path: root/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"
	"log"
	"os"
	"os/exec"
	"path"
	"path/filepath"
	"regexp"
	"runtime"
	"strings"
	"sync"
	"time"
)

var cmdBuild = &Command{
	UsageLine: "build [-o output] [build flags] [packages]",
	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.
Otherwise build compiles the packages but discards the results,
serving only as a check that the packages can be built.

The -o flag specifies the output file name.  If not specified, the
name is packagename.a (for a non-main package) or the base
name of the first source file (for a main package).

The build flags are shared by the build, install, run, and test commands:

	-a
		force rebuilding of packages that are already up-to-date.
	-n
		print the commands but do not run them.
	-p n
		the number of builds that can be run in parallel.
		The default is the number of CPUs available.
	-v
		print the names of packages as they are compiled.
	-work
		print the name of the temporary work directory and
		do not delete it when exiting.
	-x
		print the commands.

	-compiler name
		name of compiler to use, as in runtime.Compiler (gccgo or gc)
	-gccgoflags 'arg list'
		arguments to pass on each gccgo compiler/linker invocation
	-gcflags 'arg list'
		arguments to pass on each 5g, 6g, or 8g compiler invocation
	-ldflags 'flag list'
		arguments to pass on each 5l, 6l, or 8l linker invocation
	-tags 'tag list'
		a list of build tags to consider satisfied during the build.
		See the documentation for the go/build package for
		more information about build tags.

For more about specifying packages, see 'go help packages'.
For more about where packages and binaries are installed,
see 'go help gopath'.

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 buildWork bool           // -work flag
var buildGcflags []string    // -gcflags flag
var buildLdflags []string    // -ldflags flag
var buildGccgoflags []string // -gccgoflags flag

var buildContext = build.Default
var buildToolchain toolchain = noToolchain{}

// buildCompiler implements flag.Var.
// It implements Set by updating both
// buildToolchain and buildContext.Compiler.
type buildCompiler struct{}

func (c buildCompiler) Set(value string) error {
	switch value {
	case "gc":
		buildToolchain = gcToolchain{}
	case "gccgo":
		buildToolchain = gccgcToolchain{}
	default:
		return fmt.Errorf("unknown compiler %q", value)
	}
	buildContext.Compiler = value
	return nil
}

func (c buildCompiler) String() string {
	return buildContext.Compiler
}

func init() {
	switch build.Default.Compiler {
	case "gc":
		buildToolchain = gcToolchain{}
	case "gccgo":
		buildToolchain = gccgcToolchain{}
	}
}

// addBuildFlags adds the flags common to the build and install commands.
func addBuildFlags(cmd *Command) {
	// NOTE: If you add flags here, also add them to testflag.go.
	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, "")
	cmd.Flag.BoolVar(&buildWork, "work", false, "")
	cmd.Flag.Var((*stringsFlag)(&buildGcflags), "gcflags", "")
	cmd.Flag.Var((*stringsFlag)(&buildLdflags), "ldflags", "")
	cmd.Flag.Var((*stringsFlag)(&buildGccgoflags), "gccgoflags", "")
	cmd.Flag.Var((*stringsFlag)(&buildContext.BuildTags), "tags", "")
	cmd.Flag.Var(buildCompiler{}, "compiler", "")
}

type stringsFlag []string

func (v *stringsFlag) Set(s string) error {
	*v = strings.Fields(s)
	return nil
}

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

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

	pkgs := packagesForBuild(args)

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

	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 [build flags] [packages]",
	Short:     "compile and install packages and dependencies",
	Long: `
Install compiles and installs the packages named by the import paths,
along with their dependencies.

For more about the build flags, see 'go help build'.
For more about specifying packages, see 'go help packages'.

