exec.go

     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  	"internal/syscall/execenv"
    28  	"io"
    29  	"os"
    30  	"path/filepath"
    31  	"runtime"
    32  	"strconv"
    33  	"strings"
    34  	"sync"
    35  	"syscall"
    36  )
    37  
    38  // Error is returned by LookPath when it fails to classify a file as an
    39  // executable.
    40  type Error struct {
    41  	// Name is the file name for which the error occurred.
    42  	Name string
    43  	// Err is the underlying error.
    44  	Err error
    45  }
    46  
    47  func (e *Error) Error() string {
    48  	return "exec: " + strconv.Quote(e.Name) + ": " + e.Err.Error()
    49  }
    50  
    51  func (e *Error) Unwrap() error { return e.Err }
    52  
    53  // Cmd represents an external command being prepared or run.
    54  //
    55  // A Cmd cannot be reused after calling its Run, Output or CombinedOutput
    56  // methods.
    57  type Cmd struct {
    58  	// Path is the path of the command to run.
    59  	//
    60  	// This is the only field that must be set to a non-zero
    61  	// value. If Path is relative, it is evaluated relative
    62  	// to Dir.
    63  	Path string
    64  
    65  	// Args holds command line arguments, including the command as Args[0].
    66  	// If the Args field is empty or nil, Run uses {Path}.
    67  	//
    68  	// In typical use, both Path and Args are set by calling Command.
    69  	Args []string
    70  
    71  	// Env specifies the environment of the process.
    72  	// Each entry is of the form "key=value".
    73  	// If Env is nil, the new process uses the current process's
    74  	// environment.
    75  	// If Env contains duplicate environment keys, only the last
    76  	// value in the slice for each duplicate key is used.
    77  	// As a special case on Windows, SYSTEMROOT is always added if
    78  	// missing and not explicitly set to the empty string.
    79  	Env []string
    80  
    81  	// Dir specifies the working directory of the command.
    82  	// If Dir is the empty string, Run runs the command in the
    83  	// calling process's current directory.
    84  	Dir string
    85  
    86  	// Stdin specifies the process's standard input.
    87  	//
    88  	// If Stdin is nil, the process reads from the null device (os.DevNull).
    89  	//
    90  	// If Stdin is an *os.File, the process's standard input is connected
    91  	// directly to that file.
    92  	//
    93  	// Otherwise, during the execution of the command a separate
    94  	// goroutine reads from Stdin and delivers that data to the command
    95  	// over a pipe. In this case, Wait does not complete until the goroutine
    96  	// stops copying, either because it has reached the end of Stdin
    97  	// (EOF or a read error) or because writing to the pipe returned an error.
    98  	Stdin io.Reader
    99  
   100  	// Stdout and Stderr specify the process's standard output and error.
   101  	//
   102  	// If either is nil, Run connects the corresponding file descriptor
   103  	// to the null device (os.DevNull).
   104  	//
   105  	// If either is an *os.File, the corresponding output from the process
   106  	// is connected directly to that file.
   107  	//
   108  	// Otherwise, during the execution of the command a separate goroutine
   109  	// reads from the process over a pipe and delivers that data to the
   110  	// corresponding Writer. In this case, Wait does not complete until the
   111  	// goroutine reaches EOF or encounters an error.
   112  	//
   113  	// If Stdout and Stderr are the same writer, and have a type that can
   114  	// be compared with ==, at most one goroutine at a time will call Write.
   115  	Stdout io.Writer
   116  	Stderr io.Writer
   117  
   118  	// ExtraFiles specifies additional open files to be inherited by the
   119  	// new process. It does not include standard input, standard output, or
   120  	// standard error. If non-nil, entry i becomes file descriptor 3+i.
   121  	//
   122  	// ExtraFiles is not supported on Windows.
   123  	ExtraFiles []*os.File
   124  
   125  	// SysProcAttr holds optional, operating system-specific attributes.
   126  	// Run passes it to os.StartProcess as the os.ProcAttr's Sys field.
   127  	SysProcAttr *syscall.SysProcAttr
   128  
   129  	// Process is the underlying process, once started.
   130  	Process *os.Process
   131  
   132  	// ProcessState contains information about an exited process,
   133  	// available after a call to Wait or Run.
