fsys.go

Documentation: cmd/go/internal/fsys

     1  // Copyright 2020 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 fsys is an abstraction for reading files that
     6  // allows for virtual overlays on top of the files on disk.
     7  package fsys
     8  
     9  import (
    10  	"encoding/json"
    11  	"errors"
    12  	"fmt"
    13  	"internal/godebug"
    14  	"io/fs"
    15  	"log"
    16  	"os"
    17  	pathpkg "path"
    18  	"path/filepath"
    19  	"runtime"
    20  	"runtime/debug"
    21  	"sort"
    22  	"strings"
    23  	"sync"
    24  	"time"
    25  )
    26  
    27  // Trace emits a trace event for the operation and file path to the trace log,
    28  // but only when $GODEBUG contains gofsystrace=1.
    29  // The traces are appended to the file named by the $GODEBUG setting gofsystracelog, or else standard error.
    30  // For debugging, if the $GODEBUG setting gofsystracestack is non-empty, then trace events for paths
    31  // matching that glob pattern (using path.Match) will be followed by a full stack trace.
    32  func Trace(op, path string) {
    33  	if !doTrace {
    34  		return
    35  	}
    36  	traceMu.Lock()
    37  	defer traceMu.Unlock()
    38  	fmt.Fprintf(traceFile, "%d gofsystrace %s %s\n", os.Getpid(), op, path)
    39  	if pattern := gofsystracestack.Value(); pattern != "" {
    40  		if match, _ := pathpkg.Match(pattern, path); match {
    41  			traceFile.Write(debug.Stack())
    42  		}
    43  	}
    44  }
    45  
    46  var (
    47  	doTrace   bool
    48  	traceFile *os.File
    49  	traceMu   sync.Mutex
    50  
    51  	gofsystrace      = godebug.New("#gofsystrace")
    52  	gofsystracelog   = godebug.New("#gofsystracelog")
    53  	gofsystracestack = godebug.New("#gofsystracestack")
    54  )
    55  
    56  func init() {
    57  	if gofsystrace.Value() != "1" {
    58  		return
    59  	}
    60  	doTrace = true
    61  	if f := gofsystracelog.Value(); f != "" {
    62  		// Note: No buffering on writes to this file, so no need to worry about closing it at exit.
    63  		var err error
    64  		traceFile, err = os.OpenFile(f, os.O_WRONLY|os.O_APPEND|os.O_CREATE, 0666)
    65  		if err != nil {
    66  			log.Fatal(err)
    67  		}
    68  	} else {
    69  		traceFile = os.Stderr
    70  	}
    71  }
    72  
    73  // OverlayFile is the path to a text file in the OverlayJSON format.
    74  // It is the value of the -overlay flag.
    75  var OverlayFile string
    76  
    77  // OverlayJSON is the format overlay files are expected to be in.
    78  // The Replace map maps from overlaid paths to replacement paths:
    79  // the Go command will forward all reads trying to open
    80  // each overlaid path to its replacement path, or consider the overlaid
    81  // path not to exist if the replacement path is empty.
    82  type OverlayJSON struct {
    83  	Replace map[string]string
    84  }
    85  
    86  type node struct {
    87  	actualFilePath string           // empty if a directory
    88  	children       map[string]*node // path element → file or directory
    89  }
    90  
    91  func (n *node) isDir() bool {
    92  	return n.actualFilePath == "" && n.children != nil
    93  }
    94  
    95  func (n *node) isDeleted() bool {
    96  	return n.actualFilePath == "" && n.children == nil
    97  }
    98  
    99  // TODO(matloob): encapsulate these in an io/fs-like interface
   100  var overlay map[string]*node // path -> file or directory node
   101  var cwd string               // copy of base.Cwd() to avoid dependency
   102  
   103  // canonicalize a path for looking it up in the overlay.
