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Run Format

Source file src/os/exec/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	// Note that the examples in this package assume a Unix system.
    10	// They may not run on Windows, and they do not run in the Go Playground
    11	// used by golang.org and godoc.org.
    12	package exec
    13	
    14	import (
    15		"bytes"
    16		"context"
    17		"errors"
    18		"io"
    19		"os"
    20		"path/filepath"
    21		"runtime"
    22		"strconv"
    23		"strings"
    24		"sync"
    25		"syscall"
    26	)
    27	
    28	// Error records the name of a binary that failed to be executed
    29	// and the reason it failed.
    30	type Error struct {
    31		Name string
    32		Err  error
    33	}
    34	
    35	func (e *Error) Error() string {
    36		return "exec: " + strconv.Quote(e.Name) + ": " + e.Err.Error()
    37	}
    38	
    39	// Cmd represents an external command being prepared or run.
    40	//
    41	// A Cmd cannot be reused after calling its Run, Output or CombinedOutput
    42	// methods.
    43	type Cmd struct {
    44		// Path is the path of the command to run.
    45		//
    46		// This is the only field that must be set to a non-zero
    47		// value. If Path is relative, it is evaluated relative
    48		// to Dir.
    49		Path string
    50	
    51		// Args holds command line arguments, including the command as Args[0].
    52		// If the Args field is empty or nil, Run uses {Path}.
    53		//
    54		// In typical use, both Path and Args are set by calling Command.
    55		Args []string
    56	
    57		// Env specifies the environment of the process.
    58		// If Env is nil, Run uses the current process's environment.
    59		Env []string
    60	
    61		// Dir specifies the working directory of the command.
    62		// If Dir is the empty string, Run runs the command in the
    63		// calling process's current directory.
    64		Dir string
    65	
    66		// Stdin specifies the process's standard input.
    67		// If Stdin is nil, the process reads from the null device (os.DevNull).
    68		// If Stdin is an *os.File, the process's standard input is connected
    69		// directly to that file.
    70		// Otherwise, during the execution of the command a separate
    71		// goroutine reads from Stdin and delivers that data to the command
    72		// over a pipe. In this case, Wait does not complete until the goroutine
    73		// stops copying, either because it has reached the end of Stdin
    74		// (EOF or a read error) or because writing to the pipe returned an error.
    75		Stdin io.Reader
    76	
    77		// Stdout and Stderr specify the process's standard output and error.
    78		//
    79		// If either is nil, Run connects the corresponding file descriptor
    80		// to the null device (os.DevNull).
    81		//
    82		// If Stdout and Stderr are the same writer, at most one
    83		// goroutine at a time will call Write.
    84		Stdout io.Writer
    85		Stderr io.Writer
    86	
    87		// ExtraFiles specifies additional open files to be inherited by the
    88		// new process. It does not include standard input, standard output, or
    89		// standard error. If non-nil, entry i becomes file descriptor 3+i.
    90		//
    91		// BUG(rsc): On OS X 10.6, child processes may sometimes inherit unwanted fds.
    92		// https://golang.org/issue/2603
    93		ExtraFiles []*os.File
    94	
    95		// SysProcAttr holds optional, operating system-specific attributes.
    96		// Run passes it to os.StartProcess as the os.ProcAttr's Sys field.
    97		SysProcAttr *syscall.SysProcAttr
    98	
    99		// Process is the underlying process, once started.
   100		Process *os.Process
   101	
   102		// ProcessState contains information about an exited process,
   103		// available after a call to Wait or Run.
   104		ProcessState *os.ProcessState
   105	
   106		ctx             context.Context // nil means none
   107		lookPathErr     error           // LookPath error, if any.
   108		finished        bool            // when Wait was called
   109		childFiles      []*os.File
   110		closeAfterStart []io.Closer
   111		closeAfterWait  []io.Closer
   112		goroutine       []func() error
   113		errch           chan error // one send per goroutine
   114		waitDone        chan struct{}
   115	}
   116	
   117	// Command returns the Cmd struct to execute the named program with
   118	// the given arguments.
   119	//
   120	// It sets only the Path and Args in the returned structure.
   121	//
   122	// If name contains no path separators, Command uses LookPath to
   123	// resolve name to a complete path if possible. Otherwise it uses name
   124	// directly as Path.
