os: add DirFS, which is used by many programs to access readdir.

It's wafer-thin :-)

Includes smoke test from upstream.

TODO: once t.TempDir is implemented, add io/fs to the list of standard library tests to run; that's a better test.

src/os/file_go_116.go

@@ -5,6 +5,7 @@ package os
 import (
 	"io"
 	"io/fs"
+	"runtime"
 )
 
 type (
@@ -12,9 +13,56 @@ type (
 	FileInfo = fs.FileInfo
 )
 
-// The followings are copied from Go 1.16 official implementation:
+// The followings are copied from Go 1.16 or 1.17 official implementation:
 // https://github.com/golang/go/blob/go1.16/src/os/file.go
 
+// DirFS returns a file system (an fs.FS) for the tree of files rooted at the directory dir.
+//
+// Note that DirFS("/prefix") only guarantees that the Open calls it makes to the
+// operating system will begin with "/prefix": DirFS("/prefix").Open("file") is the
+// same as os.Open("/prefix/file"). So if /prefix/file is a symbolic link pointing outside
+// the /prefix tree, then using DirFS does not stop the access any more than using
+// os.Open does. DirFS is therefore not a general substitute for a chroot-style security
+// mechanism when the directory tree contains arbitrary content.
+func DirFS(dir string) fs.FS {
+	return dirFS(dir)
+}
+
+func containsAny(s, chars string) bool {
+	for i := 0; i < len(s); i++ {
+		for j := 0; j < len(chars); j++ {
+			if s[i] == chars[j] {
+				return true
+			}
+		}
+	}
+	return false
+}
+
+type dirFS string
+
+func (dir dirFS) Open(name string) (fs.File, error) {
+	if !fs.ValidPath(name) || runtime.GOOS == "windows" && containsAny(name, `\:`) {
+		return nil, &PathError{Op: "open", Path: name, Err: ErrInvalid}
+	}
+	f, err := Open(string(dir) + "/" + name)
+	if err != nil {
+		return nil, err // nil fs.File
+	}
+	return f, nil
+}
+
+func (dir dirFS) Stat(name string) (fs.FileInfo, error) {
+	if !fs.ValidPath(name) || runtime.GOOS == "windows" && containsAny(name, `\:`) {
+		return nil, &PathError{Op: "stat", Path: name, Err: ErrInvalid}
+	}
+	f, err := Stat(string(dir) + "/" + name)
+	if err != nil {
+		return nil, err
+	}
+	return f, nil
+}
+
 // ReadFile reads the named file and returns the contents.
 // A successful call returns err == nil, not err == EOF.
 // Because ReadFile reads the whole file, it does not treat an EOF from Read