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Parallel python: destroying job_servers

Remember to destroy job servers after they are done.

My nice animation generator crashed sometime over the weekend with the following death rattle:

Traceback (most recent call last):
  File "animate_array_job.py", line 168, in 
    tscale = tscale, dpi = dpi)
  File "animate_array_job.py", line 92, in process_file
    job_server = pp.Server(ppservers=ppservers)
  File "/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/site-packages/pp.py", line 336, in __init__
    self.set_ncpus(ncpus)
  File "/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/site-packages/pp.py", line 494, in set_ncpus
    range(ncpus - len(self.__workers))])
  File "/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/site-packages/pp.py", line 141, in __init__
    self.start()
  File "/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/site-packages/pp.py", line 148, in start
    shell=True)
  File "/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/subprocess.py", line 594, in __init__
    errread, errwrite)
  File "/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/subprocess.py", line 1009, in _execute_child
    errpipe_read, errpipe_write = os.pipe()
OSError: [Errno 24] Too many open files
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