code: def renew(self, cfgstr=None, product=None):
          
          products = self._rectify_products(product)
          certificate = {
              'timestamp': util_time.timestamp(),
              'product': products,
          }
          if products is not None:
              if not all(map(os.path.exists, products)):
                  raise IOError(
                      'The stamped product must exist: {}'.format(products))
              certificate['product_file_hash'] = self._product_file_hash(products)
          self.cacher.save(certificate, cfgstr=cfgstr)
          return certificate
docs: Recertify that the product has been recomputed by writing a new
          certificate to disk.