src/cmd/go/build.go

// 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 main

import (
	"bytes"
	"container/heap"
	"errors"
	"fmt"
	"go/build"
	"io"
	"io/ioutil"
	"os"
	"os/exec"
	"path"
	"path/filepath"
	"regexp"
	"runtime"
	"strings"
	"sync"
)

var cmdBuild = &Command{
	UsageLine: "build [-a] [-n] [-o output] [-p n] [-v] [-x] [importpath... | gofiles...]",
	Short:     "compile packages and dependencies",
	Long: `
Build compiles the packages named by the import paths,
along with their dependencies, but it does not install the results.

If the arguments are a list of .go files, build treats them as a list
of source files specifying a single package.

When the command line specifies a single main package,
build writes the resulting executable to output (default a.out).
Otherwise build compiles the packages but discards the results,
serving only as a check that the packages can be built.

The -a flag forces rebuilding of packages that are already up-to-date.
The -n flag prints the commands but does not run them.
The -v flag prints the names of packages as they are compiled.
The -x flag prints the commands.

The -o flag specifies the output file name.
It is an error to use -o when the command line specifies multiple packages.

The -p flag specifies the number of builds that can be run in parallel.
The default is the number of CPUs available.

For more about import paths, see 'go help importpath'.

See also: go install, go get, go clean.
	`,
}

func init() {
	// break init cycle
	cmdBuild.Run = runBuild
	cmdInstall.Run = runInstall

	addBuildFlags(cmdBuild)
	addBuildFlags(cmdInstall)
}

// Flags set by multiple commands.
var buildA bool               // -a flag
var buildN bool               // -n flag
var buildP = runtime.NumCPU() // -p flag
var buildV bool               // -v flag
var buildX bool               // -x flag
var buildO = cmdBuild.Flag.String("o", "", "output file")

var buildContext = build.DefaultContext

// addBuildFlags adds the flags common to the build and install commands.
func addBuildFlags(cmd *Command) {
	cmd.Flag.BoolVar(&buildA, "a", false, "")
	cmd.Flag.BoolVar(&buildN, "n", false, "")
	cmd.Flag.IntVar(&buildP, "p", buildP, "")
	cmd.Flag.BoolVar(&buildV, "v", false, "")
	cmd.Flag.BoolVar(&buildX, "x", false, "")

	// TODO(rsc): This -t flag is used by buildscript.sh but
	// not documented.  Should be documented but the
	// usage lines are getting too long.  Probably need to say
	// that these flags are applicable to every command and
	// document them in one help message instead of on every
	// command's help message.
	cmd.Flag.Var((*stringsFlag)(&buildContext.BuildTags), "t", "")
}

type stringsFlag []string

func (v *stringsFlag) Set(s string) error {
	*v = append(*v, s)
	return nil
}

func (v *stringsFlag) String() string {
	return "<stringsFlag>"
}

func runBuild(cmd *Command, args []string) {
	var b builder
	b.init()

	var pkgs []*Package
	if len(args) > 0 && strings.HasSuffix(args[0], ".go") {
		pkg := goFilesPackage(args, "")
		pkgs = append(pkgs, pkg)
	} else {
		pkgs = packagesForBuild(args)
	}

	if len(pkgs) == 1 && pkgs[0].Name == "main" && *buildO == "" {
		_, *buildO = path.Split(pkgs[0].ImportPath)
		if b.goos == "windows" {
			*buildO += ".exe"
		}
	}

	if *buildO != "" {
		if len(pkgs) > 1 {
			fatalf("go build: cannot use -o with multiple packages")
		}
		p := pkgs[0]
		p.target = "" // must build - not up to date
		a := b.action(modeInstall, modeBuild, p)
		a.target = *buildO
		b.do(a)
		return
	}

	a := &action{}
	for _, p := range packages(args) {
		a.deps = append(a.deps, b.action(modeBuild, modeBuild, p))
	}
	b.do(a)
}

var cmdInstall = &Command{
	UsageLine: "install [-a] [-n] [-p n] [-v] [-x] [importpath...]",
	Short:     "compile and install packages and dependencies",
	Long: `
Install compiles and installs the packages named by the import paths,
along with their dependencies.

The -a flag forces reinstallation of packages that are already up-to-date.
The -n flag prints the commands but does not run them.
The -v flag prints the names of packages as they are compiled.
The -x flag prints the commands.

The -p flag specifies the number of builds that can be run in parallel.
The default is the number of CPUs available.

For more about import paths, see 'go help importpath'.

See also: go build, go get, go clean.
	`,
}

func runInstall(cmd *Command, args []string) {
	pkgs := packagesForBuild(args)

	var b builder
	b.init()
	a := &action{}
	for _, p := range pkgs {
		a.deps = append(a.deps, b.action(modeInstall, modeInstall, p))
	}
	b.do(a)
}

