux/mips64":    true,
	"linux/mips64le":  true,
	"linux/riscv64":   true,
	"linux/s390x":     true,
	"linux/sparc64":   true,
	"android/386":     true,
	"android/amd64":   true,
	"android/arm":     true,
	"android/arm64":   true,
	"js/wasm":         false,
	"netbsd/386":      true,
	"netbsd/amd64":    true,
	"netbsd/arm":      true,
	"netbsd/arm64":    true,
	"openbsd/386":     true,
	"openbsd/amd64":   true,
	"openbsd/arm":     true,
	"openbsd/arm64":   true,
	"plan9/386":       false,
	"plan9/amd64":     false,
	"plan9/arm":       false,
	"solaris/amd64":   true,
	"windows/386":     true,
	"windows/amd64":   true,
	"windows/arm":     false,
}

// List of platforms which are supported but not complete yet. These get
// filtered out of cgoEnabled for 'dist list'. See golang.org/issue/28944
var incomplete = map[string]bool{
	"linux/riscv64": true,
	"linux/sparc64": true,
}

func needCC() bool {
	switch os.Getenv("CGO_ENABLED") {
	case "1":
		return true
	case "0":
		return false
	}
	return cgoEnabled[gohostos+"/"+gohostarch]
}

func checkCC() {
	if !needCC() {
		return
	}
	if output, err := exec.Command(defaultcc[""], "--help").CombinedOutput(); err != nil {
		outputHdr := ""
		if len(output) > 0 {
			outputHdr = "\nCommand output:\n\n"
		}
		fatalf("cannot invoke C compiler %q: %v\n\n"+
			"Go needs a system C compiler for use with cgo.\n"+
			"To set a C compiler, set CC=the-compiler.\n"+
			"To disable cgo, set CGO_ENABLED=0.\n%s%s", defaultcc[""], err, outputHdr, output)
	}
}

func defaulttarg() string {
	// xgetwd might return a path with symlinks fully resolved, and if
	// there happens to be symlinks in goroot, then the hasprefix test
	// will never succeed. Instead, we use xrealwd to get a canonical
	// goroot/src before the comparison to avoid this problem.
	pwd := xgetwd()
	src := pathf("%s/src/", goroot)
	real_src := xrealwd(src)
	if !strings.HasPrefix(pwd, real_src) {
		fatalf("current directory %s is not under %s", pwd, real_src)
	}
	pwd = pwd[len(real_src):]
	// guard against xrealwd returning the directory without the trailing /
	pwd = strings.TrimPrefix(pwd, "/")

	return pwd
}

// Install installs the list of packages named on the command line.
func cmdinstall() {
	xflagparse(-1)

	if flag.NArg() == 0 {
		install(defaulttarg())
	}

	for _, arg := range flag.Args() {
		install(arg)
	}
}

// Clean deletes temporary objects.
func cmdclean() {
	xflagparse(0)
	clean()
}

// Banner prints the 'now you've installed Go' banner.
func cmdbanner() {
	xflagparse(0)
	banner()
}

func banner() {
	if vflag > 0 {
		xprintf("\n")
	}
	xprintf("---\n")
	xprintf("Installed Go for %s/%s in %s\n", goos, goarch, goroot)
	xprintf("Installed commands in %s\n", gobin)

	if !xsamefile(goroot_final, goroot) {
		// If the files are to be moved, don't check that gobin
		// is on PATH; assume they know what they are doing.
	} else if gohostos == "plan9" {
		// Check that gobin is bound before /bin.
		pid := strings.Replace(readfile("#c/pid"), " ", "", -1)
		ns := fmt.Sprintf("/proc/%s/ns", pid)
		if !strings.Contains(readfile(ns), fmt.Sprintf("bind -b %s /bin", gobin)) {
			xprintf("*** You need to bind %s before /bin.\n", gobin)
		}
	} else {
		// Check that gobin appears in $PATH.
		pathsep := ":"
		if gohostos == "windows" {
			pathsep = ";"
		}
		if !strings.Contains(pathsep+os.Getenv("PATH")+pathsep, pathsep+gobin+pathsep) {
			xprintf("*** You need to add %s to your PATH.\n", gobin)
		}
	}

	if !xsamefile(goroot_final, goroot) {
		xprintf("\n"+
			"The binaries expect %s to be copied or moved to %s\n",
			goroot, goroot_final)
	}
}

// Version prints the Go version.
func cmdversion() {
	xflagparse(0)
	xprintf("%s\n", findgoversion())
}

// cmdlist lists all supported platforms.
func cmdlist() {
	jsonFlag := flag.Bool("json", false, "produce JSON output")
	xflagparse(0)

	var plats []string
	for p := range cgoEnabled {
		if incomplete[p] {
			continue
		}
		plats = append(plats, p)
	}
	sort.Strings(plats)

	if !*jsonFlag {
		for _, p := range plats {
			xprintf("%s\n", p)
		}
		return
	}

	type jsonResult struct {
		GOOS         string
		GOARCH       string
		CgoSupported bool
	}
	var results []jsonResult
	for _, p := range plats {
		fields := strings.Split(p, "/")
		results = append(results, jsonResult{
			GOOS:         fields[0],
			GOARCH:       fields[1],
			CgoSupported: cgoEnabled[p]})
	}
	out, err := json.MarshalIndent(results, "", "\t")
	if err != nil {
		fatalf("json marshal error: %v", err)
	}
	if _, err := os.Stdout.Write(out); err != nil {
		fatalf("write failed: %v", err)
	}
}