class_docstr: A file-like class implemented `after_fork()` hook
  
  The method :meth:`after_fork` is called in the child process after forking,
  and it closes and reopens the file object to avoid race condition caused by
  open file description.
  See: https://www.securecoding.cert.org/confluence/x/ZQG7AQ
code: def fileno(self):
      return self._fp.fileno()
  
imports: import io
  import multiprocessing.util
  import threading
  import six
  import six.moves.cPickle
  from chainer.dataset import dataset_mixin
label_desc: Use image node_id 8 for calling the _FileReader obj's underlying member method code with example usage: obj.fileno() and returns: self