   134  	ProcessState *os.ProcessState
   135  
   136  	ctx             context.Context // nil means none
   137  	lookPathErr     error           // LookPath error, if any.
   138  	finished        bool            // when Wait was called
   139  	childFiles      []*os.File
   140  	closeAfterStart []io.Closer
   141  	closeAfterWait  []io.Closer
   142  	goroutine       []func() error
   143  	errch           chan error // one send per goroutine
   144  	waitDone        chan struct{}
   145  }
   146  
   147  // Command returns the Cmd struct to execute the named program with
   148  // the given arguments.
   149  //
   150  // It sets only the Path and Args in the returned structure.
   151  //
   152  // If name contains no path separators, Command uses LookPath to
   153  // resolve name to a complete path if possible. Otherwise it uses name
   154  // directly as Path.
   155  //
   156  // The returned Cmd's Args field is constructed from the command name
   157  // followed by the elements of arg, so arg should not include the
   158  // command name itself. For example, Command("echo", "hello").
   159  // Args[0] is always name, not the possibly resolved Path.
   160  //
   161  // On Windows, processes receive the whole command line as a single string
   162  // and do their own parsing. Command combines and quotes Args into a command
   163  // line string with an algorithm compatible with applications using
   164  // CommandLineToArgvW (which is the most common way). Notable exceptions are
   165  // msiexec.exe and cmd.exe (and thus, all batch files), which have a different
   166  // unquoting algorithm. In these or other similar cases, you can do the
   167  // quoting yourself and provide the full command line in SysProcAttr.CmdLine,
   168  // leaving Args empty.
   169  func Command(name string, arg ...string) *Cmd {
   170  	cmd := &Cmd{
   171  		Path: name,
   172  		Args: append([]string{name}, arg...),
   173  	}
   174  	if filepath.Base(name) == name {
   175  		if lp, err := LookPath(name); err != nil {
   176  			cmd.lookPathErr = err
   177  		} else {
   178  			cmd.Path = lp
   179  		}
   180  	}
   181  	return cmd
   182  }
   183  
   184  // CommandContext is like Command but includes a context.
   185  //
   186  // The provided context is used to kill the process (by calling
   187  // os.Process.Kill) if the context becomes done before the command
   188  // completes on its own.
   189  func CommandContext(ctx context.Context, name string, arg ...string) *Cmd {
   190  	if ctx == nil {
   191  		panic("nil Context")
   192  	}
   193  	cmd := Command(name, arg...)
   194  	cmd.ctx = ctx
   195  	return cmd
   196  }
   197  
   198  // String returns a human-readable description of c.
   199  // It is intended only for debugging.
   200  // In particular, it is not suitable for use as input to a shell.
   201  // The output of String may vary across Go releases.
   202  func (c *Cmd) String() string {
   203  	if c.lookPathErr != nil {
   204  		// failed to resolve path; report the original requested path (plus args)
   205  		return strings.Join(c.Args, " ")
   206  	}
   207  	// report the exact executable path (plus args)
   208  	b := new(strings.Builder)
   209  	b.WriteString(c.Path)
   210  	for _, a := range c.Args[1:] {
   211  		b.WriteByte(' ')
   212  		b.WriteString(a)
   213  	}
   214  	return b.String()
   215  }
   216  
   217  // interfaceEqual protects against panics from doing equality tests on
   218  // two interfaces with non-comparable underlying types.
   219  func interfaceEqual(a, b any) bool {
   220  	defer func() {
   221  		recover()
   222  	}()
   223  	return a == b
   224  }
   225  
   226  func (c *Cmd) envv() ([]string, error) {
   227  	if c.Env != nil {
   228  		return c.Env, nil
   229  	}
   230  	return execenv.Default(c.SysProcAttr)
   231  }
   232  
   233  func (c *Cmd) argv() []string {
   234  	if len(c.Args) > 0 {
   235  		return c.Args
   236  	}
   237  	return []string{c.Path}
   238  }
   239  
   240  // skipStdinCopyError optionally specifies a function which reports
   241  // whether the provided stdin copy error should be ignored.