// A builder holds global state about a build.
// It does not hold per-package state, because eventually we will
// build packages in parallel, and the builder will be shared.
type builder struct {
	work        string               // the temporary work directory (ends in filepath.Separator)
	arch        string               // e.g., "6"
	goarch      string               // the $GOARCH
	goos        string               // the $GOOS
	exe         string               // the executable suffix - "" or ".exe"
	gcflags     []string             // additional flags for Go compiler
	actionCache map[cacheKey]*action // a cache of already-constructed actions
	mkdirCache  map[string]bool      // a cache of created directories
	print       func(args ...interface{}) (int, error)

	output    sync.Mutex
	scriptDir string // current directory in printed script

	exec      sync.Mutex
	readySema chan bool
	ready     actionQueue
}

// An action represents a single action in the action graph.
type action struct {
	p          *Package      // the package this action works on
	deps       []*action     // actions that must happen before this one
	triggers   []*action     // inverse of deps
	cgo        *action       // action for cgo binary if needed
	args       []string      // additional args for runProgram
	testOutput *bytes.Buffer // test output buffer

	f          func(*builder, *action) error // the action itself (nil = no-op)
	ignoreFail bool                          // whether to run f even if dependencies fail

	// Generated files, directories.
	link   bool   // target is executable, not just package
	pkgdir string // the -I or -L argument to use when importing this package
	objdir string // directory for intermediate objects
	objpkg string // the intermediate package .a file created during the action
	target string // goal of the action: the created package or executable

	// Execution state.
	pending  int  // number of deps yet to complete
	priority int  // relative execution priority
	failed   bool // whether the action failed
}

// cacheKey is the key for the action cache.
type cacheKey struct {
	mode buildMode
	p    *Package
}

// buildMode specifies the build mode:
// are we just building things or also installing the results?
type buildMode int

const (
	modeBuild buildMode = iota
	modeInstall
)

var (
	gobin  = build.Path[0].BinDir()
	goroot = build.Path[0].Path
)

func (b *builder) init() {
	var err error
	b.print = fmt.Print
	b.actionCache = make(map[cacheKey]*action)
	b.mkdirCache = make(map[string]bool)
	b.goarch = buildContext.GOARCH
	b.goos = buildContext.GOOS
	if b.goos == "windows" {
		b.exe = ".exe"
	}
	b.gcflags = strings.Fields(os.Getenv("GCFLAGS"))

	b.arch, err = build.ArchChar(b.goarch)
	if err != nil {
		fatalf("%s", err)
	}

	if buildN {
		b.work = "$WORK"
	} else {
		b.work, err = ioutil.TempDir("", "go-build")
		if err != nil {
			fatalf("%s", err)
		}
		if buildX {
			fmt.Printf("WORK=%s\n", b.work)
		}
		atexit(func() { os.RemoveAll(b.work) })
	}
}

// goFilesPackage creates a package for building a collection of Go files
// (typically named on the command line).  If target is given, the package
// target is target.  Otherwise, the target is named p.a for
// package p or named after the first Go file for package main.
func goFilesPackage(gofiles []string, target string) *Package {
	// TODO: Remove this restriction.
	for _, f := range gofiles {
		if !strings.HasSuffix(f, ".go") || strings.Contains(f, "/") || strings.Contains(f, string(filepath.Separator)) {
			fatalf("named files must be in current directory and .go files")
		}
	}

	// Synthesize fake "directory" that only shows those two files,
	// to make it look like this is a standard package or
	// command directory.
	var dir []os.FileInfo
	for _, file := range gofiles {
		fi, err := os.Stat(file)
		if err != nil {
			fatalf("%s", err)
		}
		if fi.IsDir() {
			fatalf("%s is a directory, should be a Go file", file)
		}
		dir = append(dir, fi)
	}
	ctxt := buildContext
	ctxt.ReadDir = func(string) ([]os.FileInfo, error) { return dir, nil }
	pwd, _ := os.Getwd()
	var stk importStack
	pkg := scanPackage(&ctxt, &build.Tree{Path: "."}, "<command line>", "<command line>", pwd+"/.", &stk)
	if pkg.Error != nil {
		fatalf("%s", pkg.Error)
	}
	printed := map[error]bool{}
	for _, err := range pkg.DepsErrors {
		// Since these are errors in dependencies,
		// the same error might show up multiple times,
		// once in each package that depends on it.
		// Only print each once.
		if !printed[err] {
			printed[err] = true
			errorf("%s", err)
		}
	}
	if target != "" {
		pkg.target = target
	} else if pkg.Name == "main" {
		pkg.target = gofiles[0][:len(gofiles[0])-len(".go")]
	} else {
		pkg.target = pkg.Name + ".a"
	}
	pkg.ImportPath = "_/" + pkg.target
	exitIfErrors()
	return pkg
}

// action returns the action for applying the given operation (mode) to the package.
// depMode is the action to use when building dependencies.
func (b *builder) action(mode buildMode, depMode buildMode, p *Package) *action {
	key := cacheKey{mode, p}
	a := b.actionCache[key]
	if a != nil {
		return a
	}

	a = &action{p: p, pkgdir: p.t.PkgDir()}
	if p.pkgdir != "" { // overrides p.t
		a.pkgdir = p.pkgdir
	}

	b.actionCache[key] = a

	for _, p1 := range p.imports {
		a.deps = append(a.deps, b.action(depMode, depMode, p1))
	}

	if len(p.CgoFiles) > 0 {
		var stk importStack
		p1 := loadPackage("cmd/cgo", &stk)
		if p1.Error != nil {
			fatalf("load cmd/cgo: %v", p1.Error)
		}
		a.cgo = b.action(depMode, depMode, p1)
		a.deps = append(a.deps, a.cgo)
	}