   104  // Important: filepath.Join(cwd, path) doesn't always produce
   105  // the correct absolute path if path is relative, because on
   106  // Windows producing the correct absolute path requires making
   107  // a syscall. So this should only be used when looking up paths
   108  // in the overlay, or canonicalizing the paths in the overlay.
   109  func canonicalize(path string) string {
   110  	if path == "" {
   111  		return ""
   112  	}
   113  	if filepath.IsAbs(path) {
   114  		return filepath.Clean(path)
   115  	}
   116  
   117  	if v := filepath.VolumeName(cwd); v != "" && path[0] == filepath.Separator {
   118  		// On Windows filepath.Join(cwd, path) doesn't always work. In general
   119  		// filepath.Abs needs to make a syscall on Windows. Elsewhere in cmd/go
   120  		// use filepath.Join(cwd, path), but cmd/go specifically supports Windows
   121  		// paths that start with "\" which implies the path is relative to the
   122  		// volume of the working directory. See golang.org/issue/8130.
   123  		return filepath.Join(v, path)
   124  	}
   125  
   126  	// Make the path absolute.
   127  	return filepath.Join(cwd, path)
   128  }
   129  
   130  // Init initializes the overlay, if one is being used.
   131  func Init(wd string) error {
   132  	if overlay != nil {
   133  		// already initialized
   134  		return nil
   135  	}
   136  
   137  	cwd = wd
   138  
   139  	if OverlayFile == "" {
   140  		return nil
   141  	}
   142  
   143  	Trace("ReadFile", OverlayFile)
   144  	b, err := os.ReadFile(OverlayFile)
   145  	if err != nil {
   146  		return fmt.Errorf("reading overlay file: %v", err)
   147  	}
   148  
   149  	var overlayJSON OverlayJSON
   150  	if err := json.Unmarshal(b, &overlayJSON); err != nil {
   151  		return fmt.Errorf("parsing overlay JSON: %v", err)
   152  	}
   153  
   154  	return initFromJSON(overlayJSON)
   155  }
   156  
   157  func initFromJSON(overlayJSON OverlayJSON) error {
   158  	// Canonicalize the paths in the overlay map.
   159  	// Use reverseCanonicalized to check for collisions:
   160  	// no two 'from' paths should canonicalize to the same path.
   161  	overlay = make(map[string]*node)
   162  	reverseCanonicalized := make(map[string]string) // inverse of canonicalize operation, to check for duplicates
   163  	// Build a table of file and directory nodes from the replacement map.
   164  
   165  	// Remove any potential non-determinism from iterating over map by sorting it.
   166  	replaceFrom := make([]string, 0, len(overlayJSON.Replace))
   167  	for k := range overlayJSON.Replace {
   168  		replaceFrom = append(replaceFrom, k)
   169  	}
   170  	sort.Strings(replaceFrom)
   171  
   172  	for _, from := range replaceFrom {
   173  		to := overlayJSON.Replace[from]
   174  		// Canonicalize paths and check for a collision.
   175  		if from == "" {
   176  			return fmt.Errorf("empty string key in overlay file Replace map")
   177  		}
   178  		cfrom := canonicalize(from)
   179  		if to != "" {
   180  			// Don't canonicalize "", meaning to delete a file, because then it will turn into ".".
   181  			to = canonicalize(to)
   182  		}
   183  		if otherFrom, seen := reverseCanonicalized[cfrom]; seen {
   184  			return fmt.Errorf(
   185  				"paths %q and %q both canonicalize to %q in overlay file Replace map", otherFrom, from, cfrom)
   186  		}
   187  		reverseCanonicalized[cfrom] = from
   188  		from = cfrom
   189  
   190  		// Create node for overlaid file.
   191  		dir, base := filepath.Dir(from), filepath.Base(from)
   192  		if n, ok := overlay[from]; ok {
   193  			// All 'from' paths in the overlay are file paths. Since the from paths
   194  			// are in a map, they are unique, so if the node already exists we added
   195  			// it below when we create parent directory nodes. That is, that
   196  			// both a file and a path to one of its parent directories exist as keys
   197  			// in the Replace map.