   242  var skipStdinCopyError func(error) bool
   243  
   244  func (c *Cmd) stdin() (f *os.File, err error) {
   245  	if c.Stdin == nil {
   246  		f, err = os.Open(os.DevNull)
   247  		if err != nil {
   248  			return
   249  		}
   250  		c.closeAfterStart = append(c.closeAfterStart, f)
   251  		return
   252  	}
   253  
   254  	if f, ok := c.Stdin.(*os.File); ok {
   255  		return f, nil
   256  	}
   257  
   258  	pr, pw, err := os.Pipe()
   259  	if err != nil {
   260  		return
   261  	}
   262  
   263  	c.closeAfterStart = append(c.closeAfterStart, pr)
   264  	c.closeAfterWait = append(c.closeAfterWait, pw)
   265  	c.goroutine = append(c.goroutine, func() error {
   266  		_, err := io.Copy(pw, c.Stdin)
   267  		if skip := skipStdinCopyError; skip != nil && skip(err) {
   268  			err = nil
   269  		}
   270  		if err1 := pw.Close(); err == nil {
   271  			err = err1
   272  		}
   273  		return err
   274  	})
   275  	return pr, nil
   276  }
   277  
   278  func (c *Cmd) stdout() (f *os.File, err error) {
   279  	return c.writerDescriptor(c.Stdout)
   280  }
   281  
   282  func (c *Cmd) stderr() (f *os.File, err error) {
   283  	if c.Stderr != nil && interfaceEqual(c.Stderr, c.Stdout) {
   284  		return c.childFiles[1], nil
   285  	}
   286  	return c.writerDescriptor(c.Stderr)
   287  }
   288  
   289  func (c *Cmd) writerDescriptor(w io.Writer) (f *os.File, err error) {
   290  	if w == nil {
   291  		f, err = os.OpenFile(os.DevNull, os.O_WRONLY, 0)
   292  		if err != nil {
   293  			return
   294  		}
   295  		c.closeAfterStart = append(c.closeAfterStart, f)
   296  		return
   297  	}
   298  
   299  	if f, ok := w.(*os.File); ok {
   300  		return f, nil
   301  	}
   302  
   303  	pr, pw, err := os.Pipe()
   304  	if err != nil {
   305  		return
   306  	}
   307  
   308  	c.closeAfterStart = append(c.closeAfterStart, pw)
   309  	c.closeAfterWait = append(c.closeAfterWait, pr)
   310  	c.goroutine = append(c.goroutine, func() error {
   311  		_, err := io.Copy(w, pr)
   312  		pr.Close() // in case io.Copy stopped due to write error
   313  		return err
   314  	})
   315  	return pw, nil
   316  }
   317  
   318  func (c *Cmd) closeDescriptors(closers []io.Closer) {
   319  	for _, fd := range closers {
   320  		fd.Close()
   321  	}
   322  }
   323  
   324  // Run starts the specified command and waits for it to complete.
   325  //
   326  // The returned error is nil if the command runs, has no problems
   327  // copying stdin, stdout, and stderr, and exits with a zero exit
   328  // status.
   329  //
   330  // If the command starts but does not complete successfully, the error is of
   331  // type *ExitError. Other error types may be returned for other situations.
   332  //
   333  // If the calling goroutine has locked the operating system thread
   334  // with runtime.LockOSThread and modified any inheritable OS-level
   335  // thread state (for example, Linux or Plan 9 name spaces), the new
   336  // process will inherit the caller's thread state.
   337  func (c *Cmd) Run() error {
   338  	if err := c.Start(); err != nil {
   339  		return err
   340  	}
   341  	return c.Wait()
   342  }
   343  
   344  // lookExtensions finds windows executable by its dir and path.
   345  // It uses LookPath to try appropriate extensions.
   346  // lookExtensions does not search PATH, instead it converts `prog` into `.\prog`.
   347  func lookExtensions(path, dir string) (string, error) {
   348  	if filepath.Base(path) == path {
   349  		path = filepath.Join(".", path)
   350  	}
   351  	if dir == "" {
   352  		return LookPath(path)
   353  	}
   354  	if filepath.VolumeName(path) != "" {
   355  		return LookPath(path)
   356  	}
   357  	if len(path) > 1 && os.IsPathSeparator(path[0]) {
   358  		return LookPath(path)
   359  	}
   360  	dirandpath := filepath.Join(dir, path)
   361  	// We assume that LookPath will only add file extension.