	if p.Standard {
		switch p.ImportPath {
		case "builtin", "unsafe":
			// Fake packages - nothing to build.
			return a
		}
		// gccgo standard library is "fake" too.
		if _, ok := buildToolchain.(gccgoToolchain); ok {
			// the target name is needed for cgo.
			a.target = p.target
			return a
		}
	}

	if !p.Stale && !buildA && p.target != "" {
		// p.Stale==false implies that p.target is up-to-date.
		// Record target name for use by actions depending on this one.
		a.target = p.target
		return a
	}

	a.objdir = filepath.Join(b.work, filepath.FromSlash(a.p.ImportPath+"/_obj")) + string(filepath.Separator)
	a.objpkg = buildToolchain.pkgpath(b.work, a.p)
	a.link = p.Name == "main"

	switch mode {
	case modeInstall:
		a.f = (*builder).install
		a.deps = []*action{b.action(modeBuild, depMode, p)}
		a.target = a.p.target
	case modeBuild:
		a.f = (*builder).build
		a.target = a.objpkg
		if a.link {
			// An executable file.
			// (This is the name of a temporary file.)
			a.target = a.objdir + "a.out" + b.exe
		}
	}

	return a
}

// actionList returns the list of actions in the dag rooted at root
// as visited in a depth-first post-order traversal.
func actionList(root *action) []*action {
	seen := map[*action]bool{}
	all := []*action{}
	var walk func(*action)
	walk = func(a *action) {
		if seen[a] {
			return
		}
		seen[a] = true
		for _, a1 := range a.deps {
			walk(a1)
		}
		all = append(all, a)
	}
	walk(root)
	return all
}

// do runs the action graph rooted at root.
func (b *builder) do(root *action) {
	// Build list of all actions, assigning depth-first post-order priority.
	// The original implementation here was a true queue
	// (using a channel) but it had the effect of getting
	// distracted by low-level leaf actions to the detriment
	// of completing higher-level actions.  The order of
	// work does not matter much to overall execution time,
	// but when running "go test std" it is nice to see each test
	// results as soon as possible.  The priorities assigned
	// ensure that, all else being equal, the execution prefers
	// to do what it would have done first in a simple depth-first
	// dependency order traversal.
	all := actionList(root)
	for i, a := range all {
		a.priority = i
	}

	b.readySema = make(chan bool, len(all))
	done := make(chan bool)

	// Initialize per-action execution state.
	for _, a := range all {
		for _, a1 := range a.deps {
			a1.triggers = append(a1.triggers, a)
		}
		a.pending = len(a.deps)
		if a.pending == 0 {
			b.ready.push(a)
			b.readySema <- true
		}
	}

	// Handle runs a single action and takes care of triggering
	// any actions that are runnable as a result.
	handle := func(a *action) {
		var err error
		if a.f != nil && (!a.failed || a.ignoreFail) {
			err = a.f(b, a)
		}

		// The actions run in parallel but all the updates to the
		// shared work state are serialized through b.exec.
		b.exec.Lock()
		defer b.exec.Unlock()

		if err != nil {
			if err == errPrintedOutput {
				setExitStatus(2)
			} else {
				errorf("%s", err)
			}
			a.failed = true
		}

		for _, a0 := range a.triggers {
			if a.failed {
				a0.failed = true
			}
			if a0.pending--; a0.pending == 0 {
				b.ready.push(a0)
				b.readySema <- true
			}
		}

		if a == root {
			close(b.readySema)
			done <- true
		}
	}

	// Kick off goroutines according to parallelism.
	// If we are using the -n flag (just printing commands)
	// drop the parallelism to 1, both to make the output
	// deterministic and because there is no real work anyway.
	par := buildP
	if buildN {
		par = 1
	}
	for i := 0; i < par; i++ {
		go func() {
			for _ = range b.readySema {
				// Receiving a value from b.sema entitles
				// us to take from the ready queue.
				b.exec.Lock()
				a := b.ready.pop()
				b.exec.Unlock()
				handle(a)
			}
		}()
	}

	<-done
}

// build is the action for building a single package or command.
func (b *builder) build(a *action) error {
	if buildN {
		// In -n mode, print a banner between packages.
		// The banner is five lines so that when changes to
		// different sections of the bootstrap script have to
		// be merged, the banners give patch something
		// to use to find its context.
		fmt.Printf("\n#\n# %s\n#\n\n", a.p.ImportPath)
	}

	if buildV {
		fmt.Fprintf(os.Stderr, "%s\n", a.p.ImportPath)
	}

	// Make build directory.
	obj := a.objdir
	if err := b.mkdir(obj); err != nil {
		return err
	}

	var gofiles, cfiles, sfiles, objects, cgoObjects []string
	gofiles = append(gofiles, a.p.GoFiles...)
	cfiles = append(cfiles, a.p.CFiles...)
	sfiles = append(sfiles, a.p.SFiles...)