   198  			//
   199  			// This only applies if the overlay directory has any files or directories
   200  			// in it: placeholder directories that only contain deleted files don't
   201  			// count. They are safe to be overwritten with actual files.
   202  			for _, f := range n.children {
   203  				if !f.isDeleted() {
   204  					return fmt.Errorf("invalid overlay: path %v is used as both file and directory", from)
   205  				}
   206  			}
   207  		}
   208  		overlay[from] = &node{actualFilePath: to}
   209  
   210  		// Add parent directory nodes to overlay structure.
   211  		childNode := overlay[from]
   212  		for {
   213  			dirNode := overlay[dir]
   214  			if dirNode == nil || dirNode.isDeleted() {
   215  				dirNode = &node{children: make(map[string]*node)}
   216  				overlay[dir] = dirNode
   217  			}
   218  			if childNode.isDeleted() {
   219  				// Only create one parent for a deleted file:
   220  				// the directory only conditionally exists if
   221  				// there are any non-deleted children, so
   222  				// we don't create their parents.
   223  				if dirNode.isDir() {
   224  					dirNode.children[base] = childNode
   225  				}
   226  				break
   227  			}
   228  			if !dirNode.isDir() {
   229  				// This path already exists as a file, so it can't be a parent
   230  				// directory. See comment at error above.
   231  				return fmt.Errorf("invalid overlay: path %v is used as both file and directory", dir)
   232  			}
   233  			dirNode.children[base] = childNode
   234  			parent := filepath.Dir(dir)
   235  			if parent == dir {
   236  				break // reached the top; there is no parent
   237  			}
   238  			dir, base = parent, filepath.Base(dir)
   239  			childNode = dirNode
   240  		}
   241  	}
   242  
   243  	return nil
   244  }
   245  
   246  // IsDir returns true if path is a directory on disk or in the
   247  // overlay.
   248  func IsDir(path string) (bool, error) {
   249  	Trace("IsDir", path)
   250  	path = canonicalize(path)
   251  
   252  	if _, ok := parentIsOverlayFile(path); ok {
   253  		return false, nil
   254  	}
   255  
   256  	if n, ok := overlay[path]; ok {
   257  		return n.isDir(), nil
   258  	}
   259  
   260  	fi, err := os.Stat(path)
   261  	if err != nil {
   262  		return false, err
   263  	}
   264  
   265  	return fi.IsDir(), nil
   266  }
   267  
   268  // parentIsOverlayFile returns whether name or any of
   269  // its parents are files in the overlay, and the first parent found,
   270  // including name itself, that's a file in the overlay.
   271  func parentIsOverlayFile(name string) (string, bool) {
   272  	if overlay != nil {
   273  		// Check if name can't possibly be a directory because
   274  		// it or one of its parents is overlaid with a file.
   275  		// TODO(matloob): Maybe save this to avoid doing it every time?
   276  		prefix := name
   277  		for {
   278  			node := overlay[prefix]
   279  			if node != nil && !node.isDir() {
   280  				return prefix, true
   281  			}
   282  			parent := filepath.Dir(prefix)
   283  			if parent == prefix {
   284  				break
   285  			}
   286  			prefix = parent
   287  		}
   288  	}
   289  
   290  	return "", false
   291  }
   292  
   293  // errNotDir is used to communicate from ReadDir to IsDirWithGoFiles
   294  // that the argument is not a directory, so that IsDirWithGoFiles doesn't
   295  // return an error.
   296  var errNotDir = errors.New("not a directory")
   297  
   298  func nonFileInOverlayError(overlayPath string) error {
   299  	return fmt.Errorf("replacement path %q is a directory, not a file", overlayPath)
   300  }
   301  
   302  // readDir reads a dir on disk, returning an error that is errNotDir if the dir is not a directory.