   125	//
   126	// The returned Cmd's Args field is constructed from the command name
   127	// followed by the elements of arg, so arg should not include the
   128	// command name itself. For example, Command("echo", "hello").
   129	// Args[0] is always name, not the possibly resolved Path.
   130	func Command(name string, arg ...string) *Cmd {
   131		cmd := &Cmd{
   132			Path: name,
   133			Args: append([]string{name}, arg...),
   134		}
   135		if filepath.Base(name) == name {
   136			if lp, err := LookPath(name); err != nil {
   137				cmd.lookPathErr = err
   138			} else {
   139				cmd.Path = lp
   140			}
   141		}
   142		return cmd
   143	}
   144	
   145	// CommandContext is like Command but includes a context.
   146	//
   147	// The provided context is used to kill the process (by calling
   148	// os.Process.Kill) if the context becomes done before the command
   149	// completes on its own.
   150	func CommandContext(ctx context.Context, name string, arg ...string) *Cmd {
   151		if ctx == nil {
   152			panic("nil Context")
   153		}
   154		cmd := Command(name, arg...)
   155		cmd.ctx = ctx
   156		return cmd
   157	}
   158	
   159	// interfaceEqual protects against panics from doing equality tests on
   160	// two interfaces with non-comparable underlying types.
   161	func interfaceEqual(a, b interface{}) bool {
   162		defer func() {
   163			recover()
   164		}()
   165		return a == b
   166	}
   167	
   168	func (c *Cmd) envv() []string {
   169		if c.Env != nil {
   170			return c.Env
   171		}
   172		return os.Environ()
   173	}
   174	
   175	func (c *Cmd) argv() []string {
   176		if len(c.Args) > 0 {
   177			return c.Args
   178		}
   179		return []string{c.Path}
   180	}
   181	
   182	// skipStdinCopyError optionally specifies a function which reports
   183	// whether the provided the stdin copy error should be ignored.
   184	// It is non-nil everywhere but Plan 9, which lacks EPIPE. See exec_posix.go.
   185	var skipStdinCopyError func(error) bool
   186	
   187	func (c *Cmd) stdin() (f *os.File, err error) {
   188		if c.Stdin == nil {
   189			f, err = os.Open(os.DevNull)
   190			if err != nil {
   191				return
   192			}
   193			c.closeAfterStart = append(c.closeAfterStart, f)
   194			return
   195		}
   196	
   197		if f, ok := c.Stdin.(*os.File); ok {
   198			return f, nil
   199		}
   200	
   201		pr, pw, err := os.Pipe()
   202		if err != nil {
   203			return
   204		}
   205	
   206		c.closeAfterStart = append(c.closeAfterStart, pr)
   207		c.closeAfterWait = append(c.closeAfterWait, pw)
   208		c.goroutine = append(c.goroutine, func() error {
   209			_, err := io.Copy(pw, c.Stdin)
   210			if skip := skipStdinCopyError; skip != nil && skip(err) {
   211				err = nil
   212			}
   213			if err1 := pw.Close(); err == nil {
   214				err = err1
   215			}
   216			return err
   217		})
   218		return pr, nil
   219	}
   220	
   221	func (c *Cmd) stdout() (f *os.File, err error) {
   222		return c.writerDescriptor(c.Stdout)
   223	}
   224	
   225	func (c *Cmd) stderr() (f *os.File, err error) {
   226		if c.Stderr != nil && interfaceEqual(c.Stderr, c.Stdout) {
   227			return c.childFiles[1], nil
   228		}
   229		return c.writerDescriptor(c.Stderr)
   230	}
   231	
   232	func (c *Cmd) writerDescriptor(w io.Writer) (f *os.File, err error) {
   233		if w == nil {
   234			f, err = os.OpenFile(os.DevNull, os.O_WRONLY, 0)
   235			if err != nil {
   236				return
   237			}
   238			c.closeAfterStart = append(c.closeAfterStart, f)
   239			return
   240		}
   241	
   242		if f, ok := w.(*os.File); ok {
   243			return f, nil
   244		}
   245	
   246		pr, pw, err := os.Pipe()
   247		if err != nil {
   248			return
   249		}
   250	
   251		c.closeAfterStart = append(c.closeAfterStart, pw)
   252		c.closeAfterWait = append(c.closeAfterWait, pr)
   253		c.goroutine = append(c.goroutine, func() error {
   254			_, err := io.Copy(w, pr)
   255			pr.Close() // in case io.Copy stopped due to write error
   256			return err
   257		})
   258		return pw, nil
   259	}
   260	
   261	func (c *Cmd) closeDescriptors(closers []io.Closer) {
   262		for _, fd := range closers {
   263			fd.Close()
   264		}
   265	}
   266	
   267	// Run starts the specified command and waits for it to complete.
