cmd/toolstash/main.go - tools - Git at Google

 // Copyright 2015 The Go Authors.  All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 // Toolstash provides a way to save, run, and restore a known good copy of the Go toolchain
 // and to compare the object files generated by two toolchains.
 //
 // Usage:
 //
 //	toolstash [-n] [-v] save [tool...]
 //	toolstash [-n] [-v] restore [tool...]
 //	toolstash [-n] [-v] [-t] go run x.go
 //	toolstash [-n] [-v] [-t] [-cmp] compile x.go
 //
 // The toolstash command manages a ``stashed'' copy of the Go toolchain
 // kept in $GOROOT/pkg/toolstash. In this case, the toolchain means the
 // tools available with the 'go tool' command as well as the go, godoc, and gofmt
 // binaries.
 //
 // The command ``toolstash save'', typically run when the toolchain is known to be working,
 // copies the toolchain from its installed location to the toolstash directory.
 // Its inverse, ``toolchain restore'', typically run when the toolchain is known to be broken,
 // copies the toolchain from the toolstash directory back to the installed locations.
 // If additional arguments are given, the save or restore applies only to the named tools.
 // Otherwise, it applies to all tools.
 //
 // Otherwise, toolstash's arguments should be a command line beginning with the
 // name of a toolchain binary, which may be a short name like compile or a complete path
 // to an installed binary. Toolstash runs the command line using the stashed
 // copy of the binary instead of the installed one.
 //
 // The -n flag causes toolstash to print the commands that would be executed
 // but not execute them. The combination -n -cmp shows the two commands
 // that would be compared and then exits successfully. A real -cmp run might
 // run additional commands for diagnosis of an output mismatch.
 //
 // The -v flag causes toolstash to print the commands being executed.
 //
 // The -t flag causes toolstash to print the time elapsed during while the
 // command ran.
 //
 // Comparing
 //
 // The -cmp flag causes toolstash to run both the installed and the stashed
 // copy of an assembler or compiler and check that they produce identical
 // object files. If not, toolstash reports the mismatch and exits with a failure status.
 // As part of reporting the mismatch, toolstash reinvokes the command with
 // the -S flag and identifies the first divergence in the assembly output.
 // If the command is a Go compiler, toolstash also determines whether the
 // difference is triggered by optimization passes.
 // On failure, toolstash leaves additional information in files named
 // similarly to the default output file. If the compilation would normally
 // produce a file x.6, the output from the stashed tool is left in x.6.stash
 // and the debugging traces are left in x.6.log and x.6.stash.log.
 //
 // The -cmp flag is a no-op when the command line is not invoking an
 // assembler or compiler.
 //
 // For example, when working on code cleanup that should not affect
 // compiler output, toolstash can be used to compare the old and new
 // compiler output:
 //
 //	toolstash save
 //	<edit compiler sources>
 //	go tool dist install cmd/compile # install compiler only
 //	toolstash -cmp compile x.go
 //
 // Go Command Integration
 //
 // The go command accepts a -toolexec flag that specifies a program
 // to use to run the build tools.
 //
 // To build with the stashed tools:
 //
 //	go build -toolexec toolstash x.go
 //
 // To build with the stashed go command and the stashed tools:
 //
 //	toolstash go build -toolexec toolstash x.go
 //
 // To verify that code cleanup in the compilers does not make any
 // changes to the objects being generated for the entire tree:
 //
 //	# Build working tree and save tools.
 //	./make.bash
 //	toolstash save
 //
 //	<edit compiler sources>
 //
 //	# Install new tools, but do not rebuild the rest of tree,
 //	# since the compilers might generate buggy code.
 //	go tool dist install cmd/compile
 //
 //	# Check that new tools behave identically to saved tools.
 //	go build -toolexec 'toolstash -cmp' -a std
 //
 //	# If not, restore, in order to keep working on Go code.
 //	toolstash restore
 //
 // Version Skew
 //
 // The Go tools write the current Go version to object files, and (outside
 // release branches) that version includes the hash and time stamp
 // of the most recent Git commit. Functionally equivalent
 // compilers built at different Git versions may produce object files that
 // differ only in the recorded version. Toolstash ignores version mismatches
 // when comparing object files, but the standard tools will refuse to compile
 // or link together packages with different object versions.
 //
 // For the full build in the final example above to work, both the stashed
 // and the installed tools must use the same version string.
 // One way to ensure this is not to commit any of the changes being
 // tested, so that the Git HEAD hash is the same for both builds.
 // A more robust way to force the tools to have the same version string
 // is to write a $GOROOT/VERSION file, which overrides the Git-based version
 // computation:
 //
 //	echo devel >$GOROOT/VERSION
 //
 // The version can be arbitrary text, but to pass all.bash's API check, it must
 // contain the substring ``devel''. The VERSION file must be created before
 // building either version of the toolchain.
 //
 package main // import "golang.org/x/tools/cmd/toolstash"