   362  	lp, err := LookPath(dirandpath)
   363  	if err != nil {
   364  		return "", err
   365  	}
   366  	ext := strings.TrimPrefix(lp, dirandpath)
   367  	return path + ext, nil
   368  }
   369  
   370  // Start starts the specified command but does not wait for it to complete.
   371  //
   372  // If Start returns successfully, the c.Process field will be set.
   373  //
   374  // The Wait method will return the exit code and release associated resources
   375  // once the command exits.
   376  func (c *Cmd) Start() error {
   377  	if c.Path == "" && c.lookPathErr == nil {
   378  		c.lookPathErr = errors.New("exec: no command")
   379  	}
   380  	if c.lookPathErr != nil {
   381  		c.closeDescriptors(c.closeAfterStart)
   382  		c.closeDescriptors(c.closeAfterWait)
   383  		return c.lookPathErr
   384  	}
   385  	if runtime.GOOS == "windows" {
   386  		lp, err := lookExtensions(c.Path, c.Dir)
   387  		if err != nil {
   388  			c.closeDescriptors(c.closeAfterStart)
   389  			c.closeDescriptors(c.closeAfterWait)
   390  			return err
   391  		}
   392  		c.Path = lp
   393  	}
   394  	if c.Process != nil {
   395  		return errors.New("exec: already started")
   396  	}
   397  	if c.ctx != nil {
   398  		select {
   399  		case <-c.ctx.Done():
   400  			c.closeDescriptors(c.closeAfterStart)
   401  			c.closeDescriptors(c.closeAfterWait)
   402  			return c.ctx.Err()
   403  		default:
   404  		}
   405  	}
   406  
   407  	c.childFiles = make([]*os.File, 0, 3+len(c.ExtraFiles))
   408  	type F func(*Cmd) (*os.File, error)
   409  	for _, setupFd := range []F{(*Cmd).stdin, (*Cmd).stdout, (*Cmd).stderr} {
   410  		fd, err := setupFd(c)
   411  		if err != nil {
   412  			c.closeDescriptors(c.closeAfterStart)
   413  			c.closeDescriptors(c.closeAfterWait)
   414  			return err
   415  		}
   416  		c.childFiles = append(c.childFiles, fd)
   417  	}
   418  	c.childFiles = append(c.childFiles, c.ExtraFiles...)
   419  
   420  	envv, err := c.envv()
   421  	if err != nil {
   422  		return err
   423  	}
   424  
   425  	c.Process, err = os.StartProcess(c.Path, c.argv(), &os.ProcAttr{
   426  		Dir:   c.Dir,
   427  		Files: c.childFiles,
   428  		Env:   addCriticalEnv(dedupEnv(envv)),
   429  		Sys:   c.SysProcAttr,
   430  	})
   431  	if err != nil {
   432  		c.closeDescriptors(c.closeAfterStart)
   433  		c.closeDescriptors(c.closeAfterWait)
   434  		return err
   435  	}
   436  
   437  	c.closeDescriptors(c.closeAfterStart)
   438  
   439  	// Don't allocate the channel unless there are goroutines to fire.
   440  	if len(c.goroutine) > 0 {
   441  		c.errch = make(chan error, len(c.goroutine))
   442  		for _, fn := range c.goroutine {
   443  			go func(fn func() error) {
   444  				c.errch <- fn()
   445  			}(fn)
   446  		}
   447  	}
   448  
   449  	if c.ctx != nil {
   450  		c.waitDone = make(chan struct{})
   451  		go func() {
   452  			select {
   453  			case <-c.ctx.Done():
   454  				c.Process.Kill()
   455  			case <-c.waitDone:
   456  			}
   457  		}()
   458  	}
   459  
   460  	return nil
   461  }
   462  
   463  // An ExitError reports an unsuccessful exit by a command.
   464  type ExitError struct {
   465  	*os.ProcessState
   466  
   467  	// Stderr holds a subset of the standard error output from the
   468  	// Cmd.Output method if standard error was not otherwise being
   469  	// collected.
