Hello Stefen, 

Yes, I was able to reproduce the issue without the patch and at the
same time, noticed that the problem was not observed in other branches
(py3k), then saw the differences where the possible socket close
delays can happen and fixed it. As it was multiple threads closely
being closed it was most probably due to delay in closing socket
connection that lead the problem in the first place.

If it is required, then manually this can be verified by setting up an
artificial delay in the network layer and trying multiple ftp
transfers. (No, I did not do that to verify. :))