 import (
 	"bufio"
 	"flag"
 	"fmt"
 	"io"
 	"io/ioutil"
 	"log"
 	"os"
 	"os/exec"
 	"path/filepath"
 	"runtime"
 	"strings"
 	"time"
 )

 var usageMessage = `usage: toolstash [-n] [-v] [-cmp] command line

 Examples:
 	toolstash save
 	toolstash restore
 	toolstash go run x.go
 	toolstash compile x.go
 	toolstash -cmp compile x.go

 For details, godoc rsc.io/toolstash
 `

 func usage() {
 	fmt.Fprint(os.Stderr, usageMessage)
 	os.Exit(2)
 }

 var (
 	norun   = flag.Bool("n", false, "print but do not run commands")
 	verbose = flag.Bool("v", false, "print commands being run")
 	cmp     = flag.Bool("cmp", false, "compare tool object files")
 	timing  = flag.Bool("t", false, "print time commands take")
 )

 var (
 	cmd       []string
 	tool      string // name of tool: "go", "compile", etc
 	toolStash string // path to stashed tool

 	goroot   string
 	toolDir  string
 	stashDir string
 	binDir   string
 )

 func canCmp(name string) bool {
 	switch name {
 	case "compile", "link", "asm":
 		return true
 	}
 	return len(name) == 2 && '0' <= name[0] && name[0] <= '9' && (name[1] == 'a' || name[1] == 'g' || name[1] == 'l')
 }

 var binTools = []string{"go", "godoc", "gofmt"}

 func isBinTool(name string) bool {
 	return strings.HasPrefix(name, "go")
 }

 func main() {
 	log.SetFlags(0)
 	log.SetPrefix("toolstash: ")

 	flag.Usage = usage
 	flag.Parse()
 	cmd = flag.Args()

 	if len(cmd) < 1 {
 		usage()
 	}

 	goroot = runtime.GOROOT()
 	toolDir = filepath.Join(goroot, fmt.Sprintf("pkg/tool/%s_%s", runtime.GOOS, runtime.GOARCH))
 	stashDir = filepath.Join(goroot, "pkg/toolstash")

 	binDir = os.Getenv("GOBIN")
 	if binDir == "" {
 		binDir = filepath.Join(goroot, "bin")
 	}

 	switch cmd[0] {
 	case "save":
 		save()
 		return

 	case "restore":
 		restore()
 		return
 	}

 	tool = cmd[0]
 	if i := strings.LastIndex(tool, "/"); i >= 0 {
 		tool = tool[i+1:]
 	}
 	if i := strings.LastIndex(tool, `\`); i >= 0 {
 		tool = tool[i+1:]
 	}

 	if !strings.HasPrefix(tool, "a.out") {
 		toolStash = filepath.Join(stashDir, tool)
 		if _, err := os.Stat(toolStash); err != nil {
 			log.Print(err)
 			os.Exit(2)
 		}

 		if *cmp && canCmp(tool) {
 			compareTool()
 			return
 		}
 		cmd[0] = toolStash
 	}

 	if *norun {
 		fmt.Printf("%s\n", strings.Join(cmd, " "))
 		return
 	}
 	if *verbose {
 		log.Print(strings.Join(cmd, " "))
 	}
 	xcmd := exec.Command(cmd[0], cmd[1:]...)
 	xcmd.Stdin = os.Stdin
 	xcmd.Stdout = os.Stdout
 	xcmd.Stderr = os.Stderr
 	err := xcmd.Run()
 	if err != nil {
 		log.Fatal(err)
 	}
 	os.Exit(0)
 }

 func compareTool() {
 	if !strings.Contains(cmd[0], "/") && !strings.Contains(cmd[0], `\`) {
 		cmd[0] = filepath.Join(toolDir, tool)
 	}

 	outfile, ok := cmpRun(false, cmd)
 	if ok {
 		os.Remove(outfile + ".stash")
 		return
 	}

 	extra := "-S"
 	switch {
 	default:
 		log.Fatalf("unknown tool %s", tool)

 	case tool == "compile" || strings.HasSuffix(tool, "g"): // compiler
 		cmdN := append([]string{cmd[0], "-N"}, cmd[1:]...)
 		_, ok := cmpRun(false, cmdN)
 		if !ok {
 			log.Printf("compiler output differs, even with optimizers disabled (-N)")
 			cmd = append([]string{cmd[0], "-v", "-N", "-m=2"}, cmd[1:]...)
 			break
 		}
 		cmd = append([]string{cmd[0], "-v", "-m=2"}, cmd[1:]...)
 		log.Printf("compiler output differs, only with optimizers enabled")