	// Run cgo.
	if len(a.p.CgoFiles) > 0 {
		// In a package using cgo, cgo compiles the C and assembly files with gcc.  
		// There is one exception: runtime/cgo's job is to bridge the
		// cgo and non-cgo worlds, so it necessarily has files in both.
		// In that case gcc only gets the gcc_* files.
		var gccfiles []string
		if a.p.Standard && a.p.ImportPath == "runtime/cgo" {
			filter := func(files, nongcc, gcc []string) ([]string, []string) {
				for _, f := range files {
					if strings.HasPrefix(f, "gcc_") {
						gcc = append(gcc, f)
					} else {
						nongcc = append(nongcc, f)
					}
				}
				return nongcc, gcc
			}
			cfiles, gccfiles = filter(cfiles, cfiles[:0], gccfiles)
			sfiles, gccfiles = filter(sfiles, sfiles[:0], gccfiles)
		} else {
			gccfiles = append(cfiles, sfiles...)
			cfiles = nil
			sfiles = nil
		}

		outGo, outObj, err := b.cgo(a.p, a.cgo.target, obj, gccfiles)
		if err != nil {
			return err
		}
		cgoObjects = append(cgoObjects, outObj...)
		gofiles = append(gofiles, outGo...)
	}

	// Prepare Go import path list.
	inc := b.includeArgs("-I", a.deps)

	// Compile Go.
	if len(gofiles) > 0 {
		if out, err := buildToolchain.gc(b, a.p, obj, inc, gofiles); err != nil {
			return err
		} else {
			objects = append(objects, out)
		}
	}

	// Copy .h files named for goos or goarch or goos_goarch
	// to names using GOOS and GOARCH.
	// For example, defs_linux_amd64.h becomes defs_GOOS_GOARCH.h.
	_goos_goarch := "_" + b.goos + "_" + b.goarch + ".h"
	_goos := "_" + b.goos + ".h"
	_goarch := "_" + b.goarch + ".h"
	for _, file := range a.p.HFiles {
		switch {
		case strings.HasSuffix(file, _goos_goarch):
			targ := file[:len(file)-len(_goos_goarch)] + "_GOOS_GOARCH.h"
			if err := b.copyFile(obj+targ, filepath.Join(a.p.Dir, file), 0666); err != nil {
				return err
			}
		case strings.HasSuffix(file, _goarch):
			targ := file[:len(file)-len(_goarch)] + "_GOARCH.h"
			if err := b.copyFile(obj+targ, filepath.Join(a.p.Dir, file), 0666); err != nil {
				return err
			}
		case strings.HasSuffix(file, _goos):
			targ := file[:len(file)-len(_goos)] + "_GOOS.h"
			if err := b.copyFile(obj+targ, filepath.Join(a.p.Dir, file), 0666); err != nil {
				return err
			}
		}
	}

	for _, file := range cfiles {
		out := file[:len(file)-len(".c")] + "." + b.arch
		if err := buildToolchain.cc(b, a.p, obj, obj+out, file); err != nil {
			return err
		}
		objects = append(objects, out)
	}

	// Assemble .s files.
	for _, file := range sfiles {
		out := file[:len(file)-len(".s")] + "." + b.arch
		if err := buildToolchain.asm(b, a.p, obj, obj+out, file); err != nil {
			return err
		}
		objects = append(objects, out)
	}

	// NOTE(rsc): On Windows, it is critically important that the
	// gcc-compiled objects (cgoObjects) be listed after the ordinary
	// objects in the archive.  I do not know why this is.
	// http://golang.org/issue/2601
	objects = append(objects, cgoObjects...)

	// Pack into archive in obj directory
	if err := buildToolchain.pack(b, a.p, obj, a.objpkg, objects); err != nil {
		return err
	}

	// Link if needed.
	if a.link {
		// The compiler only cares about direct imports, but the
		// linker needs the whole dependency tree.
		all := actionList(a)
		all = all[:len(all)-1] // drop a
		if err := buildToolchain.ld(b, a.p, a.target, all, a.objpkg, objects); err != nil {
			return err
		}
	}

	return nil
}

// install is the action for installing a single package or executable.
func (b *builder) install(a *action) error {
	a1 := a.deps[0]
	perm := os.FileMode(0666)
	if a1.link {
		perm = 0777
	}

	// make target directory
	dir, _ := filepath.Split(a.target)
	if dir != "" {
		if err := b.mkdir(dir); err != nil {
			return err
		}
	}

	// remove object dir to keep the amount of
	// garbage down in a large build.  On an operating system
	// with aggressive buffering, cleaning incrementally like
	// this keeps the intermediate objects from hitting the disk.
	defer os.RemoveAll(a1.objdir)
	defer os.Remove(a1.target)

	return b.copyFile(a.target, a1.target, perm)
}

// includeArgs returns the -I or -L directory list for access
// to the results of the list of actions.
func (b *builder) includeArgs(flag string, all []*action) []string {
	inc := []string{}
	incMap := map[string]bool{
		b.work:                 true, // handled later
		build.Path[0].PkgDir(): true, // goroot
		"":                     true, // ignore empty strings
	}

	// Look in the temporary space for results of test-specific actions.
	// This is the $WORK/my/package/_test directory for the
	// package being built, so there are few of these.
	for _, a1 := range all {
		if dir := a1.pkgdir; dir != a1.p.t.PkgDir() && !incMap[dir] {
			incMap[dir] = true
			inc = append(inc, flag, dir)
		}
	}

	// Also look in $WORK for any non-test packages that have
	// been built but not installed.
	inc = append(inc, flag, b.work)

