The last time I tested set_nonblock() as a fix for the rsync hang, I
used F_SETFD rather than F_SETFL, which resulted in no error, but also
did not set O_NONBLOCK. Turns out missing O_NONBLOCK was the problem.

The rsync hang was due to every context blocking in os.write() waiting
for either a parent or child buffer to empty, which was exacerbated by
rsync's own pipelining, that allows writes from both sides to proceed
even while reads aren't progressing. The hang was due to os.write() on a
blocking fd blocking until buffer space is available to complete the
write. Partial writes are only supported when O_NONBLOCK is enabled.

Previously _output_buf was racy. This may or may not be cheaper than
simply using a lock, but it requires much less code, so I prefer it for
now.

Defining functions in the current module was causing the entirety of
py.test and all dependencies to be sucked in.

Original commit:

commit 3535e45245f69b11fe4c9d40f5b07b4c068b0ac4
Author: David Wilson <dw@botanicus.net>
Date:   Fri Aug 26 19:35:09 2016 +0100

    Broken SSH unit test.

A  todo/six-module-import-error.py