memfs ReadDir() doesn't return an error when the directory doesn't exist

[weberc2-tempus]

Version

v5.4.1

Description

memfs.New().ReadDir("asdf") returns nil, nil instead of nil, <directory-not-found-err>.

Minimal example

func TestMemoryReadDir(t *testing.T) {
	if _, err := osfs.New("/tmp").ReadDir("asdf"); err == nil {
		t.Fatal("osfs: expected error; found <nil>")
	} else {
		t.Logf("osfs: error was correctly returned!")
	}

	if _, err := memfs.New().ReadDir("asdf"); err == nil {
		t.Fatal("memfs: expected error; found <nil>")
	} else {
		t.Logf("memfs: error was correctly returned!")
	}
}

// Output
//   osfs: error was correctly returned!
//   memfs: expected error; found <nil>

Proposed changes

func (fs *Memory) ReadDir(path string) ([]os.FileInfo, error) {
	if f, has := fs.s.Get(path); has {
		if target, isLink := fs.resolveLink(path, f); isLink {
			return fs.ReadDir(target)
		}
+	} else {
+		return nil, &fs.PathError{Op: "open", Path: path, Err: unix.ENOENT}
}

	var entries []os.FileInfo
	for _, f := range fs.s.Children(path) {
		fi, _ := f.Stat()
		entries = append(entries, fi)
	}

	sort.Sort(ByName(entries))

	return entries, nil
}

[pjbgf]

I am happy to review a PR for this @weberc2-tempus.

[weberc2-tempus]

Hi @pjbgf, I've opened #38. Thank you in advance for reviewing!