Source file src/os/exec/exec.go

Documentation: os/exec

     1  // Copyright 2009 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 exec runs external commands. It wraps os.StartProcess to make it
     6  // easier to remap stdin and stdout, connect I/O with pipes, and do other
     7  // adjustments.
     8  //
     9  // Unlike the "system" library call from C and other languages, the
    10  // os/exec package intentionally does not invoke the system shell and
    11  // does not expand any glob patterns or handle other expansions,
    12  // pipelines, or redirections typically done by shells. The package
    13  // behaves more like C's "exec" family of functions. To expand glob
    14  // patterns, either call the shell directly, taking care to escape any
    15  // dangerous input, or use the path/filepath package's Glob function.
    16  // To expand environment variables, use package os's ExpandEnv.
    17  //
    18  // Note that the examples in this package assume a Unix system.
    19  // They may not run on Windows, and they do not run in the Go Playground
    20  // used by golang.org and godoc.org.
    21  package exec
    22  
    23  import (
    24  	"bytes"
    25  	"context"
    26  	"errors"
    27  	"io"
    28  	"os"
    29  	"path/filepath"
    30  	"runtime"
    31  	"strconv"
    32  	"strings"
    33  	"sync"
    34  	"syscall"
    35  )
    36  
    37  // Error is returned by LookPath when it fails to classify a file as an
    38  // executable.
    39  type Error struct {
    40  	// Name is the file name for which the error occurred.
    41  	Name string
    42  	// Err is the underlying error.
    43  	Err error
    44  }
    45  
    46  func (e *Error) Error() string {
    47  	return "exec: " + strconv.Quote(e.Name) + ": " + e.Err.Error()
    48  }
    49  
    50  func (e *Error) Unwrap() error { return e.Err }
    51  
    52  // Cmd represents an external command being prepared or run.
    53  //
    54  // A Cmd cannot be reused after calling its Run, Output or CombinedOutput
    55  // methods.
    56  type Cmd struct {
    57  	// Path is the path of the command to run.
    58  	//
    59  	// This is the only field that must be set to a non-zero
    60  	// value. If Path is relative, it is evaluated relative
    61  	// to Dir.
    62  	Path string
    63  
    64  	// Args holds command line arguments, including the command as Args[0].
    65  	// If the Args field is empty or nil, Run uses {Path}.
    66  	//
    67  	// In typical use, both Path and Args are set by calling Command.
    68  	Args []string
    69  
    70  	// Env specifies the environment of the process.
    71  	// Each entry is of the form "key=value".
    72  	// If Env is nil, the new process uses the current process's
    73  	// environment.
    74  	// If Env contains duplicate environment keys, only the last
    75  	// value in the slice for each duplicate key is used.
    76  	// As a special case on Windows, SYSTEMROOT is always added if
    77  	// missing and not explicitly set to the empty string.
    78  	Env []string
    79  
    80  	// Dir specifies the working directory of the command.
    81  	// If Dir is the empty string, Run runs the command in the
    82  	// calling process's current directory.
    83  	Dir string
    84  
    85  	// Stdin specifies the process's standard input.
    86  	//
    87  	// If Stdin is nil, the process reads from the null device (os.DevNull).
    88  	//
    89  	// If Stdin is an *os.File, the process's standard input is connected
    90  	// directly to that file.
    91  	//
    92  	// Otherwise, during the execution of the command a separate
    93  	// goroutine reads from Stdin and delivers that data to the command
    94  	// over a pipe. In this case, Wait does not complete until the goroutine
    95  	// stops copying, either because it has reached the end of Stdin
    96  	// (EOF or a read error) or because writing to the pipe returned an error.
    97  	Stdin io.Reader
    98  
    99  	// Stdout and Stderr specify the process's standard output and error.
   100  	//
   101  	// If either is nil, Run connects the corresponding file descriptor
   102  	// to the null device (os.DevNull).
   103  	//
   104  	// If either is an *os.File, the corresponding output from the process
   105  	// is connected directly to that file.
   106  	//
   107  	// Otherwise, during the execution of the command a separate goroutine
   108  	// reads from the process over a pipe and delivers that data to the
   109  	// corresponding Writer. In this case, Wait does not complete until the
   110  	// goroutine reaches EOF or encounters an error.
