Build and return a mechanism specific credentials tuple.

def _build_credentials_tuple(mech, source, user, passwd, extra):
    """Build and return a mechanism specific credentials tuple.
    """
    user = _unicode(user) if user is not None else None
    password = passwd if passwd is None else _unicode(passwd)
    if mech == 'GSSAPI':
        properties = extra.get('authmechanismproperties', {})
        service_name = properties.get('SERVICE_NAME', 'mongodb')
        canonicalize = properties.get('CANONICALIZE_HOST_NAME', False)
        service_realm = properties.get('SERVICE_REALM')
        props = GSSAPIProperties(service_name=service_name,
                                 canonicalize_host_name=canonicalize,
                                 service_realm=service_realm)
        # Source is always $external.
        return MongoCredential(mech, '$external', user, password, props)
    elif mech == 'MONGODB-X509':
        # user can be None.
        return MongoCredential(mech, '$external', user, None, None)
    else:
        if passwd is None:
            raise ConfigurationError("A password is required.")
        return MongoCredential(mech, source, user, password, None)