title: assert self._state in (CLOSE, TERMINATE) when using python multiprocess
body: <p>I am currently trying to use python multiprocessing. The library I use is <code>multiprocess</code> (NOT <code>multiprocessing</code>). </p>
  
  <p>I have the following code, which creates a number of computing jobs, and runs it through a map operation:</p>
  
  <pre><code>pool = multiprocess.Pool(4)
  all_responses = pool.map_async(wrapper_singlerun, range(10000))
  pool.join()
  pool.close()
  </code></pre>
  
  <p>However, whenever I run this snippet of code, I get the following error:</p>
  
  <pre><code>    pool.join()
    File "/Users/davidal/miniconda3/lib/python3.6/site-packages/multiprocess/pool.py", line 509, in join
      assert self._state in (CLOSE, TERMINATE)
  AssertionError
  </code></pre>
  
  <p>Do you have any idea why this error happens? I used <code>pool.map_async</code> before, but figured that I need to have a <code>pool rendez-vous</code> command. Otherwise, my PC created something like a forkbomb, which created too many threads (at least, that's what I think it does...)</p>
  
  <p>Any ideas are appreciated!</p>