path/filepath: add WalkDir

WalkDir is like Walk but can use ReadDir to read directories,
instead of Readdirnames + Lstat on every entry,
which is usually a significant performance improvement.
(The Lstat can still happen if the walk function calls d.Info.)

Fixes #42027.

Change-Id: Ie11024b23be2656e320d41fd81ff0d8810aa729e
Reviewed-on: https://go-review.googlesource.com/c/go/+/266240
Trust: Russ Cox <[email protected]>
Run-TryBot: Russ Cox <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Rob Pike <[email protected]>

Author: rsc

src/path/filepath/export_test.go

@@ -5,3 +5,5 @@
 package filepath
 
 var LstatP = &lstat
+
+type DirEntryFromInfo = dirEntryFromInfo

src/path/filepath/path.go

@@ -336,25 +336,130 @@ func Rel(basepath, targpath string) (string, error) {
 // as an error by any function.
 var SkipDir = errors.New("skip this directory")
 
-// WalkFunc is the type of the function called for each file or directory
-// visited by Walk. The path argument contains the argument to Walk as a
-// prefix; that is, if Walk is called with "dir", which is a directory
-// containing the file "a", the walk function will be called with argument
-// "dir/a". The info argument is the fs.FileInfo for the named path.
-//
-// If there was a problem walking to the file or directory named by path, the
-// incoming error will describe the problem and the function can decide how
-// to handle that error (and Walk will not descend into that directory). In the
-// case of an error, the info argument will be nil. If an error is returned,
-// processing stops. The sole exception is when the function returns the special
-// value SkipDir. If the function returns SkipDir when invoked on a directory,
-// Walk skips the directory's contents entirely. If the function returns SkipDir
-// when invoked on a non-directory file, Walk skips the remaining files in the
-// containing directory.
+// WalkDirFunc is the type of the function called by WalkDir to visit
+// each each file or directory.
+//
+// The path argument contains the argument to Walk as a prefix.
+// That is, if Walk is called with root argument "dir" and finds a file
+// named "a" in that directory, the walk function will be called with
+// argument "dir/a".
+//
+// The directory and file are joined with Join, which may clean the
+// directory name: if Walk is called with the root argument "x/../dir"
+// and finds a file named "a" in that directory, the walk function will
+// be called with argument "dir/a", not "x/../dir/a".
+//
+// The d argument is the fs.DirEntry for the named path.
+//
+// The error result returned by the function controls how WalkDir
+// continues. If the function returns the special value SkipDir, WalkDir
+// skips the current directory (path if d.IsDir() is true, otherwise
+// path's parent directory). Otherwise, if the function returns a non-nil
+// error, WalkDir stops entirely and returns that error.
+//
+// The err argument reports an error related to path, signaling that
+// WalkDir will not walk into that directory. The function can decide how
+// to handle that error; as described earlier, returning the error will
+// cause WalkDir to stop walking the entire tree.
+//
+// WalkDir calls the function with a non-nil err argument in two cases.
+//
+// First, if the initial os.Lstat on the root directory fails, WalkDir
+// calls the function with path set to root, d set to nil, and err set to
+// the error from os.Lstat.
+//
+// Second, if a directory's ReadDir method fails, WalkDir calls the
+// function with path set to the directory's path, d set to an
+// fs.DirEntry describing the directory, and err set to the error from
+// ReadDir. In this second case, the function is called twice with the
+// path of the directory: the first call is before the directory read is
+// attempted and has err set to nil, giving the function a chance to
+// return SkipDir and avoid the ReadDir entirely. The second call is
+// after a failed ReadDir and reports the error from ReadDir.
+// (If ReadDir succeeds, there is no second call.)
+//
+// The differences between WalkDirFunc compared to WalkFunc are:
+//
+//   - The second argument has type fs.DirEntry instead of fs.FileInfo.
+//   - The function is called before reading a directory, to allow SkipDir
+//     to bypass the directory read entirely.
+//   - If a directory read fails, the function is called a second time
+//     for that directory to report the error.
+//
+type WalkDirFunc func(path string, d fs.DirEntry, err error) error
+
+// WalkFunc is the type of the function called by Walk to visit each each
+// file or directory.
+//
+// The path argument contains the argument to Walk as a prefix.
+// That is, if Walk is called with root argument "dir" and finds a file
+// named "a" in that directory, the walk function will be called with
+// argument "dir/a".
+//
+// The directory and file are joined with Join, which may clean the
+// directory name: if Walk is called with the root argument "x/../dir"
+// and finds a file named "a" in that directory, the walk function will
+// be called with argument "dir/a", not "x/../dir/a".
+//
+// The info argument is the fs.FileInfo for the named path.
+//
+// The error result returned by the function controls how Walk continues.
+// If the function returns the special value SkipDir, Walk skips the
+// current directory (path if info.IsDir() is true, otherwise path's
+// parent directory). Otherwise, if the function returns a non-nil error,
+// Walk stops entirely and returns that error.
+//
+// The err argument reports an error related to path, signaling that Walk
+// will not walk into that directory. The function can decide how to
+// handle that error; as described earlier, returning the error will
+// cause Walk to stop walking the entire tree.
+//
+// Walk calls the function with a non-nil err argument in two cases.
+//
+// First, if an os.Lstat on the root directory or any directory or file
+// in the tree fails, Walk calls the function with path set to that
+// directory or file's path, info set to nil, and err set to the error
+// from os.Lstat.
+//
+// Second, if a directory's Readdirnames method fails, Walk calls the
+// function with path set to the directory's path, info, set to an
+// fs.FileInfo describing the directory, and err set to the error from
+// Readdirnames.
 type WalkFunc func(path string, info fs.FileInfo, err error) error
 
