Source file src/os/file.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 os provides a platform-independent interface to operating system
     6  // functionality. The design is Unix-like, although the error handling is
     7  // Go-like; failing calls return values of type error rather than error numbers.
     8  // Often, more information is available within the error. For example,
     9  // if a call that takes a file name fails, such as Open or Stat, the error
    10  // will include the failing file name when printed and will be of type
    11  // *PathError, which may be unpacked for more information.
    12  //
    13  // The os interface is intended to be uniform across all operating systems.
    14  // Features not generally available appear in the system-specific package syscall.
    15  //
    16  // Here is a simple example, opening a file and reading some of it.
    17  //
    18  //	file, err := os.Open("file.go") // For read access.
    19  //	if err != nil {
    20  //		log.Fatal(err)
    21  //	}
    22  //
    23  // If the open fails, the error string will be self-explanatory, like
    24  //
    25  //	open file.go: no such file or directory
    26  //
    27  // The file's data can then be read into a slice of bytes. Read and
    28  // Write take their byte counts from the length of the argument slice.
    29  //
    30  //	data := make([]byte, 100)
    31  //	count, err := file.Read(data)
    32  //	if err != nil {
    33  //		log.Fatal(err)
    34  //	}
    35  //	fmt.Printf("read %d bytes: %q\n", count, data[:count])
    36  //
    37  // Note: The maximum number of concurrent operations on a File may be limited by
    38  // the OS or the system. The number should be high, but exceeding it may degrade
    39  // performance or cause other issues.
    40  package os
    41  
    42  import (
    43  	"errors"
    44  	"internal/poll"
    45  	"internal/testlog"
    46  	"internal/unsafeheader"
    47  	"io"
    48  	"io/fs"
    49  	"runtime"
    50  	"syscall"
    51  	"time"
    52  	"unsafe"
    53  )
    54  
    55  // Name returns the name of the file as presented to Open.
    56  func (f *File) Name() string { return f.name }
    57  
    58  // Stdin, Stdout, and Stderr are open Files pointing to the standard input,
    59  // standard output, and standard error file descriptors.
    60  //
    61  // Note that the Go runtime writes to standard error for panics and crashes;
    62  // closing Stderr may cause those messages to go elsewhere, perhaps
    63  // to a file opened later.
    64  var (
    65  	Stdin  = NewFile(uintptr(syscall.Stdin), "/dev/stdin")
    66  	Stdout = NewFile(uintptr(syscall.Stdout), "/dev/stdout")
    67  	Stderr = NewFile(uintptr(syscall.Stderr), "/dev/stderr")
    68  )
    69  
    70  // Flags to OpenFile wrapping those of the underlying system. Not all
    71  // flags may be implemented on a given system.
    72  const (
    73  	// Exactly one of O_RDONLY, O_WRONLY, or O_RDWR must be specified.
    74  	O_RDONLY int = syscall.O_RDONLY // open the file read-only.
    75  	O_WRONLY int = syscall.O_WRONLY // open the file write-only.
    76  	O_RDWR   int = syscall.O_RDWR   // open the file read-write.
    77  	// The remaining values may be or'ed in to control behavior.
    78  	O_APPEND int = syscall.O_APPEND // append data to the file when writing.
    79  	O_CREATE int = syscall.O_CREAT  // create a new file if none exists.
    80  	O_EXCL   int = syscall.O_EXCL   // used with O_CREATE, file must not exist.
    81  	O_SYNC   int = syscall.O_SYNC   // open for synchronous I/O.
    82  	O_TRUNC  int = syscall.O_TRUNC  // truncate regular writable file when opened.
    83  )
    84  
    85  // Seek whence values.
    86  //
    87  // Deprecated: Use io.SeekStart, io.SeekCurrent, and io.SeekEnd.
    88  const (
    89  	SEEK_SET int = 0 // seek relative to the origin of the file
    90  	SEEK_CUR int = 1 // seek relative to the current offset
    91  	SEEK_END int = 2 // seek relative to the end
    92  )
    93  
    94  // LinkError records an error during a link or symlink or rename
    95  // system call and the paths that caused it.