   303  // Unfortunately, the error returned by os.ReadDir if dir is not a directory
   304  // can vary depending on the OS (Linux, Mac, Windows return ENOTDIR; BSD returns EINVAL).
   305  func readDir(dir string) ([]fs.FileInfo, error) {
   306  	entries, err := os.ReadDir(dir)
   307  	if err != nil {
   308  		if os.IsNotExist(err) {
   309  			return nil, err
   310  		}
   311  		if dirfi, staterr := os.Stat(dir); staterr == nil && !dirfi.IsDir() {
   312  			return nil, &fs.PathError{Op: "ReadDir", Path: dir, Err: errNotDir}
   313  		}
   314  		return nil, err
   315  	}
   316  
   317  	fis := make([]fs.FileInfo, 0, len(entries))
   318  	for _, entry := range entries {
   319  		info, err := entry.Info()
   320  		if err != nil {
   321  			continue
   322  		}
   323  		fis = append(fis, info)
   324  	}
   325  	return fis, nil
   326  }
   327  
   328  // ReadDir provides a slice of fs.FileInfo entries corresponding
   329  // to the overlaid files in the directory.
   330  func ReadDir(dir string) ([]fs.FileInfo, error) {
   331  	Trace("ReadDir", dir)
   332  	dir = canonicalize(dir)
   333  	if _, ok := parentIsOverlayFile(dir); ok {
   334  		return nil, &fs.PathError{Op: "ReadDir", Path: dir, Err: errNotDir}
   335  	}
   336  
   337  	dirNode := overlay[dir]
   338  	if dirNode == nil {
   339  		return readDir(dir)
   340  	}
   341  	if dirNode.isDeleted() {
   342  		return nil, &fs.PathError{Op: "ReadDir", Path: dir, Err: fs.ErrNotExist}
   343  	}
   344  	diskfis, err := readDir(dir)
   345  	if err != nil && !os.IsNotExist(err) && !errors.Is(err, errNotDir) {
   346  		return nil, err
   347  	}
   348  
   349  	// Stat files in overlay to make composite list of fileinfos
   350  	files := make(map[string]fs.FileInfo)
   351  	for _, f := range diskfis {
   352  		files[f.Name()] = f
   353  	}
   354  	for name, to := range dirNode.children {
   355  		switch {
   356  		case to.isDir():
   357  			files[name] = fakeDir(name)
   358  		case to.isDeleted():
   359  			delete(files, name)
   360  		default:
   361  			// To keep the data model simple, if the overlay contains a symlink we
   362  			// always stat through it (using Stat, not Lstat). That way we don't need
   363  			// to worry about the interaction between Lstat and directories: if a
   364  			// symlink in the overlay points to a directory, we reject it like an
   365  			// ordinary directory.
   366  			fi, err := os.Stat(to.actualFilePath)
   367  			if err != nil {
   368  				files[name] = missingFile(name)
   369  				continue
   370  			} else if fi.IsDir() {
   371  				return nil, &fs.PathError{Op: "Stat", Path: filepath.Join(dir, name), Err: nonFileInOverlayError(to.actualFilePath)}
   372  			}
   373  			// Add a fileinfo for the overlaid file, so that it has
   374  			// the original file's name, but the overlaid file's metadata.
   375  			files[name] = fakeFile{name, fi}
   376  		}
   377  	}
   378  	sortedFiles := diskfis[:0]
   379  	for _, f := range files {
   380  		sortedFiles = append(sortedFiles, f)
   381  	}
   382  	sort.Slice(sortedFiles, func(i, j int) bool { return sortedFiles[i].Name() < sortedFiles[j].Name() })
   383  	return sortedFiles, nil
   384  }
   385  
   386  // OverlayPath returns the path to the overlaid contents of the
   387  // file, the empty string if the overlay deletes the file, or path
   388  // itself if the file is not in the overlay, the file is a directory
   389  // in the overlay, or there is no overlay.