   111  	//
   112  	// If Stdout and Stderr are the same writer, and have a type that can
   113  	// be compared with ==, at most one goroutine at a time will call Write.
   114  	Stdout io.Writer
   115  	Stderr io.Writer
   116  
   117  	// ExtraFiles specifies additional open files to be inherited by the
   118  	// new process. It does not include standard input, standard output, or
   119  	// standard error. If non-nil, entry i becomes file descriptor 3+i.
   120  	//
   121  	// ExtraFiles is not supported on Windows.
   122  	ExtraFiles []*os.File
   123  
   124  	// SysProcAttr holds optional, operating system-specific attributes.
   125  	// Run passes it to os.StartProcess as the os.ProcAttr's Sys field.
   126  	SysProcAttr *syscall.SysProcAttr
   127  
   128  	// Process is the underlying process, once started.
   129  	Process *os.Process
   130  
   131  	// ProcessState contains information about an exited process,
   132  	// available after a call to Wait or Run.
   133  	ProcessState *os.ProcessState
   134  
   135  	ctx             context.Context // nil means none
   136  	lookPathErr     error           // LookPath error, if any.
   137  	finished        bool            // when Wait was called
   138  	childFiles      []*os.File
   139  	closeAfterStart []io.Closer
   140  	closeAfterWait  []io.Closer
   141  	goroutine       []func() error
   142  	errch           chan error // one send per goroutine
   143  	waitDone        chan struct{}
   144  }
   145  
   146  // Command returns the Cmd struct to execute the named program with
   147  // the given arguments.
   148  //
   149  // It sets only the Path and Args in the returned structure.
   150  //
   151  // If name contains no path separators, Command uses LookPath to
   152  // resolve name to a complete path if possible. Otherwise it uses name
   153  // directly as Path.
   154  //
   155  // The returned Cmd's Args field is constructed from the command name
   156  // followed by the elements of arg, so arg should not include the
   157  // command name itself. For example, Command("echo", "hello").
   158  // Args[0] is always name, not the possibly resolved Path.
   159  //
   160  // On Windows, processes receive the whole command line as a single string
   161  // and do their own parsing. Command combines and quotes Args into a command
   162  // line string with an algorithm compatible with applications using
   163  // CommandLineToArgvW (which is the most common way). Notable exceptions are
   164  // msiexec.exe and cmd.exe (and thus, all batch files), which have a different
   165  // unquoting algorithm. In these or other similar cases, you can do the
   166  // quoting yourself and provide the full command line in SysProcAttr.CmdLine,
   167  // leaving Args empty.
   168  func Command(name string, arg ...string) *Cmd {
   169  	cmd := &Cmd{
   170  		Path: name,
   171  		Args: append([]string{name}, arg...),
   172  	}
   173  	if filepath.Base(name) == name {
   174  		if lp, err := LookPath(name); err != nil {
   175  			cmd.lookPathErr = err
   176  		} else {
   177  			cmd.Path = lp
   178  		}
   179  	}
   180  	return cmd
   181  }
   182  
   183  // CommandContext is like Command but includes a context.
   184  //
   185  // The provided context is used to kill the process (by calling
   186  // os.Process.Kill) if the context becomes done before the command
   187  // completes on its own.
   188  func CommandContext(ctx context.Context, name string, arg ...string) *Cmd {
   189  	if ctx == nil {
   190  		panic("nil Context")
   191  	}
   192  	cmd := Command(name, arg...)
   193  	cmd.ctx = ctx
   194  	return cmd
   195  }
   196  
   197  // String returns a human-readable description of c.
   198  // It is intended only for debugging.
   199  // In particular, it is not suitable for use as input to a shell.
   200  // The output of String may vary across Go releases.
   201  func (c *Cmd) String() string {
   202  	if c.lookPathErr != nil {
   203  		// failed to resolve path; report the original requested path (plus args)
   204  		return strings.Join(c.Args, " ")
   205  	}
   206  	// report the exact executable path (plus args)
   207  	b := new(strings.Builder)
   208  	b.WriteString(c.Path)
   209  	for _, a := range c.Args[1:] {
   210  		b.WriteByte(' ')
   211  		b.WriteString(a)
   212  	}
   213  	return b.String()
   214  }
   215  
   216  // interfaceEqual protects against panics from doing equality tests on
   217  // two interfaces with non-comparable underlying types.