    96  type LinkError struct {
    97  	Op  string
    98  	Old string
    99  	New string
   100  	Err error
   101  }
   102  
   103  func (e *LinkError) Error() string {
   104  	return e.Op + " " + e.Old + " " + e.New + ": " + e.Err.Error()
   105  }
   106  
   107  func (e *LinkError) Unwrap() error {
   108  	return e.Err
   109  }
   110  
   111  // Read reads up to len(b) bytes from the File and stores them in b.
   112  // It returns the number of bytes read and any error encountered.
   113  // At end of file, Read returns 0, io.EOF.
   114  func (f *File) Read(b []byte) (n int, err error) {
   115  	if err := f.checkValid("read"); err != nil {
   116  		return 0, err
   117  	}
   118  	n, e := f.read(b)
   119  	return n, f.wrapErr("read", e)
   120  }
   121  
   122  // ReadAt reads len(b) bytes from the File starting at byte offset off.
   123  // It returns the number of bytes read and the error, if any.
   124  // ReadAt always returns a non-nil error when n < len(b).
   125  // At end of file, that error is io.EOF.
   126  func (f *File) ReadAt(b []byte, off int64) (n int, err error) {
   127  	if err := f.checkValid("read"); err != nil {
   128  		return 0, err
   129  	}
   130  
   131  	if off < 0 {
   132  		return 0, &PathError{Op: "readat", Path: f.name, Err: errors.New("negative offset")}
   133  	}
   134  
   135  	for len(b) > 0 {
   136  		m, e := f.pread(b, off)
   137  		if e != nil {
   138  			err = f.wrapErr("read", e)
   139  			break
   140  		}
   141  		n += m
   142  		b = b[m:]
   143  		off += int64(m)
   144  	}
   145  	return
   146  }
   147  
   148  // ReadFrom implements io.ReaderFrom.
   149  func (f *File) ReadFrom(r io.Reader) (n int64, err error) {
   150  	if err := f.checkValid("write"); err != nil {
   151  		return 0, err
   152  	}
   153  	n, handled, e := f.readFrom(r)
   154  	if !handled {
   155  		return genericReadFrom(f, r) // without wrapping
   156  	}
   157  	return n, f.wrapErr("write", e)
   158  }
   159  
   160  func genericReadFrom(f *File, r io.Reader) (int64, error) {
   161  	return io.Copy(onlyWriter{f}, r)
   162  }
   163  
   164  type onlyWriter struct {
   165  	io.Writer
   166  }
   167  
   168  // Write writes len(b) bytes from b to the File.
   169  // It returns the number of bytes written and an error, if any.
   170  // Write returns a non-nil error when n != len(b).
   171  func (f *File) Write(b []byte) (n int, err error) {
   172  	if err := f.checkValid("write"); err != nil {
   173  		return 0, err
   174  	}
   175  	n, e := f.write(b)
   176  	if n < 0 {
   177  		n = 0
   178  	}
   179  	if n != len(b) {
   180  		err = io.ErrShortWrite
   181  	}
   182  
   183  	epipecheck(f, e)
   184  
   185  	if e != nil {
   186  		err = f.wrapErr("write", e)
   187  	}
   188  
   189  	return n, err
   190  }
   191  
   192  var errWriteAtInAppendMode = errors.New("os: invalid use of WriteAt on file opened with O_APPEND")
   193  
   194  // WriteAt writes len(b) bytes to the File starting at byte offset off.
   195  // It returns the number of bytes written and an error, if any.
   196  // WriteAt returns a non-nil error when n != len(b).
   197  //
   198  // If file was opened with the O_APPEND flag, WriteAt returns an error.
   199  func (f *File) WriteAt(b []byte, off int64) (n int, err error) {
   200  	if err := f.checkValid("write"); err != nil {
   201  		return 0, err
   202  	}
   203  	if f.appendMode {
   204  		return 0, errWriteAtInAppendMode
   205  	}
   206  
   207  	if off < 0 {
   208  		return 0, &PathError{Op: "writeat", Path: f.name, Err: errors.New("negative offset")}
   209  	}
   210  
   211  	for len(b) > 0 {
   212  		m, e := f.pwrite(b, off)
   213  		if e != nil {
   214  			err = f.wrapErr("write", e)
   215  			break
   216  		}
   217  		n += m
   218  		b = b[m:]
   219  		off += int64(m)
   220  	}
   221  	return
   222  }
   223  
   224  // Seek sets the offset for the next Read or Write on file to offset, interpreted
   225  // according to whence: 0 means relative to the origin of the file, 1 means
   226  // relative to the current offset, and 2 means relative to the end.