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

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

	for _, p := range pkgs {
		if p.Target == "" && (!p.Standard || p.ImportPath != "unsafe") {
			errorf("go install: no install location for %s", p.ImportPath)
		}
	}
	exitIfErrors()

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

// Global build parameters (used during package load)
var (
	goarch    string
	goos      string
	archChar  string
	exeSuffix string
)

func init() {
	goarch = buildContext.GOARCH
	goos = buildContext.GOOS
	if goos == "windows" {
		exeSuffix = ".exe"
	}
	var err error
	archChar, err = build.ArchChar(goarch)
	if err != nil {
		fatalf("%s", err)
	}
}

// A builder holds global state about a build.
// It does not hold per-package state, because we
// build packages in parallel, and the builder is shared.
type builder struct {
	work        string               // the temporary work directory (ends in filepath.Separator)
	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 (
	goroot       = filepath.Clean(runtime.GOROOT())
	gobin        = os.Getenv("GOBIN")
	gorootBin    = filepath.Join(goroot, "bin")
	gorootSrcPkg = filepath.Join(goroot, "src/pkg")
	gorootPkg    = filepath.Join(goroot, "pkg")
	gorootSrc    = filepath.Join(goroot, "src")
)

func (b *builder) init() {
	var err error
	b.print = fmt.Print
	b.actionCache = make(map[cacheKey]*action)
	b.mkdirCache = make(map[string]bool)

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

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

	var stk importStack
	ctxt := buildContext
	ctxt.UseAllFiles = true

	// Synthesize fake "directory" that only shows the named files,
	// to make it look like this is a standard package or
	// command directory.  So that local imports resolve
	// consistently, the files must all be in the same directory.
	var dirent []os.FileInfo
	var dir string
	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)
		}
		dir1, _ := filepath.Split(file)
		if dir == "" {
			dir = dir1
		} else if dir != dir1 {
			fatalf("named files must all be in one directory; have %s and %s", dir, dir1)
		}
		dirent = append(dirent, fi)
	}
	ctxt.ReadDir = func(string) ([]os.FileInfo, error) { return dirent, nil }

	if !filepath.IsAbs(dir) {
		dir = filepath.Join(cwd, dir)
	}

	bp, err := ctxt.ImportDir(dir, 0)
	pkg := new(Package)
	pkg.local = true
	pkg.load(&stk, bp, err)
	pkg.localPrefix = dirToImportPath(dir)
	pkg.ImportPath = "command-line-arguments"
	pkg.target = ""

	if pkg.Name == "main" {
		_, elem := filepath.Split(gofiles[0])
		exe := elem[:len(elem)-len(".go")] + exeSuffix
		if *buildO == "" {
			*buildO = exe
		}
		if gobin != "" {
			pkg.target = filepath.Join(gobin, exe)
		}
	} else {
		if *buildO == "" {
			*buildO = pkg.Name + ".a"
		}
	}
	pkg.Target = pkg.target
	pkg.Stale = true

	computeStale(pkg)
	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.build.PkgRoot}
	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 we are not doing a cross-build, then record the binary we'll
	// generate for cgo as a dependency of the build of any package
	// using cgo, to make sure we do not overwrite the binary while
	// a package is using it.  If this is a cross-build, then the cgo we
	// are writing is not the cgo we need to use.
	if goos == runtime.GOOS && goarch == runtime.GOARCH {
		if len(p.CgoFiles) > 0 || p.Standard && p.ImportPath == "runtime/cgo" {
			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.(gccgcToolchain); ok {
			// the target name is needed for cgo.
			a.target = p.target
			return a
		}
	}

	if !p.Stale && 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
	}

	if p.local && p.target == "" {
		// Imported via local path.  No permanent target.
		mode = modeBuild
	}
	a.objdir = filepath.Join(b.work, 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" + exeSuffix
		}
	}

	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) (err error) {
	defer func() {
		if err != nil && err != errPrintedOutput {
			err = fmt.Errorf("go build %s: %v", a.p.ImportPath, err)
		}
	}()
	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
		}

		cgoExe := tool("cgo")
		if a.cgo != nil && a.cgo.target != "" {
			cgoExe = a.cgo.target
		}
		outGo, outObj, err := b.cgo(a.p, cgoExe, 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 := "_" + goos + "_" + goarch + ".h"
	_goos := "_" + goos + ".h"
	_goarch := "_" + 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(a, 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(a, 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(a, obj+targ, filepath.Join(a.p.Dir, file), 0666); err != nil {
				return err
			}
		}
	}

	for _, file := range cfiles {
		out := file[:len(file)-len(".c")] + "." + archChar
		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")] + "." + archChar
		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...)

