shell.go

     1  // Copyright 2023 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package work
     6  
     7  import (
     8  	"bytes"
     9  	"cmd/go/internal/base"
    10  	"cmd/go/internal/cache"
    11  	"cmd/go/internal/cfg"
    12  	"cmd/go/internal/load"
    13  	"cmd/go/internal/str"
    14  	"cmd/internal/par"
    15  	"cmd/internal/pathcache"
    16  	"errors"
    17  	"fmt"
    18  	"io"
    19  	"io/fs"
    20  	"os"
    21  	"os/exec"
    22  	"path/filepath"
    23  	"runtime"
    24  	"strconv"
    25  	"strings"
    26  	"sync"
    27  	"time"
    28  )
    29  
    30  // A Shell runs shell commands and performs shell-like file system operations.
    31  //
    32  // Shell tracks context related to running commands, and form a tree much like
    33  // context.Context.
    34  type Shell struct {
    35  	action       *Action // nil for the root shell
    36  	*shellShared         // per-Builder state shared across Shells
    37  }
    38  
    39  // shellShared is Shell state shared across all Shells derived from a single
    40  // root shell (generally a single Builder).
    41  type shellShared struct {
    42  	workDir string // $WORK, immutable
    43  
    44  	printLock sync.Mutex
    45  	printer   load.Printer
    46  	scriptDir string // current directory in printed script
    47  
    48  	mkdirCache par.Cache[string, error] // a cache of created directories
    49  }
    50  
    51  // NewShell returns a new Shell.
    52  //
    53  // Shell will internally serialize calls to the printer.
    54  // If printer is nil, it uses load.DefaultPrinter.
    55  func NewShell(workDir string, printer load.Printer) *Shell {
    56  	if printer == nil {
    57  		printer = load.DefaultPrinter()
    58  	}
    59  	shared := &shellShared{
    60  		workDir: workDir,
    61  		printer: printer,
    62  	}
    63  	return &Shell{shellShared: shared}
    64  }
    65  
    66  func (sh *Shell) pkg() *load.Package {
    67  	if sh.action == nil {
    68  		return nil
    69  	}
    70  	return sh.action.Package
    71  }
    72  
    73  // Printf emits a to this Shell's output stream, formatting it like fmt.Printf.
    74  // It is safe to call concurrently.
    75  func (sh *Shell) Printf(format string, a ...any) {
    76  	sh.printLock.Lock()
    77  	defer sh.printLock.Unlock()
    78  	sh.printer.Printf(sh.pkg(), format, a...)
    79  }
    80  
    81  func (sh *Shell) printfLocked(format string, a ...any) {
    82  	sh.printer.Printf(sh.pkg(), format, a...)
    83  }
    84  
    85  // Errorf reports an error on sh's package and sets the process exit status to 1.
    86  func (sh *Shell) Errorf(format string, a ...any) {
    87  	sh.printLock.Lock()
    88  	defer sh.printLock.Unlock()
    89  	sh.printer.Errorf(sh.pkg(), format, a...)
    90  }
    91  
    92  // WithAction returns a Shell identical to sh, but bound to Action a.
    93  func (sh *Shell) WithAction(a *Action) *Shell {
    94  	sh2 := *sh
    95  	sh2.action = a
    96  	return &sh2
    97  }
    98  
    99  // Shell returns a shell for running commands on behalf of Action a.
   100  func (b *Builder) Shell(a *Action) *Shell {
   101  	if a == nil {
   102  		// The root shell has a nil Action. The point of this method is to
   103  		// create a Shell bound to an Action, so disallow nil Actions here.
   104  		panic("nil Action")
   105  	}
   106  	if a.sh == nil {
   107  		a.sh = b.backgroundSh.WithAction(a)
   108  	}
   109  	return a.sh
   110  }
   111  
   112  // BackgroundShell returns a Builder-wide Shell that's not bound to any Action.
   113  // Try not to use this unless there's really no sensible Action available.