	// Finally, look in the installed package directories for each action.
	for _, a1 := range all {
		if dir := a1.pkgdir; dir == a1.p.t.PkgDir() && !incMap[dir] {
			if _, ok := buildToolchain.(gccgoToolchain); ok {
				dir = filepath.Join(filepath.Dir(dir), "gccgo", filepath.Base(dir))
			}
			incMap[dir] = true
			inc = append(inc, flag, dir)
		}
	}

	return inc
}

// copyFile is like 'cp src dst'.
func (b *builder) copyFile(dst, src string, perm os.FileMode) error {
	if buildN || buildX {
		b.showcmd("", "cp %s %s", src, dst)
		if buildN {
			return nil
		}
	}

	sf, err := os.Open(src)
	if err != nil {
		return err
	}
	defer sf.Close()

	// On Windows, remove lingering ~ file from last attempt.
	if toolIsWindows {
		if _, err := os.Stat(dst + "~"); err == nil {
			os.Remove(dst + "~")
		}
	}

	os.Remove(dst)
	df, err := os.OpenFile(dst, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, perm)
	if err != nil && toolIsWindows {
		// Windows does not allow deletion of a binary file
		// while it is executing.  Try to move it out of the way.
		// If the remove fails, which is likely, we'll try again the
		// next time we do an install of this binary.
		if err := os.Rename(dst, dst+"~"); err == nil {
			os.Remove(dst + "~")
		}
		df, err = os.OpenFile(dst, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, perm)
	}
	if err != nil {
		return err
	}

	_, err = io.Copy(df, sf)
	df.Close()
	if err != nil {
		os.Remove(dst)
		return err
	}
	return nil
}

// fmtcmd formats a command in the manner of fmt.Sprintf but also:
//
//	If dir is non-empty and the script is not in dir right now,
//	fmtcmd inserts "cd dir\n" before the command.
//
//	fmtcmd replaces the value of b.work with $WORK.
//	fmtcmd replaces the value of goroot with $GOROOT.
//	fmtcmd replaces the value of b.gobin with $GOBIN.
//
//	fmtcmd replaces the name of the current directory with dot (.)
//	but only when it is at the beginning of a space-separated token.
//
func (b *builder) fmtcmd(dir string, format string, args ...interface{}) string {
	cmd := fmt.Sprintf(format, args...)
	if dir != "" {
		cmd = strings.Replace(" "+cmd, " "+dir, " .", -1)[1:]
		if b.scriptDir != dir {
			b.scriptDir = dir
			cmd = "cd " + dir + "\n" + cmd
		}
	}
	if b.work != "" {
		cmd = strings.Replace(cmd, b.work, "$WORK", -1)
	}
	cmd = strings.Replace(cmd, gobin, "$GOBIN", -1)
	cmd = strings.Replace(cmd, goroot, "$GOROOT", -1)
	return cmd
}

// showcmd prints the given command to standard output
// for the implementation of -n or -x.
func (b *builder) showcmd(dir string, format string, args ...interface{}) {
	b.output.Lock()
	defer b.output.Unlock()
	b.print(b.fmtcmd(dir, format, args...) + "\n")
}

// showOutput prints "# desc" followed by the given output.
// The output is expected to contain references to 'dir', usually
// the source directory for the package that has failed to build.
// showOutput rewrites mentions of dir with a relative path to dir
// when the relative path is shorter.  This is usually more pleasant.
// For example, if fmt doesn't compile and we are in src/pkg/html,
// the output is
//
//	$ go build
//	# fmt
//	../fmt/print.go:1090: undefined: asdf
//	$
//
// instead of
//
//	$ go build
//	# fmt
//	/usr/gopher/go/src/pkg/fmt/print.go:1090: undefined: asdf
//	$
//
// showOutput also replaces references to the work directory with $WORK.
//
func (b *builder) showOutput(dir, desc, out string) {
	prefix := "# " + desc
	suffix := "\n" + out
	pwd, _ := os.Getwd()
	if reldir, err := filepath.Rel(pwd, dir); err == nil && len(reldir) < len(dir) {
		suffix = strings.Replace(suffix, " "+dir, " "+reldir, -1)
		suffix = strings.Replace(suffix, "\n"+dir, "\n"+reldir, -1)
	}
	suffix = strings.Replace(suffix, " "+b.work, " $WORK", -1)

	b.output.Lock()
	defer b.output.Unlock()
	b.print(prefix, suffix)
}

// relPaths returns a copy of paths with absolute paths
// made relative to the current directory if they would be shorter.
func relPaths(paths []string) []string {
	var out []string
	pwd, _ := os.Getwd()
	for _, p := range paths {
		rel, err := filepath.Rel(pwd, p)
		if err == nil && len(rel) < len(p) {
			p = rel
		}
		out = append(out, p)
	}
	return out
}

// errPrintedOutput is a special error indicating that a command failed
// but that it generated output as well, and that output has already
// been printed, so there's no point showing 'exit status 1' or whatever
// the wait status was.  The main executor, builder.do, knows not to
// print this error.
var errPrintedOutput = errors.New("already printed output - no need to show error")

// run runs the command given by cmdline in the directory dir.
// If the commnd fails, run prints information about the failure
// and returns a non-nil error.
func (b *builder) run(dir string, desc string, cmdargs ...interface{}) error {
	out, err := b.runOut(dir, desc, cmdargs...)
	if len(out) > 0 {
		if out[len(out)-1] != '\n' {
			out = append(out, '\n')
		}
		if desc == "" {
			desc = b.fmtcmd(dir, "%s", strings.Join(stringList(cmdargs...), " "))
		}
		b.showOutput(dir, desc, string(out))
		if err != nil {
			err = errPrintedOutput
		}
	}
	return err
}