   470  	//
   471  	// If the error output is long, Stderr may contain only a prefix
   472  	// and suffix of the output, with the middle replaced with
   473  	// text about the number of omitted bytes.
   474  	//
   475  	// Stderr is provided for debugging, for inclusion in error messages.
   476  	// Users with other needs should redirect Cmd.Stderr as needed.
   477  	Stderr []byte
   478  }
   479  
   480  func (e *ExitError) Error() string {
   481  	return e.ProcessState.String()
   482  }
   483  
   484  // Wait waits for the command to exit and waits for any copying to
   485  // stdin or copying from stdout or stderr to complete.
   486  //
   487  // The command must have been started by Start.
   488  //
   489  // The returned error is nil if the command runs, has no problems
   490  // copying stdin, stdout, and stderr, and exits with a zero exit
   491  // status.
   492  //
   493  // If the command fails to run or doesn't complete successfully, the
   494  // error is of type *ExitError. Other error types may be
   495  // returned for I/O problems.
   496  //
   497  // If any of c.Stdin, c.Stdout or c.Stderr are not an *os.File, Wait also waits
   498  // for the respective I/O loop copying to or from the process to complete.
   499  //
   500  // Wait releases any resources associated with the Cmd.
   501  func (c *Cmd) Wait() error {
   502  	if c.Process == nil {
   503  		return errors.New("exec: not started")
   504  	}
   505  	if c.finished {
   506  		return errors.New("exec: Wait was already called")
   507  	}
   508  	c.finished = true
   509  
   510  	state, err := c.Process.Wait()
   511  	if c.waitDone != nil {
   512  		close(c.waitDone)
   513  	}
   514  	c.ProcessState = state
   515  
   516  	var copyError error
   517  	for range c.goroutine {
   518  		if err := <-c.errch; err != nil && copyError == nil {
   519  			copyError = err
   520  		}
   521  	}
   522  
   523  	c.closeDescriptors(c.closeAfterWait)
   524  
   525  	if err != nil {
   526  		return err
   527  	} else if !state.Success() {
   528  		return &ExitError{ProcessState: state}
   529  	}
   530  
   531  	return copyError
   532  }
   533  
   534  // Output runs the command and returns its standard output.
   535  // Any returned error will usually be of type *ExitError.
   536  // If c.Stderr was nil, Output populates ExitError.Stderr.
   537  func (c *Cmd) Output() ([]byte, error) {
   538  	if c.Stdout != nil {
   539  		return nil, errors.New("exec: Stdout already set")
   540  	}
   541  	var stdout bytes.Buffer
   542  	c.Stdout = &stdout
   543  
   544  	captureErr := c.Stderr == nil
   545  	if captureErr {
   546  		c.Stderr = &prefixSuffixSaver{N: 32 << 10}
   547  	}
   548  
   549  	err := c.Run()
   550  	if err != nil && captureErr {
   551  		if ee, ok := err.(*ExitError); ok {
   552  			ee.Stderr = c.Stderr.(*prefixSuffixSaver).Bytes()
   553  		}
   554  	}
   555  	return stdout.Bytes(), err
   556  }
   557  
   558  // CombinedOutput runs the command and returns its combined standard
   559  // output and standard error.
   560  func (c *Cmd) CombinedOutput() ([]byte, error) {
   561  	if c.Stdout != nil {
   562  		return nil, errors.New("exec: Stdout already set")
   563  	}
   564  	if c.Stderr != nil {
   565  		return nil, errors.New("exec: Stderr already set")
   566  	}
   567  	var b bytes.Buffer
   568  	c.Stdout = &b
   569  	c.Stderr = &b
   570  	err := c.Run()
   571  	return b.Bytes(), err
   572  }
   573  
   574  // StdinPipe returns a pipe that will be connected to the command's
   575  // standard input when the command starts.
   576  // The pipe will be closed automatically after Wait sees the command exit.
   577  // A caller need only call Close to force the pipe to close sooner.
   578  // For example, if the command being run will not exit until standard input
   579  // is closed, the caller must close the pipe.