 	case tool == "asm" || strings.HasSuffix(tool, "a"): // assembler
 		log.Printf("assembler output differs")

 	case tool == "link" || strings.HasSuffix(tool, "l"): // linker
 		log.Printf("linker output differs")
 		extra = "-v=2"
 	}

 	cmdS := append([]string{cmd[0], extra}, cmd[1:]...)
 	outfile, _ = cmpRun(true, cmdS)

 	fmt.Fprintf(os.Stderr, "\n%s\n", compareLogs(outfile))
 	os.Exit(2)
 }

 func cmpRun(keepLog bool, cmd []string) (outfile string, match bool) {
 	cmdStash := make([]string, len(cmd))
 	copy(cmdStash, cmd)
 	cmdStash[0] = toolStash
 	for i, arg := range cmdStash {
 		if arg == "-o" {
 			outfile = cmdStash[i+1]
 			cmdStash[i+1] += ".stash"
 			break
 		}
 		if strings.HasSuffix(arg, ".s") || strings.HasSuffix(arg, ".go") && '0' <= tool[0] && tool[0] <= '9' {
 			outfile = filepath.Base(arg[:strings.LastIndex(arg, ".")] + "." + tool[:1])
 			cmdStash = append([]string{cmdStash[0], "-o", outfile + ".stash"}, cmdStash[1:]...)
 			break
 		}
 	}

 	if outfile == "" {
 		log.Fatalf("cannot determine output file for command: %s", strings.Join(cmd, " "))
 	}

 	if *norun {
 		fmt.Printf("%s\n", strings.Join(cmd, " "))
 		fmt.Printf("%s\n", strings.Join(cmdStash, " "))
 		os.Exit(0)
 	}

 	out, err := runCmd(cmd, keepLog, outfile+".log")
 	if err != nil {
 		log.Printf("running: %s", strings.Join(cmd, " "))
 		os.Stderr.Write(out)
 		log.Fatal(err)
 	}

 	outStash, err := runCmd(cmdStash, keepLog, outfile+".stash.log")
 	if err != nil {
 		log.Printf("running: %s", strings.Join(cmdStash, " "))
 		log.Printf("installed tool succeeded but stashed tool failed.\n")
 		if len(out) > 0 {
 			log.Printf("installed tool output:")
 			os.Stderr.Write(out)
 		}
 		if len(outStash) > 0 {
 			log.Printf("stashed tool output:")
 			os.Stderr.Write(outStash)
 		}
 		log.Fatal(err)
 	}

 	return outfile, sameObject(outfile, outfile+".stash")
 }

 func sameObject(file1, file2 string) bool {
 	f1, err := os.Open(file1)
 	if err != nil {
 		log.Fatal(err)
 	}
 	defer f1.Close()

 	f2, err := os.Open(file2)
 	if err != nil {
 		log.Fatal(err)
 	}
 	defer f2.Close()

 	b1 := bufio.NewReader(f1)
 	b2 := bufio.NewReader(f2)

 	// Go object files and archives contain lines of the form
 	//	go object <goos> <goarch> <version>
 	// By default, the version on development branches includes
 	// the Git hash and time stamp for the most recent commit.
 	// We allow the versions to differ.
 	if !skipVersion(b1, b2, file1, file2) {
 		return false
 	}

 	lastByte := byte(0)
 	for {
 		c1, err1 := b1.ReadByte()
 		c2, err2 := b2.ReadByte()
 		if err1 == io.EOF && err2 == io.EOF {
 			return true
 		}
 		if err1 != nil {
 			log.Fatalf("reading %s: %v", file1, err1)
 		}
 		if err2 != nil {
 			log.Fatalf("reading %s: %v", file2, err1)
 		}
 		if c1 != c2 {
 			return false
 		}
 		if lastByte == '`' && c1 == '\n' {
 			if !skipVersion(b1, b2, file1, file2) {
 				return false
 			}
 		}
 		lastByte = c1
 	}
 }

 func skipVersion(b1, b2 *bufio.Reader, file1, file2 string) bool {
 	// Consume "go object " prefix, if there.
 	prefix := "go object "
 	for i := 0; i < len(prefix); i++ {
 		c1, err1 := b1.ReadByte()
 		c2, err2 := b2.ReadByte()
 		if err1 == io.EOF && err2 == io.EOF {
 			return true
 		}
 		if err1 != nil {
 			log.Fatalf("reading %s: %v", file1, err1)
 		}
 		if err2 != nil {
 			log.Fatalf("reading %s: %v", file2, err1)
 		}
 		if c1 != c2 {
 			return false
 		}
 		if c1 != prefix[i] {
 			return true // matching bytes, just not a version
 		}
 	}