   218  func interfaceEqual(a, b interface{}) bool {
   219  	defer func() {
   220  		recover()
   221  	}()
   222  	return a == b
   223  }
   224  
   225  func (c *Cmd) envv() []string {
   226  	if c.Env != nil {
   227  		return c.Env
   228  	}
   229  	return os.Environ()
   230  }
   231  
   232  func (c *Cmd) argv() []string {
   233  	if len(c.Args) > 0 {
   234  		return c.Args
   235  	}
   236  	return []string{c.Path}
   237  }
   238  
   239  // skipStdinCopyError optionally specifies a function which reports
   240  // whether the provided stdin copy error should be ignored.
   241  var skipStdinCopyError func(error) bool
   242  
   243  func (c *Cmd) stdin() (f *os.File, err error) {
   244  	if c.Stdin == nil {
   245  		f, err = os.Open(os.DevNull)
   246  		if err != nil {
   247  			return
   248  		}
   249  		c.closeAfterStart = append(c.closeAfterStart, f)
   250  		return
   251  	}
   252  
   253  	if f, ok := c.Stdin.(*os.File); ok {
   254  		return f, nil
   255  	}
   256  
   257  	pr, pw, err := os.Pipe()
   258  	if err != nil {
   259  		return
   260  	}
   261  
   262  	c.closeAfterStart = append(c.closeAfterStart, pr)
   263  	c.closeAfterWait = append(c.closeAfterWait, pw)
   264  	c.goroutine = append(c.goroutine, func() error {
   265  		_, err := io.Copy(pw, c.Stdin)
   266  		if skip := skipStdinCopyError; skip != nil && skip(err) {
   267  			err = nil
   268  		}
   269  		if err1 := pw.Close(); err == nil {
   270  			err = err1
   271  		}
   272  		return err
   273  	})
   274  	return pr, nil
   275  }
   276  
   277  func (c *Cmd) stdout() (f *os.File, err error) {
   278  	return c.writerDescriptor(c.Stdout)
   279  }
   280  
   281  func (c *Cmd) stderr() (f *os.File, err error) {
   282  	if c.Stderr != nil && interfaceEqual(c.Stderr, c.Stdout) {
   283  		return c.childFiles[1], nil
   284  	}
   285  	return c.writerDescriptor(c.Stderr)
   286  }
   287  
   288  func (c *Cmd) writerDescriptor(w io.Writer) (f *os.File, err error) {
   289  	if w == nil {
   290  		f, err = os.OpenFile(os.DevNull, os.O_WRONLY, 0)
   291  		if err != nil {
   292  			return
   293  		}
   294  		c.closeAfterStart = append(c.closeAfterStart, f)
   295  		return
   296  	}
   297  
   298  	if f, ok := w.(*os.File); ok {
   299  		return f, nil
   300  	}
   301  
   302  	pr, pw, err := os.Pipe()
   303  	if err != nil {
   304  		return
   305  	}
   306  
   307  	c.closeAfterStart = append(c.closeAfterStart, pw)
   308  	c.closeAfterWait = append(c.closeAfterWait, pr)
   309  	c.goroutine = append(c.goroutine, func() error {
   310  		_, err := io.Copy(w, pr)
   311  		pr.Close() // in case io.Copy stopped due to write error
   312  		return err
   313  	})
   314  	return pw, nil
   315  }
   316  
   317  func (c *Cmd) closeDescriptors(closers []io.Closer) {
   318  	for _, fd := range closers {
   319  		fd.Close()
   320  	}
   321  }
   322  
   323  // Run starts the specified command and waits for it to complete.
   324  //
   325  // The returned error is nil if the command runs, has no problems
   326  // copying stdin, stdout, and stderr, and exits with a zero exit
   327  // status.
   328  //
   329  // If the command starts but does not complete successfully, the error is of
   330  // type *ExitError. Other error types may be returned for other situations.
   331  //
   332  // If the calling goroutine has locked the operating system thread
   333  // with runtime.LockOSThread and modified any inheritable OS-level
   334  // thread state (for example, Linux or Plan 9 name spaces), the new
   335  // process will inherit the caller's thread state.
   336  func (c *Cmd) Run() error {
   337  	if err := c.Start(); err != nil {
   338  		return err
   339  	}
   340  	return c.Wait()
   341  }
   342  
   343  // lookExtensions finds windows executable by its dir and path.