   390  // It returns true if the path is overlaid with a regular file
   391  // or deleted, and false otherwise.
   392  func OverlayPath(path string) (string, bool) {
   393  	if p, ok := overlay[canonicalize(path)]; ok && !p.isDir() {
   394  		return p.actualFilePath, ok
   395  	}
   396  
   397  	return path, false
   398  }
   399  
   400  // Open opens the file at or overlaid on the given path.
   401  func Open(path string) (*os.File, error) {
   402  	Trace("Open", path)
   403  	return openFile(path, os.O_RDONLY, 0)
   404  }
   405  
   406  // OpenFile opens the file at or overlaid on the given path with the flag and perm.
   407  func OpenFile(path string, flag int, perm os.FileMode) (*os.File, error) {
   408  	Trace("OpenFile", path)
   409  	return openFile(path, flag, perm)
   410  }
   411  
   412  func openFile(path string, flag int, perm os.FileMode) (*os.File, error) {
   413  	cpath := canonicalize(path)
   414  	if node, ok := overlay[cpath]; ok {
   415  		// Opening a file in the overlay.
   416  		if node.isDir() {
   417  			return nil, &fs.PathError{Op: "OpenFile", Path: path, Err: errors.New("fsys.OpenFile doesn't support opening directories yet")}
   418  		}
   419  		// We can't open overlaid paths for write.
   420  		if perm != os.FileMode(os.O_RDONLY) {
   421  			return nil, &fs.PathError{Op: "OpenFile", Path: path, Err: errors.New("overlaid files can't be opened for write")}
   422  		}
   423  		return os.OpenFile(node.actualFilePath, flag, perm)
   424  	}
   425  	if parent, ok := parentIsOverlayFile(filepath.Dir(cpath)); ok {
   426  		// The file is deleted explicitly in the Replace map,
   427  		// or implicitly because one of its parent directories was
   428  		// replaced by a file.
   429  		return nil, &fs.PathError{
   430  			Op:   "Open",
   431  			Path: path,
   432  			Err:  fmt.Errorf("file %s does not exist: parent directory %s is replaced by a file in overlay", path, parent),
   433  		}
   434  	}
   435  	return os.OpenFile(cpath, flag, perm)
   436  }
   437  
   438  // IsDirWithGoFiles reports whether dir is a directory containing Go files
   439  // either on disk or in the overlay.
   440  func IsDirWithGoFiles(dir string) (bool, error) {
   441  	Trace("IsDirWithGoFiles", dir)
   442  	fis, err := ReadDir(dir)
   443  	if os.IsNotExist(err) || errors.Is(err, errNotDir) {
   444  		return false, nil
   445  	}
   446  	if err != nil {
   447  		return false, err
   448  	}
   449  
   450  	var firstErr error
   451  	for _, fi := range fis {
   452  		if fi.IsDir() {
   453  			continue
   454  		}
   455  
   456  		// TODO(matloob): this enforces that the "from" in the map
   457  		// has a .go suffix, but the actual destination file
   458  		// doesn't need to have a .go suffix. Is this okay with the
   459  		// compiler?
   460  		if !strings.HasSuffix(fi.Name(), ".go") {
   461  			continue
   462  		}
   463  		if fi.Mode().IsRegular() {
   464  			return true, nil
   465  		}
   466  
   467  		// fi is the result of an Lstat, so it doesn't follow symlinks.
   468  		// But it's okay if the file is a symlink pointing to a regular
   469  		// file, so use os.Stat to follow symlinks and check that.
   470  		actualFilePath, _ := OverlayPath(filepath.Join(dir, fi.Name()))
   471  		fi, err := os.Stat(actualFilePath)
   472  		if err == nil && fi.Mode().IsRegular() {
   473  			return true, nil
   474  		}
   475  		if err != nil && firstErr == nil {
   476  			firstErr = err
   477  		}
   478  	}
   479  
   480  	// No go files found in directory.