 	// Keep comparing until second space.
 	// Must continue to match.
 	// If we see a \n, it's not a version string after all.
 	for numSpace := 0; numSpace < 2; {
 		c1, err1 := b1.ReadByte()
 		c2, err2 := b2.ReadByte()
 		if err1 == io.EOF && err2 == io.EOF {
 			return true
 		}
 		if err1 != nil {
 			log.Fatalf("reading %s: %v", file1, err1)
 		}
 		if err2 != nil {
 			log.Fatalf("reading %s: %v", file2, err1)
 		}
 		if c1 != c2 {
 			return false
 		}
 		if c1 == '\n' {
 			return true
 		}
 		if c1 == ' ' {
 			numSpace++
 		}
 	}

 	// Have now seen 'go object goos goarch ' in both files.
 	// Now they're allowed to diverge, until the \n, which
 	// must be present.
 	for {
 		c1, err1 := b1.ReadByte()
 		if err1 == io.EOF {
 			log.Fatalf("reading %s: unexpected EOF", file1)
 		}
 		if err1 != nil {
 			log.Fatalf("reading %s: %v", file1, err1)
 		}
 		if c1 == '\n' {
 			break
 		}
 	}
 	for {
 		c2, err2 := b2.ReadByte()
 		if err2 == io.EOF {
 			log.Fatalf("reading %s: unexpected EOF", file2)
 		}
 		if err2 != nil {
 			log.Fatalf("reading %s: %v", file2, err2)
 		}
 		if c2 == '\n' {
 			break
 		}
 	}

 	// Consumed "matching" versions from both.
 	return true
 }

 func runCmd(cmd []string, keepLog bool, logName string) (output []byte, err error) {
 	if *verbose {
 		log.Print(strings.Join(cmd, " "))
 	}

 	if *timing {
 		t0 := time.Now()
 		defer func() {
 			log.Printf("%.3fs elapsed # %s\n", time.Since(t0).Seconds(), strings.Join(cmd, " "))
 		}()
 	}

 	xcmd := exec.Command(cmd[0], cmd[1:]...)
 	if !keepLog {
 		return xcmd.CombinedOutput()
 	}

 	f, err := os.Create(logName)
 	if err != nil {
 		log.Fatal(err)
 	}
 	fmt.Fprintf(f, "GOOS=%s GOARCH=%s %s\n", os.Getenv("GOOS"), os.Getenv("GOARCH"), strings.Join(cmd, " "))
 	xcmd.Stdout = f
 	xcmd.Stderr = f
 	defer f.Close()
 	return nil, xcmd.Run()
 }

 func save() {
 	if err := os.MkdirAll(stashDir, 0777); err != nil {
 		log.Fatal(err)
 	}

 	toolDir := filepath.Join(goroot, fmt.Sprintf("pkg/tool/%s_%s", runtime.GOOS, runtime.GOARCH))
 	files, err := ioutil.ReadDir(toolDir)
 	if err != nil {
 		log.Fatal(err)
 	}

 	for _, file := range files {
 		if shouldSave(file.Name()) && file.Mode().IsRegular() {
 			cp(filepath.Join(toolDir, file.Name()), filepath.Join(stashDir, file.Name()))
 		}
 	}

 	for _, name := range binTools {
 		if !shouldSave(name) {
 			continue
 		}
 		src := filepath.Join(binDir, name)
 		if _, err := os.Stat(src); err == nil {
 			cp(src, filepath.Join(stashDir, name))
 		}
 	}

 	checkShouldSave()
 }

 func restore() {
 	files, err := ioutil.ReadDir(stashDir)
 	if err != nil {
 		log.Fatal(err)
 	}

 	for _, file := range files {
 		if shouldSave(file.Name()) && file.Mode().IsRegular() {
 			targ := toolDir
 			if isBinTool(file.Name()) {
 				targ = binDir
 			}
 			cp(filepath.Join(stashDir, file.Name()), filepath.Join(targ, file.Name()))
 		}
 	}

 	checkShouldSave()
 }

 func shouldSave(name string) bool {
 	if len(cmd) == 1 {
 		return true
 	}
 	ok := false
 	for i, arg := range cmd {
 		if i > 0 && name == arg {
 			ok = true
 			cmd[i] = "DONE"
 		}
 	}
 	return ok
 }

 func checkShouldSave() {
 	var missing []string
 	for _, arg := range cmd[1:] {
 		if arg != "DONE" {
 			missing = append(missing, arg)
 		}
 	}
 	if len(missing) > 0 {
 		log.Fatalf("%s did not find tools: %s", cmd[0], strings.Join(missing, " "))
 	}
 }

