* Allow certain operations to succeed even if we don't have write

permission to the Nix store or database.  E.g., `nix-env -qa' will
  work, but `nix-env -qas' won't (the latter needs DB access).  The
  option `--readonly-mode' forces this mode; otherwise, it's only
  activated when the database cannot be opened.

src/libstore/db.hh

@@ -87,4 +87,11 @@ public:
 };
 
 
+class DbNoPermission : public Error
+{
+public:
+    DbNoPermission(const format & f) : Error(f) { };
+};
+
+
 #endif /* !__DB_H */