// runOut runs the command given by cmdline in the directory dir.
// It returns the command output and any errors that occurred.
func (b *builder) runOut(dir string, desc string, cmdargs ...interface{}) ([]byte, error) {
	cmdline := stringList(cmdargs...)
	if buildN || buildX {
		b.showcmd(dir, "%s", strings.Join(cmdline, " "))
		if buildN {
			return nil, nil
		}
	}

	var buf bytes.Buffer
	cmd := exec.Command(cmdline[0], cmdline[1:]...)
	cmd.Stdout = &buf
	cmd.Stderr = &buf
	cmd.Dir = dir
	// TODO: cmd.Env
	err := cmd.Run()
	return buf.Bytes(), err
}

// mkdir makes the named directory.
func (b *builder) mkdir(dir string) error {
	b.exec.Lock()
	defer b.exec.Unlock()
	// We can be a little aggressive about being
	// sure directories exist.  Skip repeated calls.
	if b.mkdirCache[dir] {
		return nil
	}
	b.mkdirCache[dir] = true

	if buildN || buildX {
		b.showcmd("", "mkdir -p %s", dir)
		if buildN {
			return nil
		}
	}

	if err := os.MkdirAll(dir, 0777); err != nil {
		return err
	}
	return nil
}

// mkAbs returns an absolute path corresponding to
// evaluating f in the directory dir.
// We always pass absolute paths of source files so that
// the error messages will include the full path to a file
// in need of attention.
func mkAbs(dir, f string) string {
	// Leave absolute paths alone.
	// Also, during -n mode we use the pseudo-directory $WORK
	// instead of creating an actual work directory that won't be used.
	// Leave paths beginning with $WORK alone too.
	if filepath.IsAbs(f) || strings.HasPrefix(f, "$WORK") {
		return f
	}
	return filepath.Join(dir, f)
}

type toolchain interface {
	// gc runs the compiler in a specific directory on a set of files
	// and returns the name of the generated output file. 
	gc(b *builder, p *Package, obj string, importArgs []string, gofiles []string) (ofile string, err error)
	// cc runs the toolchain's C compiler in a directory on a C file
	// to produce an output file.
	cc(b *builder, p *Package, objdir, ofile, cfile string) error
	// asm runs the assembler in a specific directory on a specific file
	// to generate the named output file. 
	asm(b *builder, p *Package, obj, ofile, sfile string) error
	// pkgpath creates the appropriate destination path for a package file.
	pkgpath(basedir string, p *Package) string
	// pack runs the archive packer in a specific directory to create
	// an archive from a set of object files.
	// typically it is run in the object directory.
	pack(b *builder, p *Package, objDir, afile string, ofiles []string) error
	// ld runs the linker to create a package starting at mainpkg.
	ld(b *builder, p *Package, out string, allactions []*action, mainpkg string, ofiles []string) error
}

type goToolchain struct{}
type gccgoToolchain struct{}

var buildToolchain toolchain

func init() {
	if os.Getenv("GC") == "gccgo" {
		buildToolchain = gccgoToolchain{}
	} else {
		buildToolchain = goToolchain{}
	}
}

// The Go toolchain.

func (goToolchain) gc(b *builder, p *Package, obj string, importArgs []string, gofiles []string) (ofile string, err error) {
	out := "_go_." + b.arch
	ofile = obj + out
	gcargs := []string{"-p", p.ImportPath}
	if p.Standard && p.ImportPath == "runtime" {
		// runtime compiles with a special 6g flag to emit
		// additional reflect type data.
		gcargs = append(gcargs, "-+")
	}

	args := stringList(tool(b.arch+"g"), "-o", ofile, b.gcflags, gcargs, importArgs)
	for _, f := range gofiles {
		args = append(args, mkAbs(p.Dir, f))
	}
	return ofile, b.run(p.Dir, p.ImportPath, args)
}

func (goToolchain) asm(b *builder, p *Package, obj, ofile, sfile string) error {
	sfile = mkAbs(p.Dir, sfile)
	return b.run(p.Dir, p.ImportPath, tool(b.arch+"a"), "-I", obj, "-o", ofile, "-DGOOS_"+b.goos, "-DGOARCH_"+b.goarch, sfile)
}

func (goToolchain) pkgpath(basedir string, p *Package) string {
	return filepath.Join(basedir, filepath.FromSlash(p.ImportPath+".a"))
}

func (goToolchain) pack(b *builder, p *Package, objDir, afile string, ofiles []string) error {
	var absOfiles []string
	for _, f := range ofiles {
		absOfiles = append(absOfiles, mkAbs(objDir, f))
	}
	return b.run(p.Dir, p.ImportPath, tool("pack"), "grc", mkAbs(objDir, afile), absOfiles)
}

func (goToolchain) ld(b *builder, p *Package, out string, allactions []*action, mainpkg string, ofiles []string) error {
	importArgs := b.includeArgs("-L", allactions)
	return b.run(p.Dir, p.ImportPath, tool(b.arch+"l"), "-o", out, importArgs, mainpkg)
}

func (goToolchain) cc(b *builder, p *Package, objdir, ofile, cfile string) error {
	inc := filepath.Join(goroot, "pkg", fmt.Sprintf("%s_%s", b.goos, b.goarch))
	cfile = mkAbs(p.Dir, cfile)
	return b.run(p.Dir, p.ImportPath, tool(b.arch+"c"), "-FVw",
		"-I", objdir, "-I", inc, "-o", ofile,
		"-DGOOS_"+b.goos, "-DGOARCH_"+b.goarch, cfile)
}