 func cp(src, dst string) {
 	if *verbose {
 		fmt.Printf("cp %s %s\n", src, dst)
 	}
 	data, err := ioutil.ReadFile(src)
 	if err != nil {
 		log.Fatal(err)
 	}
 	if err := ioutil.WriteFile(dst, data, 0777); err != nil {
 		log.Fatal(err)
 	}
 }
	// Copyright 2015 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	// Toolstash provides a way to save, run, and restore a known good copy of the Go toolchain
	// and to compare the object files generated by two toolchains.
	//
	// Usage:
	//
	// toolstash [-n] [-v] save [tool...]
	// toolstash [-n] [-v] restore [tool...]
	// toolstash [-n] [-v] [-t] go run x.go
	// toolstash [-n] [-v] [-t] [-cmp] compile x.go
	//
	// The toolstash command manages a ``stashed'' copy of the Go toolchain
	// kept in $GOROOT/pkg/toolstash. In this case, the toolchain means the
	// tools available with the 'go tool' command as well as the go, godoc, and gofmt
	// binaries.
	//
	// The command ``toolstash save'', typically run when the toolchain is known to be working,
	// copies the toolchain from its installed location to the toolstash directory.
	// Its inverse, ``toolchain restore'', typically run when the toolchain is known to be broken,
	// copies the toolchain from the toolstash directory back to the installed locations.
	// If additional arguments are given, the save or restore applies only to the named tools.
	// Otherwise, it applies to all tools.
	//
	// Otherwise, toolstash's arguments should be a command line beginning with the
	// name of a toolchain binary, which may be a short name like compile or a complete path
	// to an installed binary. Toolstash runs the command line using the stashed
	// copy of the binary instead of the installed one.
	//
	// The -n flag causes toolstash to print the commands that would be executed
	// but not execute them. The combination -n -cmp shows the two commands
	// that would be compared and then exits successfully. A real -cmp run might
	// run additional commands for diagnosis of an output mismatch.
	//
	// The -v flag causes toolstash to print the commands being executed.
	//
	// The -t flag causes toolstash to print the time elapsed during while the
	// command ran.
	//
	// Comparing
	//
	// The -cmp flag causes toolstash to run both the installed and the stashed
	// copy of an assembler or compiler and check that they produce identical
	// object files. If not, toolstash reports the mismatch and exits with a failure status.
	// As part of reporting the mismatch, toolstash reinvokes the command with
	// the -S flag and identifies the first divergence in the assembly output.
	// If the command is a Go compiler, toolstash also determines whether the
	// difference is triggered by optimization passes.
	// On failure, toolstash leaves additional information in files named
	// similarly to the default output file. If the compilation would normally
	// produce a file x.6, the output from the stashed tool is left in x.6.stash
	// and the debugging traces are left in x.6.log and x.6.stash.log.
	//
	// The -cmp flag is a no-op when the command line is not invoking an
	// assembler or compiler.
	//
	// For example, when working on code cleanup that should not affect
	// compiler output, toolstash can be used to compare the old and new
	// compiler output:
	//
	// toolstash save
	// <edit compiler sources>
	// go tool dist install cmd/compile # install compiler only
	// toolstash -cmp compile x.go
	//
	// Go Command Integration
	//
	// The go command accepts a -toolexec flag that specifies a program
	// to use to run the build tools.
	//
	// To build with the stashed tools:
	//
	// go build -toolexec toolstash x.go
	//
	// To build with the stashed go command and the stashed tools:
	//
	// toolstash go build -toolexec toolstash x.go
	//
	// To verify that code cleanup in the compilers does not make any
	// changes to the objects being generated for the entire tree:
	//
	// # Build working tree and save tools.
	// ./make.bash
	// toolstash save
	//
	// <edit compiler sources>
	//
	// # Install new tools, but do not rebuild the rest of tree,
	// # since the compilers might generate buggy code.
	// go tool dist install cmd/compile
	//
	// # Check that new tools behave identically to saved tools.
	// go build -toolexec 'toolstash -cmp' -a std
	//
	// # If not, restore, in order to keep working on Go code.
	// toolstash restore
	//
	// Version Skew
	//
	// The Go tools write the current Go version to object files, and (outside
	// release branches) that version includes the hash and time stamp
	// of the most recent Git commit. Functionally equivalent
	// compilers built at different Git versions may produce object files that
	// differ only in the recorded version. Toolstash ignores version mismatches
	// when comparing object files, but the standard tools will refuse to compile
	// or link together packages with different object versions.
	//
	// For the full build in the final example above to work, both the stashed
	// and the installed tools must use the same version string.
	// One way to ensure this is not to commit any of the changes being
	// tested, so that the Git HEAD hash is the same for both builds.
	// A more robust way to force the tools to have the same version string
	// is to write a $GOROOT/VERSION file, which overrides the Git-based version
	// computation:
	//
	// echo devel >$GOROOT/VERSION
	//
	// The version can be arbitrary text, but to pass all.bash's API check, it must
	// contain the substring ``devel''. The VERSION file must be created before
	// building either version of the toolchain.
	//
	package main // import "golang.org/x/tools/cmd/toolstash"