	// Add system object files.
	for _, syso := range a.p.SysoFiles {
		objects = append(objects, filepath.Join(a.p.Dir, syso))
	}

	// 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) (err error) {
	defer func() {
		if err != nil && err != errPrintedOutput {
			err = fmt.Errorf("go install %s: %v", a.p.ImportPath, err)
		}
	}()
	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.
	if !buildWork {
		defer os.RemoveAll(a1.objdir)
		defer os.Remove(a1.target)
	}

	return b.copyFile(a, 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
		gorootPkg: true,
		"":        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.build.PkgRoot && !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.build.PkgRoot && !incMap[dir] {
			incMap[dir] = true
			if _, ok := buildToolchain.(gccgcToolchain); ok {
				dir = filepath.Join(dir, "gccgo")
			} else {
				dir = filepath.Join(dir, goos+"_"+goarch)
			}
			inc = append(inc, flag, dir)
		}
	}

	return inc
}

// copyFile is like 'cp src dst'.
func (b *builder) copyFile(a *action, 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()

	// Be careful about removing/overwriting dst.
	// Do not remove/overwrite if dst exists and is a directory
	// or a non-object file.
	if fi, err := os.Stat(dst); err == nil {
		if fi.IsDir() {
			return fmt.Errorf("build output %q already exists and is a directory", dst)
		}
		if !isObject(dst) {
			return fmt.Errorf("build output %q already exists and is not an object file", dst)
		}
	}

	// 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 fmt.Errorf("copying %s to %s: %v", src, dst, err)
	}
	return nil
}

var objectMagic = [][]byte{
	{'!', '<', 'a', 'r', 'c', 'h', '>', '\n'},        // Package archive
	{'\x7F', 'E', 'L', 'F'},                          // ELF
	{0xFE, 0xED, 0xFA, 0xCE},                         // Mach-O big-endian 32-bit
	{0xFE, 0xED, 0xFA, 0xCF},                         // Mach-O big-endian 64-bit
	{0xCE, 0xFA, 0xED, 0xFE},                         // Mach-O little-endian 32-bit
	{0xCF, 0xFA, 0xED, 0xFE},                         // Mach-O little-endian 64-bit
	{0x4d, 0x5a, 0x90, 0x00, 0x03, 0x00, 0x04, 0x00}, // PE (Windows) as generated by 6l/8l
}

func isObject(s string) bool {
	f, err := os.Open(s)
	if err != nil {
		return false
	}
	defer f.Close()
	buf := make([]byte, 64)
	io.ReadFull(f, buf)
	for _, magic := range objectMagic {
		if bytes.HasPrefix(buf, magic) {
			return true
		}
	}
	return false
}

// 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 $GOBIN.
//
//	fmtcmd replaces the name of the current directory with dot (.)
//	but only when it is at the beginning of a space-separated token.
//
func (b *builder) fmtcmd(dir string, format string, args ...interface{}) string {
	cmd := fmt.Sprintf(format, args...)
	if dir != "" && dir != "/" {
		cmd = strings.Replace(" "+cmd, " "+dir, " .", -1)[1:]
		if b.scriptDir != dir {
			b.scriptDir = dir
			cmd = "cd " + dir + "\n" + cmd
		}
	}
	if b.work != "" {
		cmd = strings.Replace(cmd, b.work, "$WORK", -1)
	}
	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
	if reldir := shortPath(dir); reldir != 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)
}

// shortPath returns an absolute or relative name for path, whatever is shorter.
func shortPath(path string) string {
	if rel, err := filepath.Rel(cwd, path); err == nil && len(rel) < len(path) {
		return rel
	}
	return path
}

