code: def validate_absolute_path(self, roots, absolute_path):
          
          if absolute_path is None:
              raise tornado.web.HTTPError(404)
  
          for root in roots:
  
              if (absolute_path + os.path.sep).startswith(root):
                  break
          else:
              raise tornado.web.HTTPError(403, "%s is not in root static directory",
                                          self.path)
  
          if not os.path.isfile(absolute_path):
              raise tornado.web.HTTPError(403, "%s is not a file", self.path)
  
          return absolute_path
docs: Validate and return the absolute path.
  
          ``root`` is the configured path for the `StaticFileHandler`,
          and ``path`` is the result of `get_absolute_path`
  
          This is an instance method called during request processing,
          so it may raise `HTTPError` or use methods like
          `RequestHandler.redirect` (return None after redirecting to
          halt further processing).  This is where 404 errors for missing files
          are generated.