'Validate if external sources (as specified via the location
or copy-from headers) are supported. Otherwise we reject
with 400 "Bad Request".'

@staticmethod
def _validate_source(source, req):

    if store_utils.validate_external_location(source):
        return source
    else:
        if source:
            msg = (_("External    sources    are    not    supported:    '%s'") % source)
        else:
            msg = _('External    source    should    not    be    empty')
        LOG.warn(msg)
        raise HTTPBadRequest(explanation=msg, request=req, content_type='text/plain')
