security: protect multi-root virtual mounts

chfs-ng/internal/storage/multipath_test.go

@@ -24,3 +24,18 @@ func TestNewMultiKeepsDuplicateBasenames(t *testing.T) {
 		t.Fatalf("unexpected virtual names: %v", m.Names)
 	}
 }
+
+func TestMultiRejectsMutationOfVirtualRoot(t *testing.T) {
+	root1 := t.TempDir()
+	root2 := t.TempDir()
+	m := NewMulti([]string{root1, root2})
+	if err := m.Remove(m.Names[0]); err == nil {
+		t.Fatal("expected virtual root removal to be rejected")
+	}
+	if _, err := m.Create(m.Names[0]); err == nil {
+		t.Fatal("expected virtual root creation to be rejected")
+	}
+	if err := m.Rename(m.Names[0], m.Names[1]); err == nil {
+		t.Fatal("expected virtual root rename to be rejected")
+	}
+}