code: def GetFileSystemReferenceCount(self, path_spec):
      
      identifier = self._GetFileSystemCacheIdentifier(path_spec)
      cache_value = self._file_system_cache.GetCacheValue(identifier)
      if not cache_value:
        return None
  
      return cache_value.reference_count
docs: Retrieves the reference count of a cached file system object.
  
      Args:
        path_spec (PathSpec): path specification.
  
      Returns:
        int: reference count or None if there is no file system object for
            the corresponding path specification cached.