	import (
	"bufio"
	"flag"
	"fmt"
	"io"
	"io/ioutil"
	"log"
	"os"
	"os/exec"
	"path/filepath"
	"runtime"
	"strings"
	"time"
	)

	var usageMessage = `usage: toolstash [-n] [-v] [-cmp] command line

	Examples:
	toolstash save
	toolstash restore
	toolstash go run x.go
	toolstash compile x.go
	toolstash -cmp compile x.go

	For details, godoc rsc.io/toolstash
	`

	func usage() {
	fmt.Fprint(os.Stderr, usageMessage)
	os.Exit(2)
	}

	var (
	norun = flag.Bool("n", false, "print but do not run commands")
	verbose = flag.Bool("v", false, "print commands being run")
	cmp = flag.Bool("cmp", false, "compare tool object files")
	timing = flag.Bool("t", false, "print time commands take")
	)

	var (
	cmd []string
	tool string // name of tool: "go", "compile", etc
	toolStash string // path to stashed tool

	goroot string
	toolDir string
	stashDir string
	binDir string
	)

	func canCmp(name string) bool {
	switch name {
	case "compile", "link", "asm":
	return true
	}
	return len(name) == 2 && '0' <= name[0] && name[0] <= '9' && (name[1] == 'a' \|\| name[1] == 'g' \|\| name[1] == 'l')
	}

	var binTools = []string{"go", "godoc", "gofmt"}

	func isBinTool(name string) bool {
	return strings.HasPrefix(name, "go")
	}

	func main() {
	log.SetFlags(0)
	log.SetPrefix("toolstash: ")

	flag.Usage = usage
	flag.Parse()
	cmd = flag.Args()

	if len(cmd) < 1 {
	usage()
	}

	goroot = runtime.GOROOT()
	toolDir = filepath.Join(goroot, fmt.Sprintf("pkg/tool/%s_%s", runtime.GOOS, runtime.GOARCH))
	stashDir = filepath.Join(goroot, "pkg/toolstash")

	binDir = os.Getenv("GOBIN")
	if binDir == "" {
	binDir = filepath.Join(goroot, "bin")
	}

	switch cmd[0] {
	case "save":
	save()
	return

	case "restore":
	restore()
	return
	}

	tool = cmd[0]
	if i := strings.LastIndex(tool, "/"); i >= 0 {
	tool = tool[i+1:]
	}
	if i := strings.LastIndex(tool, `\`); i >= 0 {
	tool = tool[i+1:]
	}

	if !strings.HasPrefix(tool, "a.out") {
	toolStash = filepath.Join(stashDir, tool)
	if _, err := os.Stat(toolStash); err != nil {
	log.Print(err)
	os.Exit(2)
	}

	if *cmp && canCmp(tool) {
	compareTool()
	return
	}
	cmd[0] = toolStash
	}

	if *norun {
	fmt.Printf("%s\n", strings.Join(cmd, " "))
	return
	}
	if *verbose {
	log.Print(strings.Join(cmd, " "))
	}
	xcmd := exec.Command(cmd[0], cmd[1:]...)
	xcmd.Stdin = os.Stdin
	xcmd.Stdout = os.Stdout
	xcmd.Stderr = os.Stderr
	err := xcmd.Run()
	if err != nil {
	log.Fatal(err)
	}
	os.Exit(0)
	}

	func compareTool() {
	if !strings.Contains(cmd[0], "/") && !strings.Contains(cmd[0], `\`) {
	cmd[0] = filepath.Join(toolDir, tool)
	}

	outfile, ok := cmpRun(false, cmd)
	if ok {
	os.Remove(outfile + ".stash")
	return
	}

	extra := "-S"
	switch {
	default:
	log.Fatalf("unknown tool %s", tool)