   481  	return false, firstErr
   482  }
   483  
   484  // walk recursively descends path, calling walkFn. Copied, with some
   485  // modifications from path/filepath.walk.
   486  func walk(path string, info fs.FileInfo, walkFn filepath.WalkFunc) error {
   487  	if err := walkFn(path, info, nil); err != nil || !info.IsDir() {
   488  		return err
   489  	}
   490  
   491  	fis, err := ReadDir(path)
   492  	if err != nil {
   493  		return walkFn(path, info, err)
   494  	}
   495  
   496  	for _, fi := range fis {
   497  		filename := filepath.Join(path, fi.Name())
   498  		if err := walk(filename, fi, walkFn); err != nil {
   499  			if !fi.IsDir() || err != filepath.SkipDir {
   500  				return err
   501  			}
   502  		}
   503  	}
   504  	return nil
   505  }
   506  
   507  // Walk walks the file tree rooted at root, calling walkFn for each file or
   508  // directory in the tree, including root.
   509  func Walk(root string, walkFn filepath.WalkFunc) error {
   510  	Trace("Walk", root)
   511  	info, err := Lstat(root)
   512  	if err != nil {
   513  		err = walkFn(root, nil, err)
   514  	} else {
   515  		err = walk(root, info, walkFn)
   516  	}
   517  	if err == filepath.SkipDir {
   518  		return nil
   519  	}
   520  	return err
   521  }
   522  
   523  // Lstat implements a version of os.Lstat that operates on the overlay filesystem.
   524  func Lstat(path string) (fs.FileInfo, error) {
   525  	Trace("Lstat", path)
   526  	return overlayStat(path, os.Lstat, "lstat")
   527  }
   528  
   529  // Stat implements a version of os.Stat that operates on the overlay filesystem.
   530  func Stat(path string) (fs.FileInfo, error) {
   531  	Trace("Stat", path)
   532  	return overlayStat(path, os.Stat, "stat")
   533  }
   534  
   535  // overlayStat implements lstat or Stat (depending on whether os.Lstat or os.Stat is passed in).
   536  func overlayStat(path string, osStat func(string) (fs.FileInfo, error), opName string) (fs.FileInfo, error) {
   537  	cpath := canonicalize(path)
   538  
   539  	if _, ok := parentIsOverlayFile(filepath.Dir(cpath)); ok {
   540  		return nil, &fs.PathError{Op: opName, Path: cpath, Err: fs.ErrNotExist}
   541  	}
   542  
   543  	node, ok := overlay[cpath]
   544  	if !ok {
   545  		// The file or directory is not overlaid.
   546  		return osStat(path)
   547  	}
   548  
   549  	switch {
   550  	case node.isDeleted():
   551  		return nil, &fs.PathError{Op: opName, Path: cpath, Err: fs.ErrNotExist}
   552  	case node.isDir():
   553  		return fakeDir(filepath.Base(path)), nil
   554  	default:
   555  		// To keep the data model simple, if the overlay contains a symlink we
   556  		// always stat through it (using Stat, not Lstat). That way we don't need to
   557  		// worry about the interaction between Lstat and directories: if a symlink
   558  		// in the overlay points to a directory, we reject it like an ordinary
   559  		// directory.
   560  		fi, err := os.Stat(node.actualFilePath)
   561  		if err != nil {
   562  			return nil, err
   563  		}
   564  		if fi.IsDir() {
   565  			return nil, &fs.PathError{Op: opName, Path: cpath, Err: nonFileInOverlayError(node.actualFilePath)}
   566  		}
   567  		return fakeFile{name: filepath.Base(path), real: fi}, nil
   568  	}
   569  }
   570  
   571  // fakeFile provides an fs.FileInfo implementation for an overlaid file,
   572  // so that the file has the name of the overlaid file, but takes all
   573  // other characteristics of the replacement file.