   344  // It uses LookPath to try appropriate extensions.
   345  // lookExtensions does not search PATH, instead it converts `prog` into `.\prog`.
   346  func lookExtensions(path, dir string) (string, error) {
   347  	if filepath.Base(path) == path {
   348  		path = filepath.Join(".", path)
   349  	}
   350  	if dir == "" {
   351  		return LookPath(path)
   352  	}
   353  	if filepath.VolumeName(path) != "" {
   354  		return LookPath(path)
   355  	}
   356  	if len(path) > 1 && os.IsPathSeparator(path[0]) {
   357  		return LookPath(path)
   358  	}
   359  	dirandpath := filepath.Join(dir, path)
   360  	// We assume that LookPath will only add file extension.
   361  	lp, err := LookPath(dirandpath)
   362  	if err != nil {
   363  		return "", err
   364  	}
   365  	ext := strings.TrimPrefix(lp, dirandpath)
   366  	return path + ext, nil
   367  }
   368  
   369  // Start starts the specified command but does not wait for it to complete.
   370  //
   371  // If Start returns successfully, the c.Process field will be set.
   372  //
   373  // The Wait method will return the exit code and release associated resources
   374  // once the command exits.
   375  func (c *Cmd) Start() error {
   376  	if c.lookPathErr != nil {
   377  		c.closeDescriptors(c.closeAfterStart)
   378  		c.closeDescriptors(c.closeAfterWait)
   379  		return c.lookPathErr
   380  	}
   381  	if runtime.GOOS == "windows" {
   382  		lp, err := lookExtensions(c.Path, c.Dir)
   383  		if err != nil {
   384  			c.closeDescriptors(c.closeAfterStart)
   385  			c.closeDescriptors(c.closeAfterWait)
   386  			return err
   387  		}
   388  		c.Path = lp
   389  	}
   390  	if c.Process != nil {
   391  		return errors.New("exec: already started")
   392  	}
   393  	if c.ctx != nil {
   394  		select {
   395  		case <-c.ctx.Done():
   396  			c.closeDescriptors(c.closeAfterStart)
   397  			c.closeDescriptors(c.closeAfterWait)
   398  			return c.ctx.Err()
   399  		default:
   400  		}
   401  	}
   402  
   403  	c.childFiles = make([]*os.File, 0, 3+len(c.ExtraFiles))
   404  	type F func(*Cmd) (*os.File, error)
   405  	for _, setupFd := range []F{(*Cmd).stdin, (*Cmd).stdout, (*Cmd).stderr} {
   406  		fd, err := setupFd(c)
   407  		if err != nil {
   408  			c.closeDescriptors(c.closeAfterStart)
   409  			c.closeDescriptors(c.closeAfterWait)
   410  			return err
   411  		}
   412  		c.childFiles = append(c.childFiles, fd)
   413  	}
   414  	c.childFiles = append(c.childFiles, c.ExtraFiles...)
   415  
   416  	var err error
   417  	c.Process, err = os.StartProcess(c.Path, c.argv(), &os.ProcAttr{
   418  		Dir:   c.Dir,
   419  		Files: c.childFiles,
   420  		Env:   addCriticalEnv(dedupEnv(c.envv())),
   421  		Sys:   c.SysProcAttr,
   422  	})
   423  	if err != nil {
   424  		c.closeDescriptors(c.closeAfterStart)
   425  		c.closeDescriptors(c.closeAfterWait)
   426  		return err
   427  	}
   428  
   429  	c.closeDescriptors(c.closeAfterStart)
   430  
   431  	// Don't allocate the channel unless there are goroutines to fire.
   432  	if len(c.goroutine) > 0 {
   433  		c.errch = make(chan error, len(c.goroutine))
   434  		for _, fn := range c.goroutine {
   435  			go func(fn func() error) {
   436  				c.errch <- fn()
   437  			}(fn)
   438  		}
   439  	}
   440  
   441  	if c.ctx != nil {
   442  		c.waitDone = make(chan struct{})
   443  		go func() {
   444  			select {
   445  			case <-c.ctx.Done():
   446  				c.Process.Kill()
   447  			case <-c.waitDone:
   448  			}
   449  		}()
   450  	}
   451  
   452  	return nil
   453  }
   454  
   455  // An ExitError reports an unsuccessful exit by a command.