	case tool == "compile" \|\| strings.HasSuffix(tool, "g"): // compiler
	cmdN := append([]string{cmd[0], "-N"}, cmd[1:]...)
	_, ok := cmpRun(false, cmdN)
	if !ok {
	log.Printf("compiler output differs, even with optimizers disabled (-N)")
	cmd = append([]string{cmd[0], "-v", "-N", "-m=2"}, cmd[1:]...)
	break
	}
	cmd = append([]string{cmd[0], "-v", "-m=2"}, cmd[1:]...)
	log.Printf("compiler output differs, only with optimizers enabled")

	case tool == "asm" \|\| strings.HasSuffix(tool, "a"): // assembler
	log.Printf("assembler output differs")

	case tool == "link" \|\| strings.HasSuffix(tool, "l"): // linker
	log.Printf("linker output differs")
	extra = "-v=2"
	}

	cmdS := append([]string{cmd[0], extra}, cmd[1:]...)
	outfile, _ = cmpRun(true, cmdS)

	fmt.Fprintf(os.Stderr, "\n%s\n", compareLogs(outfile))
	os.Exit(2)
	}

	func cmpRun(keepLog bool, cmd []string) (outfile string, match bool) {
	cmdStash := make([]string, len(cmd))
	copy(cmdStash, cmd)
	cmdStash[0] = toolStash
	for i, arg := range cmdStash {
	if arg == "-o" {
	outfile = cmdStash[i+1]
	cmdStash[i+1] += ".stash"
	break
	}
	if strings.HasSuffix(arg, ".s") \|\| strings.HasSuffix(arg, ".go") && '0' <= tool[0] && tool[0] <= '9' {
	outfile = filepath.Base(arg[:strings.LastIndex(arg, ".")] + "." + tool[:1])
	cmdStash = append([]string{cmdStash[0], "-o", outfile + ".stash"}, cmdStash[1:]...)
	break
	}
	}

	if outfile == "" {
	log.Fatalf("cannot determine output file for command: %s", strings.Join(cmd, " "))
	}

	if *norun {
	fmt.Printf("%s\n", strings.Join(cmd, " "))
	fmt.Printf("%s\n", strings.Join(cmdStash, " "))
	os.Exit(0)
	}

	out, err := runCmd(cmd, keepLog, outfile+".log")
	if err != nil {
	log.Printf("running: %s", strings.Join(cmd, " "))
	os.Stderr.Write(out)
	log.Fatal(err)
	}

	outStash, err := runCmd(cmdStash, keepLog, outfile+".stash.log")
	if err != nil {
	log.Printf("running: %s", strings.Join(cmdStash, " "))
	log.Printf("installed tool succeeded but stashed tool failed.\n")
	if len(out) > 0 {
	log.Printf("installed tool output:")
	os.Stderr.Write(out)
	}
	if len(outStash) > 0 {
	log.Printf("stashed tool output:")
	os.Stderr.Write(outStash)
	}
	log.Fatal(err)
	}

	return outfile, sameObject(outfile, outfile+".stash")
	}

	func sameObject(file1, file2 string) bool {
	f1, err := os.Open(file1)
	if err != nil {
	log.Fatal(err)
	}
	defer f1.Close()

	f2, err := os.Open(file2)
	if err != nil {
	log.Fatal(err)
	}
	defer f2.Close()

	b1 := bufio.NewReader(f1)
	b2 := bufio.NewReader(f2)

	// Go object files and archives contain lines of the form
	// go object <goos> <goarch> <version>
	// By default, the version on development branches includes
	// the Git hash and time stamp for the most recent commit.
	// We allow the versions to differ.
	if !skipVersion(b1, b2, file1, file2) {
	return false
	}

	lastByte := byte(0)
	for {
	c1, err1 := b1.ReadByte()
	c2, err2 := b2.ReadByte()
	if err1 == io.EOF && err2 == io.EOF {
	return true
	}
	if err1 != nil {
	log.Fatalf("reading %s: %v", file1, err1)
	}
	if err2 != nil {
	log.Fatalf("reading %s: %v", file2, err1)
	}
	if c1 != c2 {
	return false
	}
	if lastByte == '`' && c1 == '\n' {
	if !skipVersion(b1, b2, file1, file2) {
	return false
	}
	}
	lastByte = c1
	}
	}

	func skipVersion(b1, b2 *bufio.Reader, file1, file2 string) bool {
	// Consume "go object " prefix, if there.
	prefix := "go object "
	for i := 0; i < len(prefix); i++ {
	c1, err1 := b1.ReadByte()
	c2, err2 := b2.ReadByte()
	if err1 == io.EOF && err2 == io.EOF {
	return true
	}
	if err1 != nil {
	log.Fatalf("reading %s: %v", file1, err1)
	}
	if err2 != nil {
	log.Fatalf("reading %s: %v", file2, err1)
	}
	if c1 != c2 {
	return false
	}
	if c1 != prefix[i] {
	return true // matching bytes, just not a version
	}
	}