   114  func (b *Builder) BackgroundShell() *Shell {
   115  	return b.backgroundSh
   116  }
   117  
   118  // moveOrCopyFile is like 'mv src dst' or 'cp src dst'.
   119  func (sh *Shell) moveOrCopyFile(dst, src string, perm fs.FileMode, force bool) error {
   120  	if cfg.BuildN {
   121  		sh.ShowCmd("", "mv %s %s", src, dst)
   122  		return nil
   123  	}
   124  
   125  	err := checkDstOverwrite(dst, force)
   126  	if err != nil {
   127  		return err
   128  	}
   129  
   130  	// If we can update the mode and rename to the dst, do it.
   131  	// Otherwise fall back to standard copy.
   132  
   133  	// If the source is in the build cache, we need to copy it.
   134  	dir, _, _ := cache.DefaultDir()
   135  	if strings.HasPrefix(src, dir) {
   136  		return sh.CopyFile(dst, src, perm, force)
   137  	}
   138  
   139  	// On Windows, always copy the file, so that we respect the NTFS
   140  	// permissions of the parent folder. https://golang.org/issue/22343.
   141  	// What matters here is not cfg.Goos (the system we are building
   142  	// for) but runtime.GOOS (the system we are building on).
   143  	if runtime.GOOS == "windows" {
   144  		return sh.CopyFile(dst, src, perm, force)
   145  	}
   146  
   147  	// If the destination directory has the group sticky bit set,
   148  	// we have to copy the file to retain the correct permissions.
   149  	// https://golang.org/issue/18878
   150  	if fi, err := os.Stat(filepath.Dir(dst)); err == nil {
   151  		if fi.IsDir() && (fi.Mode()&fs.ModeSetgid) != 0 {
   152  			return sh.CopyFile(dst, src, perm, force)
   153  		}
   154  	}
   155  
   156  	// The perm argument is meant to be adjusted according to umask,
   157  	// but we don't know what the umask is.
   158  	// Create a dummy file to find out.
   159  	// This avoids build tags and works even on systems like Plan 9
   160  	// where the file mask computation incorporates other information.
   161  	mode := perm
   162  	f, err := os.OpenFile(filepath.Clean(dst)+"-go-tmp-umask", os.O_WRONLY|os.O_CREATE|os.O_EXCL, perm)
   163  	if err == nil {
   164  		fi, err := f.Stat()
   165  		if err == nil {
   166  			mode = fi.Mode() & 0777
   167  		}
   168  		name := f.Name()
   169  		f.Close()
   170  		os.Remove(name)
   171  	}
   172  
   173  	if err := os.Chmod(src, mode); err == nil {
   174  		if err := os.Rename(src, dst); err == nil {
   175  			if cfg.BuildX {
   176  				sh.ShowCmd("", "mv %s %s", src, dst)
   177  			}
   178  			return nil
   179  		}
   180  	}
   181  
   182  	return sh.CopyFile(dst, src, perm, force)
   183  }
   184  
   185  // CopyFile is like 'cp src dst'.
   186  func (sh *Shell) CopyFile(dst, src string, perm fs.FileMode, force bool) error {
   187  	if cfg.BuildN || cfg.BuildX {
   188  		sh.ShowCmd("", "cp %s %s", src, dst)
   189  		if cfg.BuildN {
   190  			return nil
   191  		}
   192  	}
   193  
   194  	sf, err := os.Open(src)
   195  	if err != nil {
   196  		return err
   197  	}
   198  	defer sf.Close()
   199  
   200  	err = checkDstOverwrite(dst, force)
   201  	if err != nil {
   202  		return err
   203  	}
   204  
   205  	// On Windows, remove lingering ~ file from last attempt.
   206  	if runtime.GOOS == "windows" {
   207  		if _, err := os.Stat(dst + "~"); err == nil {
   208  			os.Remove(dst + "~")
   209  		}
   210  	}
   211  
   212  	mayberemovefile(dst)
   213  	df, err := os.OpenFile(dst, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, perm)
   214  	if err != nil && runtime.GOOS == "windows" {
   215  		// Windows does not allow deletion of a binary file
   216  		// while it is executing. Try to move it out of the way.