// 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 command 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
		}
	}

	nbusy := 0
	for {
		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()

		// cmd.Run will fail on Unix if some other process has the binary
		// we want to run open for writing.  This can happen here because
		// we build and install the cgo command and then run it.
		// If another command was kicked off while we were writing the
		// cgo binary, the child process for that command may be holding
		// a reference to the fd, keeping us from running exec.
		//
		// But, you might reasonably wonder, how can this happen?
		// The cgo fd, like all our fds, is close-on-exec, so that we need
		// not worry about other processes inheriting the fd accidentally.
		// The answer is that running a command is fork and exec.
		// A child forked while the cgo fd is open inherits that fd.
		// Until the child has called exec, it holds the fd open and the 
		// kernel will not let us run cgo.  Even if the child were to close
		// the fd explicitly, it would still be open from the time of the fork
		// until the time of the explicit close, and the race would remain.
		//
		// On Unix systems, this results in ETXTBSY, which formats
		// as "text file busy".  Rather than hard-code specific error cases,
		// we just look for that string.  If this happens, sleep a little
		// and try again.  We let this happen three times, with increasing
		// sleep lengths: 100+200+400 ms = 0.7 seconds.
		//
		// An alternate solution might be to split the cmd.Run into
		// separate cmd.Start and cmd.Wait, and then use an RWLock
		// to make sure that copyFile only executes when no cmd.Start
		// call is in progress.  However, cmd.Start (really syscall.forkExec)
		// only guarantees that when it returns, the exec is committed to
		// happen and succeed.  It uses a close-on-exec file descriptor
		// itself to determine this, so we know that when cmd.Start returns,
		// at least one close-on-exec file descriptor has been closed.
		// However, we cannot be sure that all of them have been closed,
		// so the program might still encounter ETXTBSY even with such
		// an RWLock.  The race window would be smaller, perhaps, but not
		// guaranteed to be gone.
		//
		// Sleeping when we observe the race seems to be the most reliable
		// option we have.
		//
		// http://golang.org/issue/3001
		//
		if err != nil && nbusy < 3 && strings.Contains(err.Error(), "text file busy") {
			time.Sleep(100 * time.Millisecond << uint(nbusy))
			nbusy++
			continue
		}

		return buf.Bytes(), err
	}
	panic("unreachable")
}

// 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.
	// The compiler runs in the directory dir.
	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 builds an appropriate path for a temporary 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

	compiler() string
	linker() string
}

type noToolchain struct{}

func noCompiler() error {
	log.Fatalf("unknown compiler %q", buildContext.Compiler)
	return nil
}

func (noToolchain) compiler() string {
	noCompiler()
	return ""
}

func (noToolchain) linker() string {
	noCompiler()
	return ""
}

func (noToolchain) gc(b *builder, p *Package, obj string, importArgs []string, gofiles []string) (ofile string, err error) {
	return "", noCompiler()
}

func (noToolchain) asm(b *builder, p *Package, obj, ofile, sfile string) error {
	return noCompiler()
}

func (noToolchain) pkgpath(basedir string, p *Package) string {
	noCompiler()
	return ""
}

func (noToolchain) pack(b *builder, p *Package, objDir, afile string, ofiles []string) error {
	return noCompiler()
}

func (noToolchain) ld(b *builder, p *Package, out string, allactions []*action, mainpkg string, ofiles []string) error {
	return noCompiler()
}

func (noToolchain) cc(b *builder, p *Package, objdir, ofile, cfile string) error {
	return noCompiler()
}

// The Go toolchain.
type gcToolchain struct{}

func (gcToolchain) compiler() string {
	return tool(archChar + "g")
}

func (gcToolchain) linker() string {
	return tool(archChar + "l")
}

func (gcToolchain) gc(b *builder, p *Package, obj string, importArgs []string, gofiles []string) (ofile string, err error) {
	out := "_go_." + archChar
	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(archChar+"g"), "-o", ofile, buildGcflags, gcargs, "-D", p.localPrefix, importArgs)
	for _, f := range gofiles {
		args = append(args, mkAbs(p.Dir, f))
	}
	return ofile, b.run(p.Dir, p.ImportPath, args)
}