   227  // It returns the new offset and an error, if any.
   228  // The behavior of Seek on a file opened with O_APPEND is not specified.
   229  //
   230  // If f is a directory, the behavior of Seek varies by operating
   231  // system; you can seek to the beginning of the directory on Unix-like
   232  // operating systems, but not on Windows.
   233  func (f *File) Seek(offset int64, whence int) (ret int64, err error) {
   234  	if err := f.checkValid("seek"); err != nil {
   235  		return 0, err
   236  	}
   237  	r, e := f.seek(offset, whence)
   238  	if e == nil && f.dirinfo != nil && r != 0 {
   239  		e = syscall.EISDIR
   240  	}
   241  	if e != nil {
   242  		return 0, f.wrapErr("seek", e)
   243  	}
   244  	return r, nil
   245  }
   246  
   247  // WriteString is like Write, but writes the contents of string s rather than
   248  // a slice of bytes.
   249  func (f *File) WriteString(s string) (n int, err error) {
   250  	var b []byte
   251  	hdr := (*unsafeheader.Slice)(unsafe.Pointer(&b))
   252  	hdr.Data = (*unsafeheader.String)(unsafe.Pointer(&s)).Data
   253  	hdr.Cap = len(s)
   254  	hdr.Len = len(s)
   255  	return f.Write(b)
   256  }
   257  
   258  // Mkdir creates a new directory with the specified name and permission
   259  // bits (before umask).
   260  // If there is an error, it will be of type *PathError.
   261  func Mkdir(name string, perm FileMode) error {
   262  	if runtime.GOOS == "windows" && isWindowsNulName(name) {
   263  		return &PathError{Op: "mkdir", Path: name, Err: syscall.ENOTDIR}
   264  	}
   265  	longName := fixLongPath(name)
   266  	e := ignoringEINTR(func() error {
   267  		return syscall.Mkdir(longName, syscallMode(perm))
   268  	})
   269  
   270  	if e != nil {
   271  		return &PathError{Op: "mkdir", Path: name, Err: e}
   272  	}
   273  
   274  	// mkdir(2) itself won't handle the sticky bit on *BSD and Solaris
   275  	if !supportsCreateWithStickyBit && perm&ModeSticky != 0 {
   276  		e = setStickyBit(name)
   277  
   278  		if e != nil {
   279  			Remove(name)
   280  			return e
   281  		}
   282  	}
   283  
   284  	return nil
   285  }
   286  
   287  // setStickyBit adds ModeSticky to the permission bits of path, non atomic.
   288  func setStickyBit(name string) error {
   289  	fi, err := Stat(name)
   290  	if err != nil {
   291  		return err
   292  	}
   293  	return Chmod(name, fi.Mode()|ModeSticky)
   294  }
   295  
   296  // Chdir changes the current working directory to the named directory.
   297  // If there is an error, it will be of type *PathError.
   298  func Chdir(dir string) error {
   299  	if e := syscall.Chdir(dir); e != nil {
   300  		testlog.Open(dir) // observe likely non-existent directory
   301  		return &PathError{Op: "chdir", Path: dir, Err: e}
   302  	}
   303  	if log := testlog.Logger(); log != nil {
   304  		wd, err := Getwd()
   305  		if err == nil {
   306  			log.Chdir(wd)
   307  		}
   308  	}
   309  	return nil
   310  }
   311  
   312  // Open opens the named file for reading. If successful, methods on
   313  // the returned file can be used for reading; the associated file
   314  // descriptor has mode O_RDONLY.
   315  // If there is an error, it will be of type *PathError.