   268	//
   269	// The returned error is nil if the command runs, has no problems
   270	// copying stdin, stdout, and stderr, and exits with a zero exit
   271	// status.
   272	//
   273	// If the command fails to run or doesn't complete successfully, the
   274	// error is of type *ExitError. Other error types may be
   275	// returned for I/O problems.
   276	func (c *Cmd) Run() error {
   277		if err := c.Start(); err != nil {
   278			return err
   279		}
   280		return c.Wait()
   281	}
   282	
   283	// lookExtensions finds windows executable by its dir and path.
   284	// It uses LookPath to try appropriate extensions.
   285	// lookExtensions does not search PATH, instead it converts `prog` into `.\prog`.
   286	func lookExtensions(path, dir string) (string, error) {
   287		if filepath.Base(path) == path {
   288			path = filepath.Join(".", path)
   289		}
   290		if dir == "" {
   291			return LookPath(path)
   292		}
   293		if filepath.VolumeName(path) != "" {
   294			return LookPath(path)
   295		}
   296		if len(path) > 1 && os.IsPathSeparator(path[0]) {
   297			return LookPath(path)
   298		}
   299		dirandpath := filepath.Join(dir, path)
   300		// We assume that LookPath will only add file extension.
   301		lp, err := LookPath(dirandpath)
   302		if err != nil {
   303			return "", err
   304		}
   305		ext := strings.TrimPrefix(lp, dirandpath)
   306		return path + ext, nil
   307	}
   308	
   309	// Start starts the specified command but does not wait for it to complete.
   310	//
   311	// The Wait method will return the exit code and release associated resources
   312	// once the command exits.
   313	func (c *Cmd) Start() error {
   314		if c.lookPathErr != nil {
   315			c.closeDescriptors(c.closeAfterStart)
   316			c.closeDescriptors(c.closeAfterWait)
   317			return c.lookPathErr
   318		}
   319		if runtime.GOOS == "windows" {
   320			lp, err := lookExtensions(c.Path, c.Dir)
   321			if err != nil {
   322				c.closeDescriptors(c.closeAfterStart)
   323				c.closeDescriptors(c.closeAfterWait)
   324				return err
   325			}
   326			c.Path = lp
   327		}
   328		if c.Process != nil {
   329			return errors.New("exec: already started")
   330		}
   331		if c.ctx != nil {
   332			select {
   333			case <-c.ctx.Done():
   334				c.closeDescriptors(c.closeAfterStart)
   335				c.closeDescriptors(c.closeAfterWait)
   336				return c.ctx.Err()
   337			default:
   338			}
   339		}
   340	
   341		type F func(*Cmd) (*os.File, error)
   342		for _, setupFd := range []F{(*Cmd).stdin, (*Cmd).stdout, (*Cmd).stderr} {
   343			fd, err := setupFd(c)
   344			if err != nil {
   345				c.closeDescriptors(c.closeAfterStart)
   346				c.closeDescriptors(c.closeAfterWait)
   347				return err
   348			}
   349			c.childFiles = append(c.childFiles, fd)
   350		}
   351		c.childFiles = append(c.childFiles, c.ExtraFiles...)
   352	
   353		var err error
   354		c.Process, err = os.StartProcess(c.Path, c.argv(), &os.ProcAttr{
   355			Dir:   c.Dir,
   356			Files: c.childFiles,
   357			Env:   c.envv(),
   358			Sys:   c.SysProcAttr,
   359		})
   360		if err != nil {
   361			c.closeDescriptors(c.closeAfterStart)
   362			c.closeDescriptors(c.closeAfterWait)
   363			return err
   364		}
   365	
   366		c.closeDescriptors(c.closeAfterStart)
   367	
   368		c.errch = make(chan error, len(c.goroutine))
   369		for _, fn := range c.goroutine {
   370			go func(fn func() error) {
   371				c.errch <- fn()
   372			}(fn)
   373		}
   374	
   375		if c.ctx != nil {
   376			c.waitDone = make(chan struct{})
   377			go func() {
   378				select {
   379				case <-c.ctx.Done():
   380					c.Process.Kill()
   381				case <-c.waitDone:
   382				}
   383			}()
   384		}
   385	
   386		return nil
   387	}
   388	
   389	// An ExitError reports an unsuccessful exit by a command.
