io/fs: set ErrInvalid for FS.Open from SubFS when it fails ValidPath

Fixes #65419

Change-Id: I8f9f82ab0387d8bb39aaca4f9e60e36ee15c587d
Reviewed-on: https://go-review.googlesource.com/c/go/+/560137
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/io/fs/sub.go

@@ -33,7 +33,7 @@ type SubFS interface {
 // chroot-style security mechanism, and Sub does not change that fact.
 func Sub(fsys FS, dir string) (FS, error) {
 	if !ValidPath(dir) {
-		return nil, &PathError{Op: "sub", Path: dir, Err: errors.New("invalid name")}
+		return nil, &PathError{Op: "sub", Path: dir, Err: ErrInvalid}
 	}
 	if dir == "." {
 		return fsys, nil
@@ -52,7 +52,7 @@ type subFS struct {
 // fullName maps name to the fully-qualified name dir/name.
 func (f *subFS) fullName(op string, name string) (string, error) {
 	if !ValidPath(name) {
-		return "", &PathError{Op: op, Path: name, Err: errors.New("invalid name")}
+		return "", &PathError{Op: op, Path: name, Err: ErrInvalid}
 	}
 	return path.Join(f.dir, name), nil
 }

src/io/fs/sub_test.go

@@ -5,6 +5,7 @@
 package fs_test
 
 import (
+	"errors"
 	. "io/fs"
 	"testing"
 )
@@ -54,4 +55,9 @@ func TestSub(t *testing.T) {
 	if pe.Path != "nonexist" {
 		t.Fatalf("Open(nonexist): err.Path = %q, want %q", pe.Path, "nonexist")
 	}
+
+	_, err = sub.Open("./")
+	if !errors.Is(err, ErrInvalid) {
+		t.Fatalf("Open(./): error is %v, want %v", err, ErrInvalid)
+	}
 }