   580  func (c *Cmd) StdinPipe() (io.WriteCloser, error) {
   581  	if c.Stdin != nil {
   582  		return nil, errors.New("exec: Stdin already set")
   583  	}
   584  	if c.Process != nil {
   585  		return nil, errors.New("exec: StdinPipe after process started")
   586  	}
   587  	pr, pw, err := os.Pipe()
   588  	if err != nil {
   589  		return nil, err
   590  	}
   591  	c.Stdin = pr
   592  	c.closeAfterStart = append(c.closeAfterStart, pr)
   593  	wc := &closeOnce{File: pw}
   594  	c.closeAfterWait = append(c.closeAfterWait, wc)
   595  	return wc, nil
   596  }
   597  
   598  type closeOnce struct {
   599  	*os.File
   600  
   601  	once sync.Once
   602  	err  error
   603  }
   604  
   605  func (c *closeOnce) Close() error {
   606  	c.once.Do(c.close)
   607  	return c.err
   608  }
   609  
   610  func (c *closeOnce) close() {
   611  	c.err = c.File.Close()
   612  }
   613  
   614  // StdoutPipe returns a pipe that will be connected to the command's
   615  // standard output when the command starts.
   616  //
   617  // Wait will close the pipe after seeing the command exit, so most callers
   618  // need not close the pipe themselves. It is thus incorrect to call Wait
   619  // before all reads from the pipe have completed.
   620  // For the same reason, it is incorrect to call Run when using StdoutPipe.
   621  // See the example for idiomatic usage.
   622  func (c *Cmd) StdoutPipe() (io.ReadCloser, error) {
   623  	if c.Stdout != nil {
   624  		return nil, errors.New("exec: Stdout already set")
   625  	}
   626  	if c.Process != nil {
   627  		return nil, errors.New("exec: StdoutPipe after process started")
   628  	}
   629  	pr, pw, err := os.Pipe()
   630  	if err != nil {
   631  		return nil, err
   632  	}
   633  	c.Stdout = pw
   634  	c.closeAfterStart = append(c.closeAfterStart, pw)
   635  	c.closeAfterWait = append(c.closeAfterWait, pr)
   636  	return pr, nil
   637  }
   638  
   639  // StderrPipe returns a pipe that will be connected to the command's
   640  // standard error when the command starts.
   641  //
   642  // Wait will close the pipe after seeing the command exit, so most callers
   643  // need not close the pipe themselves. It is thus incorrect to call Wait
   644  // before all reads from the pipe have completed.
   645  // For the same reason, it is incorrect to use Run when using StderrPipe.
   646  // See the StdoutPipe example for idiomatic usage.
   647  func (c *Cmd) StderrPipe() (io.ReadCloser, error) {
   648  	if c.Stderr != nil {
   649  		return nil, errors.New("exec: Stderr already set")
   650  	}
   651  	if c.Process != nil {
   652  		return nil, errors.New("exec: StderrPipe after process started")
   653  	}
   654  	pr, pw, err := os.Pipe()
   655  	if err != nil {
   656  		return nil, err
   657  	}
   658  	c.Stderr = pw
   659  	c.closeAfterStart = append(c.closeAfterStart, pw)
   660  	c.closeAfterWait = append(c.closeAfterWait, pr)
   661  	return pr, nil
   662  }
   663  
   664  // prefixSuffixSaver is an io.Writer which retains the first N bytes
   665  // and the last N bytes written to it. The Bytes() methods reconstructs
   666  // it with a pretty error message.
   667  type prefixSuffixSaver struct {
   668  	N         int // max size of prefix or suffix
   669  	prefix    []byte
   670  	suffix    []byte // ring buffer once len(suffix) == N
   671  	suffixOff int    // offset to write into suffix
   672  	skipped   int64
   673  
   674  	// TODO(bradfitz): we could keep one large []byte and use part of it for
   675  	// the prefix, reserve space for the '... Omitting N bytes ...' message,
   676  	// then the ring buffer suffix, and just rearrange the ring buffer
   677  	// suffix when Bytes() is called, but it doesn't seem worth it for
   678  	// now just for error messages. It's only ~64KB anyway.
   679  }
   680  
   681  func (w *prefixSuffixSaver) Write(p []byte) (n int, err error) {
   682  	lenp := len(p)
   683  	p = w.fill(&w.prefix, p)
   684  
   685  	// Only keep the last w.N bytes of suffix data.
