If large results values are returned by the subprocesses, we can hit a deadlock
where the subprocess is trying to write data back to the parent, the pipe is full
and the parent is waiting for the child to exit.
Avoid this by calling the update() method which would trigger reading a result
from the child, avoiding the deadlock. The issue is described in
https://bugs.python.org/issue8426
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