   456  type ExitError struct {
   457  	*os.ProcessState
   458  
   459  	// Stderr holds a subset of the standard error output from the
   460  	// Cmd.Output method if standard error was not otherwise being
   461  	// collected.
   462  	//
   463  	// If the error output is long, Stderr may contain only a prefix
   464  	// and suffix of the output, with the middle replaced with
   465  	// text about the number of omitted bytes.
   466  	//
   467  	// Stderr is provided for debugging, for inclusion in error messages.
   468  	// Users with other needs should redirect Cmd.Stderr as needed.
   469  	Stderr []byte
   470  }
   471  
   472  func (e *ExitError) Error() string {
   473  	return e.ProcessState.String()
   474  }
   475  
   476  // Wait waits for the command to exit and waits for any copying to
   477  // stdin or copying from stdout or stderr to complete.
   478  //
   479  // The command must have been started by Start.
   480  //
   481  // The returned error is nil if the command runs, has no problems
   482  // copying stdin, stdout, and stderr, and exits with a zero exit
   483  // status.
   484  //
   485  // If the command fails to run or doesn't complete successfully, the
   486  // error is of type *ExitError. Other error types may be
   487  // returned for I/O problems.
   488  //
   489  // If any of c.Stdin, c.Stdout or c.Stderr are not an *os.File, Wait also waits
   490  // for the respective I/O loop copying to or from the process to complete.
   491  //
   492  // Wait releases any resources associated with the Cmd.
   493  func (c *Cmd) Wait() error {
   494  	if c.Process == nil {
   495  		return errors.New("exec: not started")
   496  	}
   497  	if c.finished {
   498  		return errors.New("exec: Wait was already called")
   499  	}
   500  	c.finished = true
   501  
   502  	state, err := c.Process.Wait()
   503  	if c.waitDone != nil {
   504  		close(c.waitDone)
   505  	}
   506  	c.ProcessState = state
   507  
   508  	var copyError error
   509  	for range c.goroutine {
   510  		if err := <-c.errch; err != nil && copyError == nil {
   511  			copyError = err
   512  		}
   513  	}
   514  
   515  	c.closeDescriptors(c.closeAfterWait)
   516  
   517  	if err != nil {
   518  		return err
   519  	} else if !state.Success() {
   520  		return &ExitError{ProcessState: state}
   521  	}
   522  
   523  	return copyError
   524  }
   525  
   526  // Output runs the command and returns its standard output.
   527  // Any returned error will usually be of type *ExitError.
   528  // If c.Stderr was nil, Output populates ExitError.Stderr.
   529  func (c *Cmd) Output() ([]byte, error) {
   530  	if c.Stdout != nil {
   531  		return nil, errors.New("exec: Stdout already set")
   532  	}
   533  	var stdout bytes.Buffer
   534  	c.Stdout = &stdout
   535  
   536  	captureErr := c.Stderr == nil
   537  	if captureErr {
   538  		c.Stderr = &prefixSuffixSaver{N: 32 << 10}
   539  	}
   540  
   541  	err := c.Run()
   542  	if err != nil && captureErr {
   543  		if ee, ok := err.(*ExitError); ok {
   544  			ee.Stderr = c.Stderr.(*prefixSuffixSaver).Bytes()
   545  		}
   546  	}
   547  	return stdout.Bytes(), err
   548  }
   549  
   550  // CombinedOutput runs the command and returns its combined standard
   551  // output and standard error.
   552  func (c *Cmd) CombinedOutput() ([]byte, error) {
   553  	if c.Stdout != nil {
   554  		return nil, errors.New("exec: Stdout already set")
   555  	}
   556  	if c.Stderr != nil {
   557  		return nil, errors.New("exec: Stderr already set")
   558  	}
   559  	var b bytes.Buffer
   560  	c.Stdout = &b
   561  	c.Stderr = &b
   562  	err := c.Run()
   563  	return b.Bytes(), err
   564  }
   565  
   566  // StdinPipe returns a pipe that will be connected to the command's
   567  // standard input when the command starts.
   568  // The pipe will be closed automatically after Wait sees the command exit.
   569  // A caller need only call Close to force the pipe to close sooner.
   570  // For example, if the command being run will not exit until standard input
   571  // is closed, the caller must close the pipe.
