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