code: def test_after_fork(self):
      writer = datasets.PickleDatasetWriter(self.io)
      writer.write(1)
      writer.flush()
  
      reader = ReaderMock(self.io)
      # Assign to avoid destruction of the instance
      # before creation a child process
      dataset = datasets.PickleDataset(reader)
  
      assert reader.n_hook_called == 0
      ctx = multiprocessing.get_context("fork")
      p = ctx.Process()
      p.start()
      p.join()
      assert reader.n_hook_called == 1
      assert reader.last_caller_pid == p.pid
  
      # Touch to suppress "unused variable' warning
      del dataset
  
imports: import ctypes
  import io
  import multiprocessing
  import os
  import pickle
  import platform
  import sys
  import unittest
  import mock
  from chainer import datasets
  from chainer.datasets import pickle_dataset
  from chainer import testing
  from chainer import utils
label_desc: Use image node_id 4 for calling the TestPickleDataset obj's underlying member method code with example usage: obj.test_after_fork() without return types