   217  		// If the move fails, which is likely, we'll try again the
   218  		// next time we do an install of this binary.
   219  		if err := os.Rename(dst, dst+"~"); err == nil {
   220  			os.Remove(dst + "~")
   221  		}
   222  		df, err = os.OpenFile(dst, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, perm)
   223  	}
   224  	if err != nil {
   225  		return fmt.Errorf("copying %s: %w", src, err) // err should already refer to dst
   226  	}
   227  
   228  	_, err = io.Copy(df, sf)
   229  	df.Close()
   230  	if err != nil {
   231  		mayberemovefile(dst)
   232  		return fmt.Errorf("copying %s to %s: %v", src, dst, err)
   233  	}
   234  	return nil
   235  }
   236  
   237  // mayberemovefile removes a file only if it is a regular file
   238  // When running as a user with sufficient privileges, we may delete
   239  // even device files, for example, which is not intended.
   240  func mayberemovefile(s string) {
   241  	if fi, err := os.Lstat(s); err == nil && !fi.Mode().IsRegular() {
   242  		return
   243  	}
   244  	os.Remove(s)
   245  }
   246  
   247  // Be careful about removing/overwriting dst.
   248  // Do not remove/overwrite if dst exists and is a directory
   249  // or a non-empty non-object file.
   250  func checkDstOverwrite(dst string, force bool) error {
   251  	if fi, err := os.Stat(dst); err == nil {
   252  		if fi.IsDir() {
   253  			return fmt.Errorf("build output %q already exists and is a directory", dst)
   254  		}
   255  		if !force && fi.Mode().IsRegular() && fi.Size() != 0 && !isObject(dst) {
   256  			return fmt.Errorf("build output %q already exists and is not an object file", dst)
   257  		}
   258  	}
   259  	return nil
   260  }
   261  
   262  // writeFile writes the text to file.
   263  func (sh *Shell) writeFile(file string, text []byte) error {
   264  	if cfg.BuildN || cfg.BuildX {
   265  		switch {
   266  		case len(text) == 0:
   267  			sh.ShowCmd("", "echo -n > %s # internal", file)
   268  		case bytes.IndexByte(text, '\n') == len(text)-1:
   269  			// One line. Use a simpler "echo" command.
   270  			sh.ShowCmd("", "echo '%s' > %s # internal", bytes.TrimSuffix(text, []byte("\n")), file)
   271  		default:
   272  			// Use the most general form.
   273  			sh.ShowCmd("", "cat >%s << 'EOF' # internal\n%sEOF", file, text)
   274  		}
   275  	}
   276  	if cfg.BuildN {
   277  		return nil
   278  	}
   279  	return os.WriteFile(file, text, 0666)
   280  }
   281  
   282  // Mkdir makes the named directory.
   283  func (sh *Shell) Mkdir(dir string) error {
   284  	// Make Mkdir(a.Objdir) a no-op instead of an error when a.Objdir == "".
   285  	if dir == "" {
   286  		return nil
   287  	}
   288  
   289  	// We can be a little aggressive about being
   290  	// sure directories exist. Skip repeated calls.
   291  	return sh.mkdirCache.Do(dir, func() error {
   292  		if cfg.BuildN || cfg.BuildX {
   293  			sh.ShowCmd("", "mkdir -p %s", dir)
   294  			if cfg.BuildN {
   295  				return nil
   296  			}
   297  		}
   298  
   299  		return os.MkdirAll(dir, 0777)
   300  	})
   301  }
   302  
   303  // RemoveAll is like 'rm -rf'. It attempts to remove all paths even if there's
   304  // an error, and returns the first error.
   305  func (sh *Shell) RemoveAll(paths ...string) error {
   306  	if cfg.BuildN || cfg.BuildX {
   307  		// Don't say we are removing the directory if we never created it.