   316  func Open(name string) (*File, error) {
   317  	return OpenFile(name, O_RDONLY, 0)
   318  }
   319  
   320  // Create creates or truncates the named file. If the file already exists,
   321  // it is truncated. If the file does not exist, it is created with mode 0666
   322  // (before umask). If successful, methods on the returned File can
   323  // be used for I/O; the associated file descriptor has mode O_RDWR.
   324  // If there is an error, it will be of type *PathError.
   325  func Create(name string) (*File, error) {
   326  	return OpenFile(name, O_RDWR|O_CREATE|O_TRUNC, 0666)
   327  }
   328  
   329  // OpenFile is the generalized open call; most users will use Open
   330  // or Create instead. It opens the named file with specified flag
   331  // (O_RDONLY etc.). If the file does not exist, and the O_CREATE flag
   332  // is passed, it is created with mode perm (before umask). If successful,
   333  // methods on the returned File can be used for I/O.
   334  // If there is an error, it will be of type *PathError.
   335  func OpenFile(name string, flag int, perm FileMode) (*File, error) {
   336  	testlog.Open(name)
   337  	f, err := openFileNolog(name, flag, perm)
   338  	if err != nil {
   339  		return nil, err
   340  	}
   341  	f.appendMode = flag&O_APPEND != 0
   342  
   343  	return f, nil
   344  }
   345  
   346  // lstat is overridden in tests.
   347  var lstat = Lstat
   348  
   349  // Rename renames (moves) oldpath to newpath.
   350  // If newpath already exists and is not a directory, Rename replaces it.
   351  // OS-specific restrictions may apply when oldpath and newpath are in different directories.
   352  // If there is an error, it will be of type *LinkError.
   353  func Rename(oldpath, newpath string) error {
   354  	return rename(oldpath, newpath)
   355  }
   356  
   357  // Many functions in package syscall return a count of -1 instead of 0.
   358  // Using fixCount(call()) instead of call() corrects the count.
   359  func fixCount(n int, err error) (int, error) {
   360  	if n < 0 {
   361  		n = 0
   362  	}
   363  	return n, err
   364  }
   365  
   366  // wrapErr wraps an error that occurred during an operation on an open file.
   367  // It passes io.EOF through unchanged, otherwise converts
   368  // poll.ErrFileClosing to ErrClosed and wraps the error in a PathError.
   369  func (f *File) wrapErr(op string, err error) error {
   370  	if err == nil || err == io.EOF {
   371  		return err
   372  	}
   373  	if err == poll.ErrFileClosing {
   374  		err = ErrClosed
   375  	}
   376  	return &PathError{Op: op, Path: f.name, Err: err}
   377  }
   378  
   379  // TempDir returns the default directory to use for temporary files.
   380  //
   381  // On Unix systems, it returns $TMPDIR if non-empty, else /tmp.
   382  // On Windows, it uses GetTempPath, returning the first non-empty
   383  // value from %TMP%, %TEMP%, %USERPROFILE%, or the Windows directory.
   384  // On Plan 9, it returns /tmp.
   385  //
   386  // The directory is neither guaranteed to exist nor have accessible
   387  // permissions.
   388  func TempDir() string {
   389  	return tempDir()
   390  }
   391  
   392  // UserCacheDir returns the default root directory to use for user-specific
   393  // cached data. Users should create their own application-specific subdirectory
   394  // within this one and use that.
   395  //
   396  // On Unix systems, it returns $XDG_CACHE_HOME as specified by
   397  // https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html if
   398  // non-empty, else $HOME/.cache.
   399  // On Darwin, it returns $HOME/Library/Caches.
   400  // On Windows, it returns %LocalAppData%.
   401  // On Plan 9, it returns $home/lib/cache.
   402  //
   403  // If the location cannot be determined (for example, $HOME is not defined),
   404  // then it will return an error.