   390	type ExitError struct {
   391		*os.ProcessState
   392	
   393		// Stderr holds a subset of the standard error output from the
   394		// Cmd.Output method if standard error was not otherwise being
   395		// collected.
   396		//
   397		// If the error output is long, Stderr may contain only a prefix
   398		// and suffix of the output, with the middle replaced with
   399		// text about the number of omitted bytes.
   400		//
   401		// Stderr is provided for debugging, for inclusion in error messages.
   402		// Users with other needs should redirect Cmd.Stderr as needed.
   403		Stderr []byte
   404	}
   405	
   406	func (e *ExitError) Error() string {
   407		return e.ProcessState.String()
   408	}
   409	
   410	// Wait waits for the command to exit.
   411	// It must have been started by Start.
   412	//
   413	// The returned error is nil if the command runs, has no problems
   414	// copying stdin, stdout, and stderr, and exits with a zero exit
   415	// status.
   416	//
   417	// If the command fails to run or doesn't complete successfully, the
   418	// error is of type *ExitError. Other error types may be
   419	// returned for I/O problems.
   420	//
   421	// If c.Stdin is not an *os.File, Wait also waits for the I/O loop
   422	// copying from c.Stdin into the process's standard input
   423	// to complete.
   424	//
   425	// Wait releases any resources associated with the Cmd.
   426	func (c *Cmd) Wait() error {
   427		if c.Process == nil {
   428			return errors.New("exec: not started")
   429		}
   430		if c.finished {
   431			return errors.New("exec: Wait was already called")
   432		}
   433		c.finished = true
   434	
   435		state, err := c.Process.Wait()
   436		if c.waitDone != nil {
   437			close(c.waitDone)
   438		}
   439		c.ProcessState = state
   440	
   441		var copyError error
   442		for range c.goroutine {
   443			if err := <-c.errch; err != nil && copyError == nil {
   444				copyError = err
   445			}
   446		}
   447	
   448		c.closeDescriptors(c.closeAfterWait)
   449	
   450		if err != nil {
   451			return err
   452		} else if !state.Success() {
   453			return &ExitError{ProcessState: state}
   454		}
   455	
   456		return copyError
   457	}
   458	
   459	// Output runs the command and returns its standard output.
   460	// Any returned error will usually be of type *ExitError.
   461	// If c.Stderr was nil, Output populates ExitError.Stderr.
   462	func (c *Cmd) Output() ([]byte, error) {
   463		if c.Stdout != nil {
   464			return nil, errors.New("exec: Stdout already set")
   465		}
   466		var stdout bytes.Buffer
   467		c.Stdout = &stdout
   468	
   469		captureErr := c.Stderr == nil
   470		if captureErr {
   471			c.Stderr = &prefixSuffixSaver{N: 32 << 10}
   472		}
   473	
   474		err := c.Run()
   475		if err != nil && captureErr {
   476			if ee, ok := err.(*ExitError); ok {
   477				ee.Stderr = c.Stderr.(*prefixSuffixSaver).Bytes()
   478			}
   479		}
   480		return stdout.Bytes(), err
   481	}
   482	
   483	// CombinedOutput runs the command and returns its combined standard
   484	// output and standard error.
   485	func (c *Cmd) CombinedOutput() ([]byte, error) {
   486		if c.Stdout != nil {
   487			return nil, errors.New("exec: Stdout already set")
   488		}
   489		if c.Stderr != nil {
   490			return nil, errors.New("exec: Stderr already set")
   491		}
   492		var b bytes.Buffer
   493		c.Stdout = &b
   494		c.Stderr = &b
   495		err := c.Run()
   496		return b.Bytes(), err
   497	}
   498	
   499	// StdinPipe returns a pipe that will be connected to the command's
   500	// standard input when the command starts.
   501	// The pipe will be closed automatically after Wait sees the command exit.
   502	// A caller need only call Close to force the pipe to close sooner.
