code: public static File maybeMarkUnreadable(File path)
      {
          File directory = getDirectory(path);
          if (instance.unreadableDirectories.add(directory))
          {
              logger.warn("Blacklisting {} for reads", directory);
              return directory;
          }
          return null;
      }
docs: Adds parent directory of the file (or the file itself, if it is a directory)
  to the set of unreadable directories.
  
  @return the blacklisted directory or null if nothing has been added to the list.