   405  func UserCacheDir() (string, error) {
   406  	var dir string
   407  
   408  	switch runtime.GOOS {
   409  	case "windows":
   410  		dir = Getenv("LocalAppData")
   411  		if dir == "" {
   412  			return "", errors.New("%LocalAppData% is not defined")
   413  		}
   414  
   415  	case "darwin", "ios":
   416  		dir = Getenv("HOME")
   417  		if dir == "" {
   418  			return "", errors.New("$HOME is not defined")
   419  		}
   420  		dir += "/Library/Caches"
   421  
   422  	case "plan9":
   423  		dir = Getenv("home")
   424  		if dir == "" {
   425  			return "", errors.New("$home is not defined")
   426  		}
   427  		dir += "/lib/cache"
   428  
   429  	default: // Unix
   430  		dir = Getenv("XDG_CACHE_HOME")
   431  		if dir == "" {
   432  			dir = Getenv("HOME")
   433  			if dir == "" {
   434  				return "", errors.New("neither $XDG_CACHE_HOME nor $HOME are defined")
   435  			}
   436  			dir += "/.cache"
   437  		}
   438  	}
   439  
   440  	return dir, nil
   441  }
   442  
   443  // UserConfigDir returns the default root directory to use for user-specific
   444  // configuration data. Users should create their own application-specific
   445  // subdirectory within this one and use that.
   446  //
   447  // On Unix systems, it returns $XDG_CONFIG_HOME as specified by
   448  // https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html if
   449  // non-empty, else $HOME/.config.
   450  // On Darwin, it returns $HOME/Library/Application Support.
   451  // On Windows, it returns %AppData%.
   452  // On Plan 9, it returns $home/lib.
   453  //
   454  // If the location cannot be determined (for example, $HOME is not defined),
   455  // then it will return an error.
   456  func UserConfigDir() (string, error) {
   457  	var dir string
   458  
   459  	switch runtime.GOOS {
   460  	case "windows":
   461  		dir = Getenv("AppData")
   462  		if dir == "" {
   463  			return "", errors.New("%AppData% is not defined")
   464  		}
   465  
   466  	case "darwin", "ios":
   467  		dir = Getenv("HOME")
   468  		if dir == "" {
   469  			return "", errors.New("$HOME is not defined")
   470  		}
   471  		dir += "/Library/Application Support"
   472  
   473  	case "plan9":
   474  		dir = Getenv("home")
   475  		if dir == "" {
   476  			return "", errors.New("$home is not defined")
   477  		}
   478  		dir += "/lib"
   479  
   480  	default: // Unix
   481  		dir = Getenv("XDG_CONFIG_HOME")
   482  		if dir == "" {
   483  			dir = Getenv("HOME")
   484  			if dir == "" {
   485  				return "", errors.New("neither $XDG_CONFIG_HOME nor $HOME are defined")
   486  			}
   487  			dir += "/.config"
   488  		}
   489  	}
   490  
   491  	return dir, nil
   492  }
   493  
   494  // UserHomeDir returns the current user's home directory.
   495  //
   496  // On Unix, including macOS, it returns the $HOME environment variable.
   497  // On Windows, it returns %USERPROFILE%.
   498  // On Plan 9, it returns the $home environment variable.
   499  func UserHomeDir() (string, error) {
   500  	env, enverr := "HOME", "$HOME"
   501  	switch runtime.GOOS {
   502  	case "windows":
   503  		env, enverr = "USERPROFILE", "%userprofile%"
   504  	case "plan9":
   505  		env, enverr = "home", "$home"
   506  	}
   507  	if v := Getenv(env); v != "" {
   508  		return v, nil
   509  	}
   510  	// On some geese the home directory is not always defined.
   511  	switch runtime.GOOS {
   512  	case "android":
   513  		return "/sdcard", nil
   514  	case "ios":
   515  		return "/", nil
   516  	}
   517  	return "", errors.New(enverr + " is not defined")
   518  }
   519  
   520  // Chmod changes the mode of the named file to mode.
   521  // If the file is a symbolic link, it changes the mode of the link's target.
   522  // If there is an error, it will be of type *PathError.
   523  //
   524  // A different subset of the mode bits are used, depending on the
   525  // operating system.
   526  //
   527  // On Unix, the mode's permission bits, ModeSetuid, ModeSetgid, and
   528  // ModeSticky are used.
   529  //
   530  // On Windows, only the 0200 bit (owner writable) of mode is used; it
   531  // controls whether the file's read-only attribute is set or cleared.