   503	// For example, if the command being run will not exit until standard input
   504	// is closed, the caller must close the pipe.
   505	func (c *Cmd) StdinPipe() (io.WriteCloser, error) {
   506		if c.Stdin != nil {
   507			return nil, errors.New("exec: Stdin already set")
   508		}
   509		if c.Process != nil {
   510			return nil, errors.New("exec: StdinPipe after process started")
   511		}
   512		pr, pw, err := os.Pipe()
   513		if err != nil {
   514			return nil, err
   515		}
   516		c.Stdin = pr
   517		c.closeAfterStart = append(c.closeAfterStart, pr)
   518		wc := &closeOnce{File: pw}
   519		c.closeAfterWait = append(c.closeAfterWait, closerFunc(wc.safeClose))
   520		return wc, nil
   521	}
   522	
   523	type closeOnce struct {
   524		*os.File
   525	
   526		writers sync.RWMutex // coordinate safeClose and Write
   527		once    sync.Once
   528		err     error
   529	}
   530	
   531	func (c *closeOnce) Close() error {
   532		c.once.Do(c.close)
   533		return c.err
   534	}
   535	
   536	func (c *closeOnce) close() {
   537		c.err = c.File.Close()
   538	}
   539	
   540	type closerFunc func() error
   541	
   542	func (f closerFunc) Close() error { return f() }
   543	
   544	// safeClose closes c being careful not to race with any calls to c.Write.
   545	// See golang.org/issue/9307 and TestEchoFileRace in exec_test.go.
   546	// In theory other calls could also be excluded (by writing appropriate
   547	// wrappers like c.Write's implementation below), but since c is most
   548	// commonly used as a WriteCloser, Write is the main one to worry about.
   549	// See also #7970, for which this is a partial fix for this specific instance.
   550	// The idea is that we return a WriteCloser, and so the caller can be
   551	// relied upon not to call Write and Close simultaneously, but it's less
   552	// obvious that cmd.Wait calls Close and that the caller must not call
   553	// Write and cmd.Wait simultaneously. In fact that seems too onerous.
   554	// So we change the use of Close in cmd.Wait to use safeClose, which will
   555	// synchronize with any Write.
   556	//
   557	// It's important that we know this won't block forever waiting for the
   558	// operations being excluded. At the point where this is called,
   559	// the invoked command has exited and the parent copy of the read side
   560	// of the pipe has also been closed, so there should really be no read side
   561	// of the pipe left. Any active writes should return very shortly with an EPIPE,
   562	// making it reasonable to wait for them.
   563	// Technically it is possible that the child forked a sub-process or otherwise
   564	// handed off the read side of the pipe before exiting and the current holder
   565	// is not reading from the pipe, and the pipe is full, in which case the close here
   566	// might block waiting for the write to complete. That's probably OK.
   567	// It's a small enough problem to be outweighed by eliminating the race here.
   568	func (c *closeOnce) safeClose() error {
   569		c.writers.Lock()
   570		err := c.Close()
   571		c.writers.Unlock()
   572		return err
   573	}
   574	
   575	func (c *closeOnce) Write(b []byte) (int, error) {
   576		c.writers.RLock()
   577		n, err := c.File.Write(b)
   578		c.writers.RUnlock()
   579		return n, err
   580	}
   581	
   582	func (c *closeOnce) WriteString(s string) (int, error) {
   583		c.writers.RLock()
   584		n, err := c.File.WriteString(s)
   585		c.writers.RUnlock()
   586		return n, err
   587	}
   588	
   589	// StdoutPipe returns a pipe that will be connected to the command's
   590	// standard output when the command starts.
   591	//
   592	// Wait will close the pipe after seeing the command exit, so most callers
   593	// need not close the pipe themselves; however, an implication is that
   594	// it is incorrect to call Wait before all reads from the pipe have completed.
   595	// For the same reason, it is incorrect to call Run when using StdoutPipe.
   596	// See the example for idiomatic usage.
   597	func (c *Cmd) StdoutPipe() (io.ReadCloser, error) {
   598		if c.Stdout != nil {
   599			return nil, errors.New("exec: Stdout already set")
   600		}
   601		if c.Process != nil {
   602			return nil, errors.New("exec: StdoutPipe after process started")
   603		}
   604		pr, pw, err := os.Pipe()
   605		if err != nil {
   606			return nil, err
   607		}
   608		c.Stdout = pw
   609		c.closeAfterStart = append(c.closeAfterStart, pw)
   610		c.closeAfterWait = append(c.closeAfterWait, pr)
   611		return pr, nil
   612	}
   613	
   614	// StderrPipe returns a pipe that will be connected to the command's
   615	// standard error 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; however, an implication is that
   619	// it is incorrect to call Wait before all reads from the pipe have completed.