   686  	if overage := len(p) - w.N; overage > 0 {
   687  		p = p[overage:]
   688  		w.skipped += int64(overage)
   689  	}
   690  	p = w.fill(&w.suffix, p)
   691  
   692  	// w.suffix is full now if p is non-empty. Overwrite it in a circle.
   693  	for len(p) > 0 { // 0, 1, or 2 iterations.
   694  		n := copy(w.suffix[w.suffixOff:], p)
   695  		p = p[n:]
   696  		w.skipped += int64(n)
   697  		w.suffixOff += n
   698  		if w.suffixOff == w.N {
   699  			w.suffixOff = 0
   700  		}
   701  	}
   702  	return lenp, nil
   703  }
   704  
   705  // fill appends up to len(p) bytes of p to *dst, such that *dst does not
   706  // grow larger than w.N. It returns the un-appended suffix of p.
   707  func (w *prefixSuffixSaver) fill(dst *[]byte, p []byte) (pRemain []byte) {
   708  	if remain := w.N - len(*dst); remain > 0 {
   709  		add := minInt(len(p), remain)
   710  		*dst = append(*dst, p[:add]...)
   711  		p = p[add:]
   712  	}
   713  	return p
   714  }
   715  
   716  func (w *prefixSuffixSaver) Bytes() []byte {
   717  	if w.suffix == nil {
   718  		return w.prefix
   719  	}
   720  	if w.skipped == 0 {
   721  		return append(w.prefix, w.suffix...)
   722  	}
   723  	var buf bytes.Buffer
   724  	buf.Grow(len(w.prefix) + len(w.suffix) + 50)
   725  	buf.Write(w.prefix)
   726  	buf.WriteString("\n... omitting ")
   727  	buf.WriteString(strconv.FormatInt(w.skipped, 10))
   728  	buf.WriteString(" bytes ...\n")
   729  	buf.Write(w.suffix[w.suffixOff:])
   730  	buf.Write(w.suffix[:w.suffixOff])
   731  	return buf.Bytes()
   732  }
   733  
   734  func minInt(a, b int) int {
   735  	if a < b {
   736  		return a
   737  	}
   738  	return b
   739  }
   740  
   741  // dedupEnv returns a copy of env with any duplicates removed, in favor of
   742  // later values.
   743  // Items not of the normal environment "key=value" form are preserved unchanged.
   744  func dedupEnv(env []string) []string {
   745  	return dedupEnvCase(runtime.GOOS == "windows", env)
   746  }
   747  
   748  // dedupEnvCase is dedupEnv with a case option for testing.
   749  // If caseInsensitive is true, the case of keys is ignored.
   750  func dedupEnvCase(caseInsensitive bool, env []string) []string {
   751  	out := make([]string, 0, len(env))
   752  	saw := make(map[string]int, len(env)) // key => index into out
   753  	for _, kv := range env {
   754  		k, _, ok := strings.Cut(kv, "=")
   755  		if !ok {
   756  			out = append(out, kv)
   757  			continue
   758  		}
   759  		if caseInsensitive {
   760  			k = strings.ToLower(k)
   761  		}
   762  		if dupIdx, isDup := saw[k]; isDup {
   763  			out[dupIdx] = kv
   764  			continue
   765  		}
   766  		saw[k] = len(out)
   767  		out = append(out, kv)
   768  	}
   769  	return out
   770  }
   771  
   772  // addCriticalEnv adds any critical environment variables that are required
   773  // (or at least almost always required) on the operating system.
   774  // Currently this is only used for Windows.
   775  func addCriticalEnv(env []string) []string {
   776  	if runtime.GOOS != "windows" {
   777  		return env
   778  	}
   779  	for _, kv := range env {
   780  		k, _, ok := strings.Cut(kv, "=")
   781  		if !ok {
   782  			continue
   783  		}
   784  		if strings.EqualFold(k, "SYSTEMROOT") {
   785  			// We already have it.
   786  			return env
   787  		}
   788  	}
   789  	return append(env, "SYSTEMROOT="+os.Getenv("SYSTEMROOT"))
   790  }
   791
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