   532  // The other bits are currently unused. For compatibility with Go 1.12
   533  // and earlier, use a non-zero mode. Use mode 0400 for a read-only
   534  // file and 0600 for a readable+writable file.
   535  //
   536  // On Plan 9, the mode's permission bits, ModeAppend, ModeExclusive,
   537  // and ModeTemporary are used.
   538  func Chmod(name string, mode FileMode) error { return chmod(name, mode) }
   539  
   540  // Chmod changes the mode of the file to mode.
   541  // If there is an error, it will be of type *PathError.
   542  func (f *File) Chmod(mode FileMode) error { return f.chmod(mode) }
   543  
   544  // SetDeadline sets the read and write deadlines for a File.
   545  // It is equivalent to calling both SetReadDeadline and SetWriteDeadline.
   546  //
   547  // Only some kinds of files support setting a deadline. Calls to SetDeadline
   548  // for files that do not support deadlines will return ErrNoDeadline.
   549  // On most systems ordinary files do not support deadlines, but pipes do.
   550  //
   551  // A deadline is an absolute time after which I/O operations fail with an
   552  // error instead of blocking. The deadline applies to all future and pending
   553  // I/O, not just the immediately following call to Read or Write.
   554  // After a deadline has been exceeded, the connection can be refreshed
   555  // by setting a deadline in the future.
   556  //
   557  // If the deadline is exceeded a call to Read or Write or to other I/O
   558  // methods will return an error that wraps ErrDeadlineExceeded.
   559  // This can be tested using errors.Is(err, os.ErrDeadlineExceeded).
   560  // That error implements the Timeout method, and calling the Timeout
   561  // method will return true, but there are other possible errors for which
   562  // the Timeout will return true even if the deadline has not been exceeded.
   563  //
   564  // An idle timeout can be implemented by repeatedly extending
   565  // the deadline after successful Read or Write calls.
   566  //
   567  // A zero value for t means I/O operations will not time out.
   568  func (f *File) SetDeadline(t time.Time) error {
   569  	return f.setDeadline(t)
   570  }
   571  
   572  // SetReadDeadline sets the deadline for future Read calls and any
   573  // currently-blocked Read call.
   574  // A zero value for t means Read will not time out.
   575  // Not all files support setting deadlines; see SetDeadline.
   576  func (f *File) SetReadDeadline(t time.Time) error {
   577  	return f.setReadDeadline(t)
   578  }
   579  
   580  // SetWriteDeadline sets the deadline for any future Write calls and any
   581  // currently-blocked Write call.
   582  // Even if Write times out, it may return n > 0, indicating that
   583  // some of the data was successfully written.
   584  // A zero value for t means Write will not time out.
   585  // Not all files support setting deadlines; see SetDeadline.
   586  func (f *File) SetWriteDeadline(t time.Time) error {
   587  	return f.setWriteDeadline(t)
   588  }
   589  
   590  // SyscallConn returns a raw file.
   591  // This implements the syscall.Conn interface.
   592  func (f *File) SyscallConn() (syscall.RawConn, error) {
   593  	if err := f.checkValid("SyscallConn"); err != nil {
   594  		return nil, err
   595  	}
   596  	return newRawConn(f)
   597  }
   598  
   599  // isWindowsNulName reports whether name is os.DevNull ('NUL') on Windows.
   600  // True is returned if name is 'NUL' whatever the case.
   601  func isWindowsNulName(name string) bool {
   602  	if len(name) != 3 {
   603  		return false
   604  	}
   605  	if name[0] != 'n' && name[0] != 'N' {
   606  		return false
   607  	}
   608  	if name[1] != 'u' && name[1] != 'U' {
   609  		return false
   610  	}
   611  	if name[2] != 'l' && name[2] != 'L' {
   612  		return false
   613  	}
   614  	return true
   615  }
   616  
   617  // DirFS returns a file system (an fs.FS) for the tree of files rooted at the directory dir.