   572  func (c *Cmd) StdinPipe() (io.WriteCloser, error) {
   573  	if c.Stdin != nil {
   574  		return nil, errors.New("exec: Stdin already set")
   575  	}
   576  	if c.Process != nil {
   577  		return nil, errors.New("exec: StdinPipe after process started")
   578  	}
   579  	pr, pw, err := os.Pipe()
   580  	if err != nil {
   581  		return nil, err
   582  	}
   583  	c.Stdin = pr
   584  	c.closeAfterStart = append(c.closeAfterStart, pr)
   585  	wc := &closeOnce{File: pw}
   586  	c.closeAfterWait = append(c.closeAfterWait, wc)
   587  	return wc, nil
   588  }
   589  
   590  type closeOnce struct {
   591  	*os.File
   592  
   593  	once sync.Once
   594  	err  error
   595  }
   596  
   597  func (c *closeOnce) Close() error {
   598  	c.once.Do(c.close)
   599  	return c.err
   600  }
   601  
   602  func (c *closeOnce) close() {
   603  	c.err = c.File.Close()
   604  }
   605  
   606  // StdoutPipe returns a pipe that will be connected to the command's
   607  // standard output when the command starts.
   608  //
   609  // Wait will close the pipe after seeing the command exit, so most callers
   610  // need not close the pipe themselves. It is thus incorrect to call Wait
   611  // before all reads from the pipe have completed.
   612  // For the same reason, it is incorrect to call Run when using StdoutPipe.
   613  // See the example for idiomatic usage.
   614  func (c *Cmd) StdoutPipe() (io.ReadCloser, error) {
   615  	if c.Stdout != nil {
   616  		return nil, errors.New("exec: Stdout already set")
   617  	}
   618  	if c.Process != nil {
   619  		return nil, errors.New("exec: StdoutPipe after process started")
   620  	}
   621  	pr, pw, err := os.Pipe()
   622  	if err != nil {
   623  		return nil, err
   624  	}
   625  	c.Stdout = pw
   626  	c.closeAfterStart = append(c.closeAfterStart, pw)
   627  	c.closeAfterWait = append(c.closeAfterWait, pr)
   628  	return pr, nil
   629  }
   630  
   631  // StderrPipe returns a pipe that will be connected to the command's
   632  // standard error when the command starts.
   633  //
   634  // Wait will close the pipe after seeing the command exit, so most callers
   635  // need not close the pipe themselves. It is thus incorrect to call Wait
   636  // before all reads from the pipe have completed.
   637  // For the same reason, it is incorrect to use Run when using StderrPipe.
   638  // See the StdoutPipe example for idiomatic usage.
   639  func (c *Cmd) StderrPipe() (io.ReadCloser, error) {
   640  	if c.Stderr != nil {
   641  		return nil, errors.New("exec: Stderr already set")
   642  	}
   643  	if c.Process != nil {
   644  		return nil, errors.New("exec: StderrPipe after process started")
   645  	}
   646  	pr, pw, err := os.Pipe()
   647  	if err != nil {
   648  		return nil, err
   649  	}
   650  	c.Stderr = pw
   651  	c.closeAfterStart = append(c.closeAfterStart, pw)
   652  	c.closeAfterWait = append(c.closeAfterWait, pr)
   653  	return pr, nil
   654  }
   655  
   656  // prefixSuffixSaver is an io.Writer which retains the first N bytes
   657  // and the last N bytes written to it. The Bytes() methods reconstructs
   658  // it with a pretty error message.
   659  type prefixSuffixSaver struct {
   660  	N         int // max size of prefix or suffix
   661  	prefix    []byte
   662  	suffix    []byte // ring buffer once len(suffix) == N
   663  	suffixOff int    // offset to write into suffix
   664  	skipped   int64
   665  
   666  	// TODO(bradfitz): we could keep one large []byte and use part of it for
   667  	// the prefix, reserve space for the '... Omitting N bytes ...' message,
   668  	// then the ring buffer suffix, and just rearrange the ring buffer
   669  	// suffix when Bytes() is called, but it doesn't seem worth it for
   670  	// now just for error messages. It's only ~64KB anyway.
