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 file_exists
returns
`Unknown
.
when_file_changes file
polls file
using stat
and writes file
's mtime to the
pipe every time it changes. The first time in the pipe will be file
's current
mtime. To stop polling, close the pipe.