multiprocessing signal defect

[Adam Tauno Williams]

On Fri, 2010-10-29 at 08:12 -0400, Neal Becker wrote: 
> Seems multiprocessing doesn't behave well with signals:
> ---------
> from multiprocessing import Pool
> import time
> def sleep (dummy):
>     time.sleep (10)    
> if __name__ == '__main__':
>     pool = Pool (processes=2)
>     result = pool.map (sleep, range (4))   
> -------------
> start it up
> $ python test_multip.py 
> ----------------------
> ps auxf | grep python
> nbecker   6605  1.6  0.1 338192  6952 pts/1    Sl+  08:03   0:00  |       \_ 
> python test_multip.py
> nbecker   6606  0.0  0.1 186368  4760 pts/1    S+   08:03   0:00  |           
> \_ python test_multip.py
> nbecker   6607  0.0  0.1 186372  4740 pts/1    S+   08:03   0:00  |           
> \_ python test_multip.py
> kill 6607
>  ps auxf | grep python
> nbecker   6605  0.5  0.1 338192  6952 pts/1    Sl+  08:03   0:00  |       \_ 
> python test_multip.py
> nbecker   6606  0.0  0.1 186368  4760 pts/1    S+   08:03   0:00  |           
> \_ python test_multip.py
> nbecker   6607  0.0  0.0      0     0 pts/1    Z+   08:03   0:00  |           
> \_ [python] <defunct>
>  kill 6606
> ps auxf | grep python
> nbecker   6605  0.3  0.1 338192  6952 pts/1    Sl+  08:03   0:00  |       \_ 
> python test_multip.py
> nbecker   6606  0.0  0.0      0     0 pts/1    Z+   08:03   0:00  |           
> \_ [python] <defunct>
> nbecker   6607  0.0  0.0      0     0 pts/1    Z+   08:03   0:00  |           
> \_ [python] <defunct>
> Now we have 2 dead children and the parent is hung forever.
> Isn't this a serious defect?

No, I think this is just POSIX/UNIX process behavior.  If the parent
never joins on the child the child can never exit [which is what a
Zombie process is].

For example, see the do_verify_workers method in
<http://coils.hg.sourceforge.net/hgweb/coils/coils/file/6ab5ade3e488/src/coils/logic/workflow/services/executor.py> 

A parent process needs to make some effort to reap its children.