Multiprocessing pool.py gets SimpleQueue objects as inqueue and outqueue.
when it terminates, it doesn't call the close() method of the queues' readers and writers.
As a results, 4 file pipes leak in one pool termination.

Expected: The pool closes reader and writer pipes of the inqueue and outqueue when it terminates.
What did happen: the pool doesn't close the pipes. 4 pipes leak.

Interestingly, there is no close() method on SimpleQueue...

I did some investigating using a test script and Python 3.7.0a0

from multiprocessing import Pool
import os
import time

def f(x):
	time.sleep(30)
	return x*x

if __name__=='__main__':
	print('Main pid {0}'.format(os.getpid()))
	p = Pool(5)
	p.map(f, [1,2,3])
	print('Returned')
	time.sleep(30)
and grepping for pipe and the parentpid in the output from lsof ( lsof | grep python.*<parentpid>.*pipe ). The pipes opened at the start of the script are still open even after the line print('Returned') is executed. I suppose this is expected because I did not call *p.close()*. All pipes are cleaned up after the parent process finishes.

When I repeat the experiment calling p.close() after p.map returns, all that is left is the 9 pipes opened by the parent. All pipes are cleaned up after parent script exits. 

@shani - could you please clarify how you were able to detect the leaking pipes?

I think this issue is mistaken.  The reader and writer objects are closed automatically when they are destroyed (see Connection.__del__).  The only thing that may lack is a way to close them more eagerly.

In any case, I'm closing as a duplicate of issue 30966.