// The Gccgo toolchain.

func (gccgoToolchain) gc(b *builder, p *Package, obj string, importArgs []string, gofiles []string) (ofile string, err error) {
	out := p.Name + ".o"
	ofile = obj + out
	gcargs := []string{"-g"}
	if p.Name != "main" {
		if p.fake {
			gcargs = append(gcargs, "-fgo-prefix=fake_"+p.ImportPath)
		} else {
			gcargs = append(gcargs, "-fgo-prefix=go_"+p.ImportPath)
		}
	}
	args := stringList("gccgo", importArgs, "-c", b.gcflags, gcargs, "-o", ofile)
	for _, f := range gofiles {
		args = append(args, mkAbs(p.Dir, f))
	}
	return ofile, b.run(p.Dir, p.ImportPath, args)
}

func (gccgoToolchain) asm(b *builder, p *Package, obj, ofile, sfile string) error {
	sfile = mkAbs(p.Dir, sfile)
	return b.run(p.Dir, p.ImportPath, "gccgo", "-I", obj, "-o", ofile, "-DGOOS_"+b.goos, "-DGOARCH_"+b.goarch, sfile)
}

func (gccgoToolchain) pkgpath(basedir string, p *Package) string {
	afile := filepath.Join(basedir, filepath.FromSlash(p.ImportPath+".a"))
	// prepend "lib" to the basename
	return filepath.Join(filepath.Dir(afile), "lib"+filepath.Base(afile))
}

func (gccgoToolchain) pack(b *builder, p *Package, objDir, afile string, ofiles []string) error {
	var absOfiles []string
	for _, f := range ofiles {
		absOfiles = append(absOfiles, mkAbs(objDir, f))
	}
	return b.run(p.Dir, p.ImportPath, "ar", "cru", mkAbs(objDir, afile), absOfiles)
}

func (tools gccgoToolchain) ld(b *builder, p *Package, out string, allactions []*action, mainpkg string, ofiles []string) error {
	// gccgo needs explicit linking with all package dependencies,
	// and all LDFLAGS from cgo dependencies
	afiles := []string{}
	ldflags := []string{}
	seen := map[*Package]bool{}
	for _, a := range allactions {
		if a.p != nil && !seen[a.p] {
			seen[a.p] = true
			if !a.p.Standard {
				afiles = append(afiles, a.target)
			}
			ldflags = append(ldflags, a.p.CgoLDFLAGS...)
		}
	}
	return b.run(p.Dir, p.ImportPath, "gccgo", "-o", out, ofiles, "-Wl,-(", afiles, ldflags, "-Wl,-)")
}

func (gccgoToolchain) cc(b *builder, p *Package, objdir, ofile, cfile string) error {
	inc := filepath.Join(goroot, "pkg", fmt.Sprintf("%s_%s", b.goos, b.goarch))
	cfile = mkAbs(p.Dir, cfile)
	return b.run(p.Dir, p.ImportPath, "gcc", "-Wall", "-g",
		"-I", objdir, "-I", inc, "-o", ofile,
		"-DGOOS_"+b.goos, "-DGOARCH_"+b.goarch, "-c", cfile)
}

// gcc runs the gcc C compiler to create an object from a single C file.
func (b *builder) gcc(p *Package, out string, flags []string, cfile string) error {
	cfile = mkAbs(p.Dir, cfile)
	return b.run(p.Dir, p.ImportPath, b.gccCmd(p.Dir), flags, "-o", out, "-c", cfile)
}

// gccld runs the gcc linker to create an executable from a set of object files
func (b *builder) gccld(p *Package, out string, flags []string, obj []string) error {
	return b.run(p.Dir, p.ImportPath, b.gccCmd(p.Dir), "-o", out, obj, flags)
}

// gccCmd returns a gcc command line prefix
func (b *builder) gccCmd(objdir string) []string {
	// TODO: HOST_CC?
	a := []string{"gcc", "-I", objdir, "-g", "-O2"}

	// Definitely want -fPIC but on Windows gcc complains
	// "-fPIC ignored for target (all code is position independent)"
	if b.goos != "windows" {
		a = append(a, "-fPIC")
	}
	switch b.arch {
	case "8":
		a = append(a, "-m32")
	case "6":
		a = append(a, "-m64")
	}
	// gcc-4.5 and beyond require explicit "-pthread" flag
	// for multithreading with pthread library.
	if buildContext.CgoEnabled {
		switch b.goos {
		case "windows":
			a = append(a, "-mthreads")
		default:
			a = append(a, "-pthread")
		}
	}
	return a
}

func envList(key string) []string {
	return strings.Fields(os.Getenv(key))
}

var cgoRe = regexp.MustCompile(`[/\\:]`)

func (b *builder) cgo(p *Package, cgoExe, obj string, gccfiles []string) (outGo, outObj []string, err error) {
	if b.goos != toolGOOS {
		return nil, nil, errors.New("cannot use cgo when compiling for a different operating system")
	}