   308  		show := func() bool {
   309  			for _, path := range paths {
   310  				if _, ok := sh.mkdirCache.Get(path); ok {
   311  					return true
   312  				}
   313  				if _, err := os.Stat(path); !os.IsNotExist(err) {
   314  					return true
   315  				}
   316  			}
   317  			return false
   318  		}
   319  		if show() {
   320  			sh.ShowCmd("", "rm -rf %s", strings.Join(paths, " "))
   321  		}
   322  	}
   323  	if cfg.BuildN {
   324  		return nil
   325  	}
   326  
   327  	var err error
   328  	for _, path := range paths {
   329  		if err2 := os.RemoveAll(path); err2 != nil && err == nil {
   330  			err = err2
   331  		}
   332  	}
   333  	return err
   334  }
   335  
   336  // Symlink creates a symlink newname -> oldname.
   337  func (sh *Shell) Symlink(oldname, newname string) error {
   338  	// It's not an error to try to recreate an existing symlink.
   339  	if link, err := os.Readlink(newname); err == nil && link == oldname {
   340  		return nil
   341  	}
   342  
   343  	if cfg.BuildN || cfg.BuildX {
   344  		sh.ShowCmd("", "ln -s %s %s", oldname, newname)
   345  		if cfg.BuildN {
   346  			return nil
   347  		}
   348  	}
   349  	return os.Symlink(oldname, newname)
   350  }
   351  
   352  // fmtCmd formats a command in the manner of fmt.Sprintf but also:
   353  //
   354  //	fmtCmd replaces the value of b.WorkDir with $WORK.
   355  func (sh *Shell) fmtCmd(dir string, format string, args ...any) string {
   356  	cmd := fmt.Sprintf(format, args...)
   357  	if sh.workDir != "" && !strings.HasPrefix(cmd, "cat ") {
   358  		cmd = strings.ReplaceAll(cmd, sh.workDir, "$WORK")
   359  		escaped := strconv.Quote(sh.workDir)
   360  		escaped = escaped[1 : len(escaped)-1] // strip quote characters
   361  		if escaped != sh.workDir {
   362  			cmd = strings.ReplaceAll(cmd, escaped, "$WORK")
   363  		}
   364  	}
   365  	return cmd
   366  }
   367  
   368  // ShowCmd prints the given command to standard output
   369  // for the implementation of -n or -x.
   370  //
   371  // ShowCmd also replaces the name of the current script directory with dot (.)
   372  // but only when it is at the beginning of a space-separated token.
   373  //
   374  // If dir is not "" or "/" and not the current script directory, ShowCmd first
   375  // prints a "cd" command to switch to dir and updates the script directory.
   376  func (sh *Shell) ShowCmd(dir string, format string, args ...any) {
   377  	// Use the output lock directly so we can manage scriptDir.
   378  	sh.printLock.Lock()
   379  	defer sh.printLock.Unlock()
   380  
   381  	cmd := sh.fmtCmd(dir, format, args...)
   382  
   383  	if dir != "" && dir != "/" {
   384  		if dir != sh.scriptDir {
   385  			// Show changing to dir and update the current directory.
   386  			sh.printfLocked("%s", sh.fmtCmd("", "cd %s\n", dir))
   387  			sh.scriptDir = dir
   388  		}
   389  		// Replace scriptDir is our working directory. Replace it
   390  		// with "." in the command.
   391  		dot := " ."
   392  		if dir[len(dir)-1] == filepath.Separator {
   393  			dot += string(filepath.Separator)
   394  		}
   395  		cmd = strings.ReplaceAll(" "+cmd, " "+dir, dot)[1:]
   396  	}
   397  
   398  	sh.printfLocked("%s\n", cmd)
   399  }
   400  
   401  // reportCmd reports the output and exit status of a command. The cmdOut and
   402  // cmdErr arguments are the output and exit error of the command, respectively.
