Function walk_auth_chain 

pub(crate) fn walk_auth_chain<'a, I>(
    services: &'a Services,
    room_id: &'a RoomId,
    room_version: &'a RoomVersionId,
    heads: I,
) -> impl Stream<Item = OwnedEventId> + Send + 'a
where
    I: Iterator<Item = &'a EventId> + Clone + ExactSizeIterator + Send + 'a,

Streams the auth chain of the state a federation response serves.

A walk that fails is logged and yields no events, so the response goes out without the chain rather than failing whole.