	outObj = append(outObj, "") // for importObj, at end of function

	cgoCFLAGS := stringList(envList("CGO_CFLAGS"), p.info.CgoCFLAGS)
	cgoLDFLAGS := stringList(envList("CGO_LDFLAGS"), p.info.CgoLDFLAGS)

	if pkgs := p.info.CgoPkgConfig; len(pkgs) > 0 {
		out, err := b.runOut(p.Dir, p.ImportPath, "pkg-config", "--cflags", pkgs)
		if err != nil {
			b.showOutput(p.Dir, "pkg-config --cflags "+strings.Join(pkgs, " "), string(out))
			b.print(err.Error() + "\n")
			return nil, nil, errPrintedOutput
		}
		if len(out) > 0 {
			cgoCFLAGS = append(cgoCFLAGS, strings.Fields(string(out))...)
		}
		out, err = b.runOut(p.Dir, p.ImportPath, "pkg-config", "--libs", pkgs)
		if err != nil {
			b.showOutput(p.Dir, "pkg-config --libs "+strings.Join(pkgs, " "), string(out))
			b.print(err.Error() + "\n")
			return nil, nil, errPrintedOutput
		}
		if len(out) > 0 {
			cgoLDFLAGS = append(cgoLDFLAGS, strings.Fields(string(out))...)
		}
	}

	// Allows including _cgo_export.h from .[ch] files in the package.
	cgoCFLAGS = append(cgoCFLAGS, "-I", obj)

	// cgo
	// TODO: CGOPKGPATH, CGO_FLAGS?
	gofiles := []string{obj + "_cgo_gotypes.go"}
	cfiles := []string{"_cgo_main.c", "_cgo_export.c"}
	for _, fn := range p.CgoFiles {
		f := cgoRe.ReplaceAllString(fn[:len(fn)-2], "_")
		gofiles = append(gofiles, obj+f+"cgo1.go")
		cfiles = append(cfiles, f+"cgo2.c")
	}
	defunC := obj + "_cgo_defun.c"

	cgoflags := []string{}
	// TODO: make cgo not depend on $GOARCH?

	if p.Standard && p.ImportPath == "runtime/cgo" {
		cgoflags = append(cgoflags, "-import_runtime_cgo=false")
	}
	if _, ok := buildToolchain.(gccgoToolchain); ok {
		cgoflags = append(cgoflags, "-gccgo")
	}
	if err := b.run(p.Dir, p.ImportPath, cgoExe, "-objdir", obj, cgoflags, "--", cgoCFLAGS, p.CgoFiles); err != nil {
		return nil, nil, err
	}
	outGo = append(outGo, gofiles...)

	// cc _cgo_defun.c
	defunObj := obj + "_cgo_defun." + b.arch
	if err := buildToolchain.cc(b, p, obj, defunObj, defunC); err != nil {
		return nil, nil, err
	}
	outObj = append(outObj, defunObj)

	// gcc
	var linkobj []string
	for _, cfile := range cfiles {
		ofile := obj + cfile[:len(cfile)-1] + "o"
		if err := b.gcc(p, ofile, cgoCFLAGS, obj+cfile); err != nil {
			return nil, nil, err
		}
		linkobj = append(linkobj, ofile)
		if !strings.HasSuffix(ofile, "_cgo_main.o") {
			outObj = append(outObj, ofile)
		}
	}
	for _, file := range gccfiles {
		ofile := obj + cgoRe.ReplaceAllString(file[:len(file)-1], "_") + "o"
		if err := b.gcc(p, ofile, cgoCFLAGS, file); err != nil {
			return nil, nil, err
		}
		linkobj = append(linkobj, ofile)
		outObj = append(outObj, ofile)
	}
	dynobj := obj + "_cgo_.o"
	if err := b.gccld(p, dynobj, cgoLDFLAGS, linkobj); err != nil {
		return nil, nil, err
	}

	// cgo -dynimport
	importC := obj + "_cgo_import.c"
	if err := b.run(p.Dir, p.ImportPath, cgoExe, "-objdir", obj, "-dynimport", dynobj, "-dynout", importC); err != nil {
		return nil, nil, err
	}

	// cc _cgo_import.ARCH
	importObj := obj + "_cgo_import." + b.arch
	if err := buildToolchain.cc(b, p, obj, importObj, importC); err != nil {
		return nil, nil, err
	}

	// NOTE(rsc): The importObj is a 5c/6c/8c object and on Windows
	// must be processed before the gcc-generated objects.
	// Put it first.  We left room above.  http://golang.org/issue/2601
	outObj[0] = importObj

	return outGo, outObj, nil
}

// An actionQueue is a priority queue of actions.
type actionQueue []*action

// Implement heap.Interface
func (q *actionQueue) Len() int           { return len(*q) }
func (q *actionQueue) Swap(i, j int)      { (*q)[i], (*q)[j] = (*q)[j], (*q)[i] }
func (q *actionQueue) Less(i, j int) bool { return (*q)[i].priority < (*q)[j].priority }
func (q *actionQueue) Push(x interface{}) { *q = append(*q, x.(*action)) }
func (q *actionQueue) Pop() interface{} {
	n := len(*q) - 1
	x := (*q)[n]
	*q = (*q)[:n]
	return x
}

func (q *actionQueue) push(a *action) {
	heap.Push(q, a)
}

func (q *actionQueue) pop() *action {
	return heap.Pop(q).(*action)
}