when_file_exists ?poll_delay file returns a deferred that becomes determined when
file exists. The default poll delay is 0.5 seconds. It raises an exception if it
can not check whether the file is there, in the same cases
when_file_changes file polls
stat and writes
file's mtime to the
pipe every time it changes. The first time in the pipe will be
mtime. To stop polling, close the pipe.