	// Keep comparing until second space.
	// Must continue to match.
	// If we see a \n, it's not a version string after all.
	for numSpace := 0; numSpace < 2; {
	c1, err1 := b1.ReadByte()
	c2, err2 := b2.ReadByte()
	if err1 == io.EOF && err2 == io.EOF {
	return true
	}
	if err1 != nil {
	log.Fatalf("reading %s: %v", file1, err1)
	}
	if err2 != nil {
	log.Fatalf("reading %s: %v", file2, err1)
	}
	if c1 != c2 {
	return false
	}
	if c1 == '\n' {
	return true
	}
	if c1 == ' ' {
	numSpace++
	}
	}

	// Have now seen 'go object goos goarch ' in both files.
	// Now they're allowed to diverge, until the \n, which
	// must be present.
	for {
	c1, err1 := b1.ReadByte()
	if err1 == io.EOF {
	log.Fatalf("reading %s: unexpected EOF", file1)
	}
	if err1 != nil {
	log.Fatalf("reading %s: %v", file1, err1)
	}
	if c1 == '\n' {
	break
	}
	}
	for {
	c2, err2 := b2.ReadByte()
	if err2 == io.EOF {
	log.Fatalf("reading %s: unexpected EOF", file2)
	}
	if err2 != nil {
	log.Fatalf("reading %s: %v", file2, err2)
	}
	if c2 == '\n' {
	break
	}
	}

	// Consumed "matching" versions from both.
	return true
	}

	func runCmd(cmd []string, keepLog bool, logName string) (output []byte, err error) {
	if *verbose {
	log.Print(strings.Join(cmd, " "))
	}

	if *timing {
	t0 := time.Now()
	defer func() {
	log.Printf("%.3fs elapsed # %s\n", time.Since(t0).Seconds(), strings.Join(cmd, " "))
	}()
	}

	xcmd := exec.Command(cmd[0], cmd[1:]...)
	if !keepLog {
	return xcmd.CombinedOutput()
	}

	f, err := os.Create(logName)
	if err != nil {
	log.Fatal(err)
	}
	fmt.Fprintf(f, "GOOS=%s GOARCH=%s %s\n", os.Getenv("GOOS"), os.Getenv("GOARCH"), strings.Join(cmd, " "))
	xcmd.Stdout = f
	xcmd.Stderr = f
	defer f.Close()
	return nil, xcmd.Run()
	}

	func save() {
	if err := os.MkdirAll(stashDir, 0777); err != nil {
	log.Fatal(err)
	}

	toolDir := filepath.Join(goroot, fmt.Sprintf("pkg/tool/%s_%s", runtime.GOOS, runtime.GOARCH))
	files, err := ioutil.ReadDir(toolDir)
	if err != nil {
	log.Fatal(err)
	}

	for _, file := range files {
	if shouldSave(file.Name()) && file.Mode().IsRegular() {
	cp(filepath.Join(toolDir, file.Name()), filepath.Join(stashDir, file.Name()))
	}
	}

	for _, name := range binTools {
	if !shouldSave(name) {
	continue
	}
	src := filepath.Join(binDir, name)
	if _, err := os.Stat(src); err == nil {
	cp(src, filepath.Join(stashDir, name))
	}
	}

	checkShouldSave()
	}

	func restore() {
	files, err := ioutil.ReadDir(stashDir)
	if err != nil {
	log.Fatal(err)
	}

	for _, file := range files {
	if shouldSave(file.Name()) && file.Mode().IsRegular() {
	targ := toolDir
	if isBinTool(file.Name()) {
	targ = binDir
	}
	cp(filepath.Join(stashDir, file.Name()), filepath.Join(targ, file.Name()))
	}
	}

	checkShouldSave()
	}

	func shouldSave(name string) bool {
	if len(cmd) == 1 {
	return true
	}
	ok := false
	for i, arg := range cmd {
	if i > 0 && name == arg {
	ok = true
	cmd[i] = "DONE"
	}
	}
	return ok
	}

	func checkShouldSave() {
	var missing []string
	for _, arg := range cmd[1:] {
	if arg != "DONE" {
	missing = append(missing, arg)
	}
	}
	if len(missing) > 0 {
	log.Fatalf("%s did not find tools: %s", cmd[0], strings.Join(missing, " "))
	}
	}

	func cp(src, dst string) {
	if *verbose {
	fmt.Printf("cp %s %s\n", src, dst)
	}
	data, err := ioutil.ReadFile(src)
	if err != nil {
	log.Fatal(err)
	}
	if err := ioutil.WriteFile(dst, data, 0777); err != nil {
	log.Fatal(err)
	}
	}