   574  type fakeFile struct {
   575  	name string
   576  	real fs.FileInfo
   577  }
   578  
   579  func (f fakeFile) Name() string       { return f.name }
   580  func (f fakeFile) Size() int64        { return f.real.Size() }
   581  func (f fakeFile) Mode() fs.FileMode  { return f.real.Mode() }
   582  func (f fakeFile) ModTime() time.Time { return f.real.ModTime() }
   583  func (f fakeFile) IsDir() bool        { return f.real.IsDir() }
   584  func (f fakeFile) Sys() any           { return f.real.Sys() }
   585  
   586  func (f fakeFile) String() string {
   587  	return fs.FormatFileInfo(f)
   588  }
   589  
   590  // missingFile provides an fs.FileInfo for an overlaid file where the
   591  // destination file in the overlay doesn't exist. It returns zero values
   592  // for the fileInfo methods other than Name, set to the file's name, and Mode
   593  // set to ModeIrregular.
   594  type missingFile string
   595  
   596  func (f missingFile) Name() string       { return string(f) }
   597  func (f missingFile) Size() int64        { return 0 }
   598  func (f missingFile) Mode() fs.FileMode  { return fs.ModeIrregular }
   599  func (f missingFile) ModTime() time.Time { return time.Unix(0, 0) }
   600  func (f missingFile) IsDir() bool        { return false }
   601  func (f missingFile) Sys() any           { return nil }
   602  
   603  func (f missingFile) String() string {
   604  	return fs.FormatFileInfo(f)
   605  }
   606  
   607  // fakeDir provides an fs.FileInfo implementation for directories that are
   608  // implicitly created by overlaid files. Each directory in the
   609  // path of an overlaid file is considered to exist in the overlay filesystem.
   610  type fakeDir string
   611  
   612  func (f fakeDir) Name() string       { return string(f) }
   613  func (f fakeDir) Size() int64        { return 0 }
   614  func (f fakeDir) Mode() fs.FileMode  { return fs.ModeDir | 0500 }
   615  func (f fakeDir) ModTime() time.Time { return time.Unix(0, 0) }
   616  func (f fakeDir) IsDir() bool        { return true }
   617  func (f fakeDir) Sys() any           { return nil }
   618  
   619  func (f fakeDir) String() string {
   620  	return fs.FormatFileInfo(f)
   621  }
   622  
   623  // Glob is like filepath.Glob but uses the overlay file system.
   624  func Glob(pattern string) (matches []string, err error) {
   625  	Trace("Glob", pattern)
   626  	// Check pattern is well-formed.
   627  	if _, err := filepath.Match(pattern, ""); err != nil {
   628  		return nil, err
   629  	}
   630  	if !hasMeta(pattern) {
   631  		if _, err = Lstat(pattern); err != nil {
   632  			return nil, nil
   633  		}
   634  		return []string{pattern}, nil
   635  	}
   636  
   637  	dir, file := filepath.Split(pattern)
   638  	volumeLen := 0
   639  	if runtime.GOOS == "windows" {
   640  		volumeLen, dir = cleanGlobPathWindows(dir)
   641  	} else {
   642  		dir = cleanGlobPath(dir)
   643  	}
   644  
   645  	if !hasMeta(dir[volumeLen:]) {
   646  		return glob(dir, file, nil)
   647  	}
   648  
   649  	// Prevent infinite recursion. See issue 15879.
   650  	if dir == pattern {
   651  		return nil, filepath.ErrBadPattern
   652  	}
   653  
   654  	var m []string
   655  	m, err = Glob(dir)
   656  	if err != nil {
   657  		return
   658  	}
   659  	for _, d := range m {
   660  		matches, err = glob(d, file, matches)
   661  		if err != nil {
   662  			return
   663  		}
   664  	}
   665  	return
   666  }
   667  
   668  // cleanGlobPath prepares path for glob matching.
   669  func cleanGlobPath(path string) string {
   670  	switch path {
   671  	case "":
   672  		return "."