   403  //
   404  // The exact reporting behavior is as follows:
   405  //
   406  //	cmdOut  cmdErr  Result
   407  //	""      nil     print nothing, return nil
   408  //	!=""    nil     print output, return nil
   409  //	""      !=nil   print nothing, return cmdErr (later printed)
   410  //	!=""    !=nil   print nothing, ignore err, return output as error (later printed)
   411  //
   412  // reportCmd returns a non-nil error if and only if cmdErr != nil. It assumes
   413  // that the command output, if non-empty, is more detailed than the command
   414  // error (which is usually just an exit status), so prefers using the output as
   415  // the ultimate error. Typically, the caller should return this error from an
   416  // Action, which it will be printed by the Builder.
   417  //
   418  // reportCmd formats the output as "# desc" followed by the given output. The
   419  // output is expected to contain references to 'dir', usually the source
   420  // directory for the package that has failed to build. reportCmd rewrites
   421  // mentions of dir with a relative path to dir when the relative path is
   422  // shorter. This is usually more pleasant. For example, if fmt doesn't compile
   423  // and we are in src/html, the output is
   424  //
   425  //	$ go build
   426  //	# fmt
   427  //	../fmt/print.go:1090: undefined: asdf
   428  //	$
   429  //
   430  // instead of
   431  //
   432  //	$ go build
   433  //	# fmt
   434  //	/usr/gopher/go/src/fmt/print.go:1090: undefined: asdf
   435  //	$
   436  //
   437  // reportCmd also replaces references to the work directory with $WORK, replaces
   438  // cgo file paths with the original file path, and replaces cgo-mangled names
   439  // with "C.name".
   440  //
   441  // desc is optional. If "", a.Package.Desc() is used.
   442  //
   443  // dir is optional. If "", a.Package.Dir is used.
   444  func (sh *Shell) reportCmd(desc, dir string, cmdOut []byte, cmdErr error) error {
   445  	if len(cmdOut) == 0 && cmdErr == nil {
   446  		// Common case
   447  		return nil
   448  	}
   449  	if len(cmdOut) == 0 && cmdErr != nil {
   450  		// Just return the error.
   451  		//
   452  		// TODO: This is what we've done for a long time, but it may be a
   453  		// mistake because it loses all of the extra context and results in
   454  		// ultimately less descriptive output. We should probably just take the
   455  		// text of cmdErr as the output in this case and do everything we
   456  		// otherwise would. We could chain the errors if we feel like it.
   457  		return cmdErr
   458  	}
   459  
   460  	// Fetch defaults from the package.
   461  	var p *load.Package
   462  	a := sh.action
   463  	if a != nil {
   464  		p = a.Package
   465  	}
   466  	var importPath string
   467  	if p != nil {
   468  		importPath = p.ImportPath
   469  		if desc == "" {
   470  			desc = p.Desc()
   471  		}
   472  		if dir == "" {
   473  			dir = p.Dir
   474  		}
   475  	}
   476  
   477  	out := string(cmdOut)
   478  
   479  	if !strings.HasSuffix(out, "\n") {
   480  		out = out + "\n"
   481  	}
   482  
   483  	// Replace workDir with $WORK
   484  	out = replacePrefix(out, sh.workDir, "$WORK")
   485  
   486  	// Rewrite mentions of dir with a relative path to dir
   487  	// when the relative path is shorter.
   488  	for {
   489  		// Note that dir starts out long, something like
   490  		// /foo/bar/baz/root/a
   491  		// The target string to be reduced is something like
   492  		// (blah-blah-blah) /foo/bar/baz/root/sibling/whatever.go:blah:blah
   493  		// /foo/bar/baz/root/a doesn't match /foo/bar/baz/root/sibling, but the prefix
   494  		// /foo/bar/baz/root does.  And there may be other niblings sharing shorter
   495  		// prefixes, the only way to find them is to look.
   496  		// This doesn't always produce a relative path --
   497  		// /foo is shorter than ../../.., for example.
   498  		if reldir := base.ShortPath(dir); reldir != dir {
   499  			out = replacePrefix(out, dir, reldir)
   500  			if filepath.Separator == '\\' {
   501  				// Don't know why, sometimes this comes out with slashes, not backslashes.
