Add Walk function.

walk.go

@@ -0,0 +1,80 @@
+package vfs
+
+import (
+	"os"
+	"path/filepath"
+	"sort"
+	"strings"
+)
+
+// Walk walks the file tree rooted at root, calling walkFunc 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.
+// Walk does not follow symbolic links.
+func Walk(fs Filesystem, root string, walkFunc filepath.WalkFunc) error {
+	info, err := fs.Lstat(root)
+	if err != nil {
+		err = walkFunc(root, nil, err)
+	} else {
+		err = walk(fs, root, info, walkFunc)
+	}
+	if err == filepath.SkipDir {
+		return nil
+	}
+	return err
+}
+
+// readDirNames reads the directory named by dirname and returns
+// a sorted list of directory entries.
+func readDirNames(fs Filesystem, dirname string) ([]string, error) {
+	infos, err := fs.ReadDir(dirname)
+	if err != nil {
+		return nil, err
+	}
+	names := make([]string, 0, len(infos))
+	for _, info := range infos {
+		names = append(names, info.Name())
+	}
+	sort.Strings(names)
+	return names, nil
+}
+
+// walk recursively descends path, calling walkFunc.
+func walk(fs Filesystem, path string, info os.FileInfo, walkFunc filepath.WalkFunc) error {
+	if !info.IsDir() {
+		return walkFunc(path, info, nil)
+	}
+
+	names, err := readDirNames(fs, path)
+	err1 := walkFunc(path, info, err)
+	// If err != nil, walk can't walk into this directory.
+	// err1 != nil means walkFn want walk to skip this directory or stop walking.
+	// Therefore, if one of err and err1 isn't nil, walk will return.
+	if err != nil || err1 != nil {
+		// The caller's behavior is controlled by the return value, which is decided
+		// by walkFn. walkFn may ignore err and return nil.
+		// If walkFn returns SkipDir, it will be handled by the caller.
+		// So walk should return whatever walkFn returns.
+		return err1
+	}
+
+	for _, name := range names {
+		filename := strings.Join([]string{path, name}, string(fs.PathSeparator()))
+		fileInfo, err := fs.Lstat(filename)
+		if err != nil {
+			if err := walkFunc(filename, fileInfo, err); err != nil && err != filepath.SkipDir {
+				return err
+			}
+		} else {
+			err = walk(fs, filename, fileInfo, walkFunc)
+			if err != nil {
+				if !fileInfo.IsDir() || err != filepath.SkipDir {
+					return err
+				}
+			}
+		}
+	}
+	return nil
+}