   673  	case string(filepath.Separator):
   674  		// do nothing to the path
   675  		return path
   676  	default:
   677  		return path[0 : len(path)-1] // chop off trailing separator
   678  	}
   679  }
   680  
   681  func volumeNameLen(path string) int {
   682  	isSlash := func(c uint8) bool {
   683  		return c == '\\' || c == '/'
   684  	}
   685  	if len(path) < 2 {
   686  		return 0
   687  	}
   688  	// with drive letter
   689  	c := path[0]
   690  	if path[1] == ':' && ('a' <= c && c <= 'z' || 'A' <= c && c <= 'Z') {
   691  		return 2
   692  	}
   693  	// is it UNC? https://learn.microsoft.com/en-us/windows/win32/fileio/naming-a-file
   694  	if l := len(path); l >= 5 && isSlash(path[0]) && isSlash(path[1]) &&
   695  		!isSlash(path[2]) && path[2] != '.' {
   696  		// first, leading `\\` and next shouldn't be `\`. its server name.
   697  		for n := 3; n < l-1; n++ {
   698  			// second, next '\' shouldn't be repeated.
   699  			if isSlash(path[n]) {
   700  				n++
   701  				// third, following something characters. its share name.
   702  				if !isSlash(path[n]) {
   703  					if path[n] == '.' {
   704  						break
   705  					}
   706  					for ; n < l; n++ {
   707  						if isSlash(path[n]) {
   708  							break
   709  						}
   710  					}
   711  					return n
   712  				}
   713  				break
   714  			}
   715  		}
   716  	}
   717  	return 0
   718  }
   719  
   720  // cleanGlobPathWindows is windows version of cleanGlobPath.
   721  func cleanGlobPathWindows(path string) (prefixLen int, cleaned string) {
   722  	vollen := volumeNameLen(path)
   723  	switch {
   724  	case path == "":
   725  		return 0, "."
   726  	case vollen+1 == len(path) && os.IsPathSeparator(path[len(path)-1]): // /, \, C:\ and C:/
   727  		// do nothing to the path
   728  		return vollen + 1, path
   729  	case vollen == len(path) && len(path) == 2: // C:
   730  		return vollen, path + "." // convert C: into C:.
   731  	default:
   732  		if vollen >= len(path) {
   733  			vollen = len(path) - 1
   734  		}
   735  		return vollen, path[0 : len(path)-1] // chop off trailing separator
   736  	}
   737  }
   738  
   739  // glob searches for files matching pattern in the directory dir
   740  // and appends them to matches. If the directory cannot be
   741  // opened, it returns the existing matches. New matches are
   742  // added in lexicographical order.
   743  func glob(dir, pattern string, matches []string) (m []string, e error) {
   744  	m = matches
   745  	fi, err := Stat(dir)
   746  	if err != nil {
   747  		return // ignore I/O error
   748  	}
   749  	if !fi.IsDir() {
   750  		return // ignore I/O error
   751  	}
   752  
   753  	list, err := ReadDir(dir)
   754  	if err != nil {
   755  		return // ignore I/O error
   756  	}
   757  
   758  	var names []string
   759  	for _, info := range list {
   760  		names = append(names, info.Name())
   761  	}
   762  	sort.Strings(names)
   763  
   764  	for _, n := range names {
   765  		matched, err := filepath.Match(pattern, n)
   766  		if err != nil {
   767  			return m, err
   768  		}
   769  		if matched {
   770  			m = append(m, filepath.Join(dir, n))
   771  		}
   772  	}
   773  	return
   774  }
   775  
   776  // hasMeta reports whether path contains any of the magic characters
   777  // recognized by filepath.Match.
   778  func hasMeta(path string) bool {
   779  	magicChars := `*?[`
   780  	if runtime.GOOS != "windows" {
   781  		magicChars = `*?[\`
   782  	}
   783  	return strings.ContainsAny(path, magicChars)
   784  }
   785
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