Python multiprocessing hangs indefinitely on completion

Question

I'm trying to use Python's multiprocessing module to run an analysis on multiple samples in parallel. I'm using pool.map_async to spawn the function (called crispr_analysis) on a tuple of tuples for arguments (zipped_args). Each tuple within zipped_args is not empty, as that can cause multiprocessing to hang. Upon completion of the pool, it hangs and fails to move on to the rest of the script. I know that crispr_analysis finishes as it creates output files (generated with with statements, so they're closing properly); I can browse said files and they are complete. I never see the debug message for sorting the results, and the program never terminates.

try:
    #   Use map_async and get with a large timeout
    #   to allow for KeyboardInterrupts to be caught
    #   and handled with the try/except
    timeout = max((9999, 600 * len(fastq_list)))
    logging.debug("Setting timeout to %s seconds", timeout)
    res = pool.map_async(crispr_analysis, zipped_args) # type: multiprocessing.pool.MapResult
    pool.close()
    results = res.get(timeout)
except (KeyboardInterrupt, ExitPool) as error: # Handle ctrl+c or custom ExitPool
    pool.terminate()
    # pool.join()
    if isinstance(error, KeyboardInterrupt): # ctrl+c
        sys.exit('\nkilled')
    elif isinstance(error, ExitPool): # My way of handling SystemExits
        sys.exit(error.msg)
    else: # Shouldn't happen, but you know...
        raise
except:
    pool.terminate(); pool.join()
    raise
else:
    pool.join()
try:
    logging.debug("Sorting results into alignments and summaries")
    sort_start = time.time() # type: float
    alignments, summaries = zip(*results) # type: Tuple[Tuple[alignment.Alignment]], Tuple[Dict[str, Any]]
    logging.debug("Sorting results took %s seconds", round(time.time() - sort_start, 3))
except ExitPool as error: # Handle ExitPool calls for single-threaded map
    sys.exit(error.msg)

Does anyone have any idea why multiprocessing is hanging and how I can fix it?

Extra information:

• I'm using Python 2.7.8 on CentOS 7.3.1611; platform and Python version are not changeable
• crispr_analysis returns a tuple and dictionary, that are each either empty or have some length based on the inputs
• I have tried omitting the pool.join() statements, to no avail
• ExitPool is an error that I throw to stop the entire pool in place ofSystemExits; multiprocessing normally swallows SystemExits, but I want them to bubble up
• This entire snippet is called from within a function (called main)

• This analysis program is called from an easy-install entry script, where the entry point is the main function that starts the multiprocessing pool

  #!/usr/bin/python
  # EASY-INSTALL-ENTRY-SCRIPT: 
  'EdiTyper==1.0.0','console_scripts','EdiTyper'
  __requires__ = 'EdiTyper==1.0.0'
  import sys 
  from pkg_resources import load_entry_point
  
  if __name__ == '__main__':
      sys.exit(
          load_entry_point('EdiTyper==1.0.0', 'console_scripts', 'EdiTyper')()
      )