   671  }
   672  
   673  func (w *prefixSuffixSaver) Write(p []byte) (n int, err error) {
   674  	lenp := len(p)
   675  	p = w.fill(&w.prefix, p)
   676  
   677  	// Only keep the last w.N bytes of suffix data.
   678  	if overage := len(p) - w.N; overage > 0 {
   679  		p = p[overage:]
   680  		w.skipped += int64(overage)
   681  	}
   682  	p = w.fill(&w.suffix, p)
   683  
   684  	// w.suffix is full now if p is non-empty. Overwrite it in a circle.
   685  	for len(p) > 0 { // 0, 1, or 2 iterations.
   686  		n := copy(w.suffix[w.suffixOff:], p)
   687  		p = p[n:]
   688  		w.skipped += int64(n)
   689  		w.suffixOff += n
   690  		if w.suffixOff == w.N {
   691  			w.suffixOff = 0
   692  		}
   693  	}
   694  	return lenp, nil
   695  }
   696  
   697  // fill appends up to len(p) bytes of p to *dst, such that *dst does not
   698  // grow larger than w.N. It returns the un-appended suffix of p.
   699  func (w *prefixSuffixSaver) fill(dst *[]byte, p []byte) (pRemain []byte) {
   700  	if remain := w.N - len(*dst); remain > 0 {
   701  		add := minInt(len(p), remain)
   702  		*dst = append(*dst, p[:add]...)
   703  		p = p[add:]
   704  	}
   705  	return p
   706  }
   707  
   708  func (w *prefixSuffixSaver) Bytes() []byte {
   709  	if w.suffix == nil {
   710  		return w.prefix
   711  	}
   712  	if w.skipped == 0 {
   713  		return append(w.prefix, w.suffix...)
   714  	}
   715  	var buf bytes.Buffer
   716  	buf.Grow(len(w.prefix) + len(w.suffix) + 50)
   717  	buf.Write(w.prefix)
   718  	buf.WriteString("\n... omitting ")
   719  	buf.WriteString(strconv.FormatInt(w.skipped, 10))
   720  	buf.WriteString(" bytes ...\n")
   721  	buf.Write(w.suffix[w.suffixOff:])
   722  	buf.Write(w.suffix[:w.suffixOff])
   723  	return buf.Bytes()
   724  }
   725  
   726  func minInt(a, b int) int {
   727  	if a < b {
   728  		return a
   729  	}
   730  	return b
   731  }
   732  
   733  // dedupEnv returns a copy of env with any duplicates removed, in favor of
   734  // later values.
   735  // Items not of the normal environment "key=value" form are preserved unchanged.
   736  func dedupEnv(env []string) []string {
   737  	return dedupEnvCase(runtime.GOOS == "windows", env)
   738  }
   739  
   740  // dedupEnvCase is dedupEnv with a case option for testing.
   741  // If caseInsensitive is true, the case of keys is ignored.
   742  func dedupEnvCase(caseInsensitive bool, env []string) []string {
   743  	out := make([]string, 0, len(env))
   744  	saw := make(map[string]int, len(env)) // key => index into out
   745  	for _, kv := range env {
   746  		eq := strings.Index(kv, "=")
   747  		if eq < 0 {
   748  			out = append(out, kv)
   749  			continue
   750  		}
   751  		k := kv[:eq]
   752  		if caseInsensitive {
   753  			k = strings.ToLower(k)
   754  		}
   755  		if dupIdx, isDup := saw[k]; isDup {
   756  			out[dupIdx] = kv
   757  			continue
   758  		}
   759  		saw[k] = len(out)
   760  		out = append(out, kv)
   761  	}
   762  	return out
   763  }
   764  
   765  // addCriticalEnv adds any critical environment variables that are required
   766  // (or at least almost always required) on the operating system.
   767  // Currently this is only used for Windows.
   768  func addCriticalEnv(env []string) []string {
   769  	if runtime.GOOS != "windows" {
   770  		return env
   771  	}
   772  	for _, kv := range env {
   773  		eq := strings.Index(kv, "=")
   774  		if eq < 0 {
   775  			continue
   776  		}
   777  		k := kv[:eq]
   778  		if strings.EqualFold(k, "SYSTEMROOT") {
   779  			// We already have it.
   780  			return env
   781  		}
   782  	}
   783  	return append(env, "SYSTEMROOT="+os.Getenv("SYSTEMROOT"))
   784  }
   785  

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