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

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

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

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

// The Gccgo toolchain.
type gccgcToolchain struct{}

var gccgoBin, _ = exec.LookPath("gccgo")

func (gccgcToolchain) compiler() string {
	return gccgoBin
}

func (gccgcToolchain) linker() string {
	return gccgoBin
}

func (gccgcToolchain) 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 prefix := gccgoPrefix(p); prefix != "" {
		gcargs = append(gcargs, "-fgo-prefix="+gccgoPrefix(p))
	}
	args := stringList("gccgo", importArgs, "-c", gcargs, "-o", ofile, buildGccgoflags)
	for _, f := range gofiles {
		args = append(args, mkAbs(p.Dir, f))
	}
	return ofile, b.run(p.Dir, p.ImportPath, args)
}

func (gccgcToolchain) 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_"+goos, "-DGOARCH_"+goarch, sfile)
}

func (gccgcToolchain) pkgpath(basedir string, p *Package) string {
	end := filepath.FromSlash(p.ImportPath + ".a")
	afile := filepath.Join(basedir, end)
	// add "lib" to the final element
	return filepath.Join(filepath.Dir(afile), "lib"+filepath.Base(afile))
}

func (gccgcToolchain) 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 gccgcToolchain) 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 := make(map[*Package]string)
	ldflags := []string{}
	cgoldflags := []string{}
	for _, a := range allactions {
		if a.p != nil {
			if !a.p.Standard {
				if afiles[a.p] == "" || a.objpkg != a.target {
					afiles[a.p] = a.target
				}
			}
			cgoldflags = append(cgoldflags, a.p.CgoLDFLAGS...)
		}
	}
	for _, afile := range afiles {
		ldflags = append(ldflags, afile)
	}
	ldflags = append(ldflags, cgoldflags...)
	return b.run(".", p.ImportPath, "gccgo", "-o", out, buildGccgoflags, ofiles, "-Wl,-(", ldflags, "-Wl,-)")
}

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

func gccgoPrefix(p *Package) string {
	switch {
	case p.build.IsCommand() && !p.forceLibrary:
		return ""
	case p.fake:
		return "fake_" + p.ImportPath
	}
	return "go_" + p.ImportPath
}

// 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 {
	// NOTE: env.go's mkEnv knows that the first three
	// strings returned are "gcc", "-I", objdir (and cuts them off).

	// 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 goos != "windows" {
		a = append(a, "-fPIC")
	}
	switch archChar {
	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 goos {
		case "windows":
			a = append(a, "-mthreads")
		default:
			a = append(a, "-pthread")
		}
	}

	// On OS X, some of the compilers behave as if -fno-common
	// is always set, and the Mach-O linker in 6l/8l assumes this.
	// See http://golang.org/issue/3253.
	if goos == "darwin" {
		a = append(a, "-fno-common")
	}

	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 goos != toolGOOS {
		return nil, nil, errors.New("cannot use cgo when compiling for a different operating system")
	}

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

	if pkgs := p.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?

	objExt := archChar

	if p.Standard && p.ImportPath == "runtime/cgo" {
		cgoflags = append(cgoflags, "-import_runtime_cgo=false")
	}
	if _, ok := buildToolchain.(gccgcToolchain); ok {
		cgoflags = append(cgoflags, "-gccgo")
		if prefix := gccgoPrefix(p); prefix != "" {
			cgoflags = append(cgoflags, "-gccgoprefix="+gccgoPrefix(p))
		}
		objExt = "o"
	}
	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." + objExt
	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
	}

	if _, ok := buildToolchain.(gccgcToolchain); ok {
		// we don't use dynimport when using gccgo.
		return outGo, outObj, nil
	}

	// 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." + objExt
	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.  http://golang.org/issue/2601
	outObj = append([]string{importObj}, outObj...)

	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)
}