   620	// For the same reason, it is incorrect to use Run when using StderrPipe.
   621	// See the StdoutPipe example for idiomatic usage.
   622	func (c *Cmd) StderrPipe() (io.ReadCloser, error) {
   623		if c.Stderr != nil {
   624			return nil, errors.New("exec: Stderr already set")
   625		}
   626		if c.Process != nil {
   627			return nil, errors.New("exec: StderrPipe after process started")
   628		}
   629		pr, pw, err := os.Pipe()
   630		if err != nil {
   631			return nil, err
   632		}
   633		c.Stderr = pw
   634		c.closeAfterStart = append(c.closeAfterStart, pw)
   635		c.closeAfterWait = append(c.closeAfterWait, pr)
   636		return pr, nil
   637	}
   638	
   639	// prefixSuffixSaver is an io.Writer which retains the first N bytes
   640	// and the last N bytes written to it. The Bytes() methods reconstructs
   641	// it with a pretty error message.
   642	type prefixSuffixSaver struct {
   643		N         int // max size of prefix or suffix
   644		prefix    []byte
   645		suffix    []byte // ring buffer once len(suffix) == N
   646		suffixOff int    // offset to write into suffix
   647		skipped   int64
   648	
   649		// TODO(bradfitz): we could keep one large []byte and use part of it for
   650		// the prefix, reserve space for the '... Omitting N bytes ...' message,
   651		// then the ring buffer suffix, and just rearrange the ring buffer
   652		// suffix when Bytes() is called, but it doesn't seem worth it for
   653		// now just for error messages. It's only ~64KB anyway.
   654	}
   655	
   656	func (w *prefixSuffixSaver) Write(p []byte) (n int, err error) {
   657		lenp := len(p)
   658		p = w.fill(&w.prefix, p)
   659	
   660		// Only keep the last w.N bytes of suffix data.
   661		if overage := len(p) - w.N; overage > 0 {
   662			p = p[overage:]
   663			w.skipped += int64(overage)
   664		}
   665		p = w.fill(&w.suffix, p)
   666	
   667		// w.suffix is full now if p is non-empty. Overwrite it in a circle.
   668		for len(p) > 0 { // 0, 1, or 2 iterations.
   669			n := copy(w.suffix[w.suffixOff:], p)
   670			p = p[n:]
   671			w.skipped += int64(n)
   672			w.suffixOff += n
   673			if w.suffixOff == w.N {
   674				w.suffixOff = 0
   675			}
   676		}
   677		return lenp, nil
   678	}
   679	
   680	// fill appends up to len(p) bytes of p to *dst, such that *dst does not
   681	// grow larger than w.N. It returns the un-appended suffix of p.
   682	func (w *prefixSuffixSaver) fill(dst *[]byte, p []byte) (pRemain []byte) {
   683		if remain := w.N - len(*dst); remain > 0 {
   684			add := minInt(len(p), remain)
   685			*dst = append(*dst, p[:add]...)
   686			p = p[add:]
   687		}
   688		return p
   689	}
   690	
   691	func (w *prefixSuffixSaver) Bytes() []byte {
   692		if w.suffix == nil {
   693			return w.prefix
   694		}
   695		if w.skipped == 0 {
   696			return append(w.prefix, w.suffix...)
   697		}
   698		var buf bytes.Buffer
   699		buf.Grow(len(w.prefix) + len(w.suffix) + 50)
   700		buf.Write(w.prefix)
   701		buf.WriteString("\n... omitting ")
   702		buf.WriteString(strconv.FormatInt(w.skipped, 10))
   703		buf.WriteString(" bytes ...\n")
   704		buf.Write(w.suffix[w.suffixOff:])
   705		buf.Write(w.suffix[:w.suffixOff])
   706		return buf.Bytes()
   707	}
   708	
   709	func minInt(a, b int) int {
   710		if a < b {
   711			return a
   712		}
   713		return b
   714	}
   715	

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