os: set full name for Roots created with Root.OpenRoot

Set the Name for a Root created within a Root to be the
concatenation of the parent's path and the name used to open the child.

This matches the behavior for files opened within a Root
with Root.Open.

Fixes #73868

Change-Id: Idf4021602ac25556721b7ef6924dec652c7bf4db
Reviewed-on: https://go-review.googlesource.com/c/go/+/698376
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/os/root_test.go

@@ -1919,3 +1919,36 @@ func TestRootWriteReadFile(t *testing.T) {
 		t.Fatalf("root.ReadFile(%q) = %q, %v; want %q, nil", name, got, err, want)
 	}
 }
+
+func TestRootName(t *testing.T) {
+	dir := t.TempDir()
+	root, err := os.OpenRoot(dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer root.Close()
+	if got, want := root.Name(), dir; got != want {
+		t.Errorf("root.Name() = %q, want %q", got, want)
+	}
+
+	f, err := root.Create("file")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+	if got, want := f.Name(), filepath.Join(dir, "file"); got != want {
+		t.Errorf(`root.Create("file").Name() = %q, want %q`, got, want)
+	}
+
+	if err := root.Mkdir("dir", 0o777); err != nil {
+		t.Fatal(err)
+	}
+	subroot, err := root.OpenRoot("dir")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer subroot.Close()
+	if got, want := subroot.Name(), filepath.Join(dir, "dir"); got != want {
+		t.Errorf(`root.OpenRoot("dir").Name() = %q, want %q`, got, want)
+	}
+}

src/os/root_unix.go

@@ -75,7 +75,7 @@ func openRootInRoot(r *Root, name string) (*Root, error) {
 	if err != nil {
 		return nil, &PathError{Op: "openat", Path: name, Err: err}
 	}
-	return newRoot(fd, name)
+	return newRoot(fd, joinPath(r.Name(), name))
 }
 
 // rootOpenFileNolog is Root.OpenFile.

src/os/root_windows.go

@@ -123,7 +123,7 @@ func openRootInRoot(r *Root, name string) (*Root, error) {
 	if err != nil {
 		return nil, &PathError{Op: "openat", Path: name, Err: err}
 	}
-	return newRoot(fd, name)
+	return newRoot(fd, joinPath(r.Name(), name))
 }
 
 // rootOpenFileNolog is Root.OpenFile.