   502  				wdir := strings.ReplaceAll(dir, "\\", "/")
   503  				out = replacePrefix(out, wdir, reldir)
   504  			}
   505  		}
   506  		dirP := filepath.Dir(dir)
   507  		if dir == dirP {
   508  			break
   509  		}
   510  		dir = dirP
   511  	}
   512  
   513  	// Usually desc is already p.Desc(), but if not, signal cmdError.Error to
   514  	// add a line explicitly mentioning the import path.
   515  	needsPath := importPath != "" && p != nil && desc != p.Desc()
   516  
   517  	err := &cmdError{desc, out, importPath, needsPath}
   518  	if cmdErr != nil {
   519  		// The command failed. Report the output up as an error.
   520  		return err
   521  	}
   522  	// The command didn't fail, so just print the output as appropriate.
   523  	if a != nil && a.output != nil {
   524  		// The Action is capturing output.
   525  		a.output = append(a.output, err.Error()...)
   526  	} else {
   527  		// Write directly to the Builder output.
   528  		sh.Printf("%s", err)
   529  	}
   530  	return nil
   531  }
   532  
   533  // replacePrefix is like strings.ReplaceAll, but only replaces instances of old
   534  // that are preceded by ' ', '\t', or appear at the beginning of a line.
   535  func replacePrefix(s, old, new string) string {
   536  	n := strings.Count(s, old)
   537  	if n == 0 {
   538  		return s
   539  	}
   540  
   541  	s = strings.ReplaceAll(s, " "+old, " "+new)
   542  	s = strings.ReplaceAll(s, "\n"+old, "\n"+new)
   543  	s = strings.ReplaceAll(s, "\n\t"+old, "\n\t"+new)
   544  	if strings.HasPrefix(s, old) {
   545  		s = new + s[len(old):]
   546  	}
   547  	return s
   548  }
   549  
   550  type cmdError struct {
   551  	desc       string
   552  	text       string
   553  	importPath string
   554  	needsPath  bool // Set if desc does not already include the import path
   555  }
   556  
   557  func (e *cmdError) Error() string {
   558  	var msg string
   559  	if e.needsPath {
   560  		// Ensure the import path is part of the message.
   561  		// Clearly distinguish the description from the import path.
   562  		msg = fmt.Sprintf("# %s\n# [%s]\n", e.importPath, e.desc)
   563  	} else {
   564  		msg = "# " + e.desc + "\n"
   565  	}
   566  	return msg + e.text
   567  }
   568  
   569  func (e *cmdError) ImportPath() string {
   570  	return e.importPath
   571  }
   572  
   573  // run runs the command given by cmdline in the directory dir.
   574  // If the command fails, run prints information about the failure
   575  // and returns a non-nil error.
   576  func (sh *Shell) run(dir string, desc string, env []string, cmdargs ...any) error {
   577  	out, err := sh.runOut(dir, env, cmdargs...)
   578  	if desc == "" {
   579  		desc = sh.fmtCmd(dir, "%s", strings.Join(str.StringList(cmdargs...), " "))
   580  	}
   581  	return sh.reportCmd(desc, dir, out, err)
   582  }
   583  
   584  // runOut runs the command given by cmdline in the directory dir.
   585  // It returns the command output and any errors that occurred.
   586  // It accumulates execution time in a.
   587  func (sh *Shell) runOut(dir string, env []string, cmdargs ...any) ([]byte, error) {
   588  	a := sh.action
   589  
   590  	cmdline := str.StringList(cmdargs...)
   591  
   592  	for _, arg := range cmdline {
   593  		// GNU binutils commands, including gcc and gccgo, interpret an argument
   594  		// @foo anywhere in the command line (even following --) as meaning
   595  		// "read and insert arguments from the file named foo."
   596  		// Don't say anything that might be misinterpreted that way.