   618  //
   619  // Note that DirFS("/prefix") only guarantees that the Open calls it makes to the
   620  // operating system will begin with "/prefix": DirFS("/prefix").Open("file") is the
   621  // same as os.Open("/prefix/file"). So if /prefix/file is a symbolic link pointing outside
   622  // the /prefix tree, then using DirFS does not stop the access any more than using
   623  // os.Open does. Additionally, the root of the fs.FS returned for a relative path,
   624  // DirFS("prefix"), will be affected by later calls to Chdir. DirFS is therefore not
   625  // a general substitute for a chroot-style security mechanism when the directory tree
   626  // contains arbitrary content.
   627  //
   628  // The result implements fs.StatFS.
   629  func DirFS(dir string) fs.FS {
   630  	return dirFS(dir)
   631  }
   632  
   633  func containsAny(s, chars string) bool {
   634  	for i := 0; i < len(s); i++ {
   635  		for j := 0; j < len(chars); j++ {
   636  			if s[i] == chars[j] {
   637  				return true
   638  			}
   639  		}
   640  	}
   641  	return false
   642  }
   643  
   644  type dirFS string
   645  
   646  func (dir dirFS) Open(name string) (fs.File, error) {
   647  	if !fs.ValidPath(name) || runtime.GOOS == "windows" && containsAny(name, `\:`) {
   648  		return nil, &PathError{Op: "open", Path: name, Err: ErrInvalid}
   649  	}
   650  	f, err := Open(string(dir) + "/" + name)
   651  	if err != nil {
   652  		return nil, err // nil fs.File
   653  	}
   654  	return f, nil
   655  }
   656  
   657  func (dir dirFS) Stat(name string) (fs.FileInfo, error) {
   658  	if !fs.ValidPath(name) || runtime.GOOS == "windows" && containsAny(name, `\:`) {
   659  		return nil, &PathError{Op: "stat", Path: name, Err: ErrInvalid}
   660  	}
   661  	f, err := Stat(string(dir) + "/" + name)
   662  	if err != nil {
   663  		return nil, err
   664  	}
   665  	return f, nil
   666  }
   667  
   668  // ReadFile reads the named file and returns the contents.
   669  // A successful call returns err == nil, not err == EOF.
   670  // Because ReadFile reads the whole file, it does not treat an EOF from Read
   671  // as an error to be reported.
   672  func ReadFile(name string) ([]byte, error) {
   673  	f, err := Open(name)
   674  	if err != nil {
   675  		return nil, err
   676  	}
   677  	defer f.Close()
   678  
   679  	var size int
   680  	if info, err := f.Stat(); err == nil {
   681  		size64 := info.Size()
   682  		if int64(int(size64)) == size64 {
   683  			size = int(size64)
   684  		}
   685  	}
   686  	size++ // one byte for final read at EOF
   687  
   688  	// If a file claims a small size, read at least 512 bytes.
   689  	// In particular, files in Linux's /proc claim size 0 but
   690  	// then do not work right if read in small pieces,
   691  	// so an initial read of 1 byte would not work correctly.
   692  	if size < 512 {
   693  		size = 512
   694  	}
   695  
   696  	data := make([]byte, 0, size)
   697  	for {
   698  		if len(data) >= cap(data) {
   699  			d := append(data[:cap(data)], 0)
   700  			data = d[:len(data)]
   701  		}
   702  		n, err := f.Read(data[len(data):cap(data)])
   703  		data = data[:len(data)+n]
   704  		if err != nil {
   705  			if err == io.EOF {
   706  				err = nil
   707  			}
   708  			return data, err
   709  		}
   710  	}
   711  }
   712  
   713  // WriteFile writes data to the named file, creating it if necessary.
   714  // If the file does not exist, WriteFile creates it with permissions perm (before umask);
   715  // otherwise WriteFile truncates it before writing, without changing permissions.
   716  func WriteFile(name string, data []byte, perm FileMode) error {
   717  	f, err := OpenFile(name, O_WRONLY|O_CREATE|O_TRUNC, perm)
   718  	if err != nil {
   719  		return err
   720  	}
   721  	_, err = f.Write(data)
   722  	if err1 := f.Close(); err1 != nil && err == nil {
   723  		err = err1
   724  	}
   725  	return err
   726  }
   727  

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