def find_urls(self, site, frametype, gpsstart, gpsend,
                  match=None, on_gaps='warn'):
        """Find all files of the given type in the [start, end) GPS interval.
        """
        span = Segment(gpsstart, gpsend)
        cache = [e for e in self._read_ffl_cache(site, frametype) if
                 e.observatory == site and e.description == frametype and
                 e.segment.intersects(span)]
        urls = [e.path for e in cache]
        missing = SegmentList([span]) - cache_segments(cache)

        if match:
            match = re.compile(match)
            urls = list(filter(match.search, urls))

        # no missing data or don't care, return
        if on_gaps == 'ignore' or not missing:
            return urls

        # handle missing data
        msg = 'Missing segments: \n{0}'.format('\n'.join(map(str, missing)))
        if on_gaps == 'warn':
            warnings.warn(msg)
            return urls
        raise RuntimeError(msg)
Find all files of the given type in the [start, end) GPS interval.