   597  		if strings.HasPrefix(arg, "@") {
   598  			return nil, fmt.Errorf("invalid command-line argument %s in command: %s", arg, joinUnambiguously(cmdline))
   599  		}
   600  	}
   601  
   602  	if cfg.BuildN || cfg.BuildX {
   603  		var envcmdline string
   604  		for _, e := range env {
   605  			if j := strings.IndexByte(e, '='); j != -1 {
   606  				if strings.ContainsRune(e[j+1:], '\'') {
   607  					envcmdline += fmt.Sprintf("%s=%q", e[:j], e[j+1:])
   608  				} else {
   609  					envcmdline += fmt.Sprintf("%s='%s'", e[:j], e[j+1:])
   610  				}
   611  				envcmdline += " "
   612  			}
   613  		}
   614  		envcmdline += joinUnambiguously(cmdline)
   615  		sh.ShowCmd(dir, "%s", envcmdline)
   616  		if cfg.BuildN {
   617  			return nil, nil
   618  		}
   619  	}
   620  
   621  	var buf bytes.Buffer
   622  	path, err := pathcache.LookPath(cmdline[0])
   623  	if err != nil {
   624  		return nil, err
   625  	}
   626  	cmd := exec.Command(path, cmdline[1:]...)
   627  	if cmd.Path != "" {
   628  		cmd.Args[0] = cmd.Path
   629  	}
   630  	cmd.Stdout = &buf
   631  	cmd.Stderr = &buf
   632  	cleanup := passLongArgsInResponseFiles(cmd)
   633  	defer cleanup()
   634  	if dir != "." {
   635  		cmd.Dir = dir
   636  	}
   637  	cmd.Env = cmd.Environ() // Pre-allocate with correct PWD.
   638  
   639  	// Add the TOOLEXEC_IMPORTPATH environment variable for -toolexec tools.
   640  	// It doesn't really matter if -toolexec isn't being used.
   641  	// Note that a.Package.Desc is not really an import path,
   642  	// but this is consistent with 'go list -f {{.ImportPath}}'.
   643  	// Plus, it is useful to uniquely identify packages in 'go list -json'.
   644  	if a != nil && a.Package != nil {
   645  		cmd.Env = append(cmd.Env, "TOOLEXEC_IMPORTPATH="+a.Package.Desc())
   646  	}
   647  
   648  	cmd.Env = append(cmd.Env, env...)
   649  	start := time.Now()
   650  	err = cmd.Run()
   651  	if a != nil && a.json != nil {
   652  		aj := a.json
   653  		aj.Cmd = append(aj.Cmd, joinUnambiguously(cmdline))
   654  		aj.CmdReal += time.Since(start)
   655  		if ps := cmd.ProcessState; ps != nil {
   656  			aj.CmdUser += ps.UserTime()
   657  			aj.CmdSys += ps.SystemTime()
   658  		}
   659  	}
   660  
   661  	// err can be something like 'exit status 1'.
   662  	// Add information about what program was running.
   663  	// Note that if buf.Bytes() is non-empty, the caller usually
   664  	// shows buf.Bytes() and does not print err at all, so the
   665  	// prefix here does not make most output any more verbose.
   666  	if err != nil {
   667  		err = errors.New(cmdline[0] + ": " + err.Error())
   668  	}
   669  	return buf.Bytes(), err
   670  }
   671  
   672  // joinUnambiguously prints the slice, quoting where necessary to make the
   673  // output unambiguous.
   674  // TODO: See issue 5279. The printing of commands needs a complete redo.
   675  func joinUnambiguously(a []string) string {
   676  	var buf strings.Builder
   677  	for i, s := range a {
   678  		if i > 0 {
   679  			buf.WriteByte(' ')
   680  		}
   681  		q := strconv.Quote(s)
   682  		// A gccgo command line can contain -( and -).
   683  		// Make sure we quote them since they are special to the shell.
   684  		// The trimpath argument can also contain > (part of =>) and ;. Quote those too.
   685  		if s == "" || strings.ContainsAny(s, " ()>;") || len(q) > len(s)+2 {
   686  			buf.WriteString(q)
   687  		} else {
   688  			buf.WriteString(s)
   689  		}
   690  	}
   691  	return buf.String()
   692  }
   693
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