'Register any function that will be called with given key each
interval seconds (interval is defined at __init__). Its result is then
passed to ``function``, but only if the result is true.
:param function condition_function:
Function which will be called each ``interval`` seconds. All
exceptions from it will be logged and ignored. IOError exception
will be ignored without logging.
:param function function:
Function which will be called if condition_function returns
something that is true. Accepts result of condition_function as an
argument.
:param str key:
Any value, it will be passed to condition_function on each call.
Note: registered functions will be automatically removed if
condition_function results in something true.'

def register_missing(self, condition_function, function, key):

    with self.lock:
        self.missing[key].add((condition_function, function))