 var lstat = os.Lstat // for testing
 
+// walkDir recursively descends path, calling walkDirFn.
+func walkDir(path string, d fs.DirEntry, walkDirFn WalkDirFunc) error {
+	if err := walkDirFn(path, d, nil); err != nil || !d.IsDir() {
+		if err == SkipDir && d.IsDir() {
+			// Successfully skipped directory.
+			err = nil
+		}
+		return err
+	}
+
+	dirs, err := readDir(path)
+	if err != nil {
+		// Second call, to report ReadDir error.
+		err = walkDirFn(path, d, err)
+		if err != nil {
+			return err
+		}
+	}
+
+	for _, d1 := range dirs {
+		path1 := Join(path, d1.Name())
+		if err := walkDir(path1, d1, walkDirFn); err != nil {
+			if err == SkipDir {
+				break
+			}
+			return err
+		}
+	}
+	return nil
+}
+
 // walk recursively descends path, calling walkFn.
 func walk(path string, info fs.FileInfo, walkFn WalkFunc) error {
 	if !info.IsDir() {
@@ -393,27 +498,87 @@ func walk(path string, info fs.FileInfo, walkFn WalkFunc) error {
 	return nil
 }
 
-// Walk walks the file tree rooted at root, calling walkFn for each file or
-// directory in the tree, including root. All errors that arise visiting files
-// and directories are filtered by walkFn. The files are walked in lexical
-// order, which makes the output deterministic but means that for very
-// large directories Walk can be inefficient.
+// WalkDir walks the file tree rooted at root, calling fn for each file or
+// directory in the tree, including root.
+//
+// All errors that arise visiting files and directories are filtered by fn:
+// see the WalkDirFunc documentation for details.
+//
+// The files are walked in lexical order, which makes the output deterministic
+// but requires WalkDir to read an entire directory into memory before proceeding
+// to walk that directory.
+//
+// WalkDir does not follow symbolic links.
+func WalkDir(root string, fn WalkDirFunc) error {
+	info, err := os.Lstat(root)
+	if err != nil {
+		err = fn(root, nil, err)
+	} else {
+		err = walkDir(root, &dirEntryFromInfo{info}, fn)
+	}
+	if err == SkipDir {
+		return nil
+	}
+	return err
+}
+
+type dirEntryFromInfo struct {
+	fs.FileInfo
+}
+
+func (e *dirEntryFromInfo) Type() fs.FileMode {
+	return e.Mode().Type()
+}
+
+func (e *dirEntryFromInfo) Info() (fs.FileInfo, error) {
+	return e.FileInfo, nil
+}
+
+// Walk walks the file tree rooted at root, calling fn for each file or
+// directory in the tree, including root.
+//
+// All errors that arise visiting files and directories are filtered by fn:
+// see the WalkFunc documentation for details.
+//
+// The files are walked in lexical order, which makes the output deterministic
+// but requires Walk to read an entire directory into memory before proceeding
+// to walk that directory.
+//
 // Walk does not follow symbolic links.
-func Walk(root string, walkFn WalkFunc) error {
+//
+// Walk is less efficient than WalkDir, introduced in Go 1.16,
+// which avoids calling os.Lstat on every visited file or directory.
+func Walk(root string, fn WalkFunc) error {
 	info, err := os.Lstat(root)
 	if err != nil {
-		err = walkFn(root, nil, err)
+		err = fn(root, nil, err)
 	} else {
-		err = walk(root, info, walkFn)
+		err = walk(root, info, fn)
 	}
 	if err == SkipDir {
 		return nil
 	}
 	return err
 }
 
-// readDirNames reads the directory named by dirname and returns
+// readDir reads the directory named by dirname and returns
 // a sorted list of directory entries.
+func readDir(dirname string) ([]fs.DirEntry, error) {
+	f, err := os.Open(dirname)
+	if err != nil {
+		return nil, err
+	}
+	dirs, err := f.ReadDir(-1)
+	f.Close()
+	if err != nil {
+		return nil, err
+	}
+	sort.Slice(dirs, func(i, j int) bool { return dirs[i].Name() < dirs[j].Name() })
+	return dirs, nil
+}
+
+// readDirNames reads the directory named by dirname and returns
+// a sorted list of directory entry names.
 func readDirNames(dirname string) ([]string, error) {
 	f, err := os.Open(dirname)
 	if err != nil {