Function walk_state_auth_chain 

pub(crate) fn walk_state_auth_chain<'a>(
    services: &'a Services,
    room_id: &'a RoomId,
    room_version: &'a RoomVersionId,
    state: &'a [OwnedEventId],
    event_id: &'a EventId,
) -> impl Stream<Item = OwnedEventId> + Send + 'a

Streams the auth chain of the state at an event and of the event itself.

Once the room has forked, the event’s own auth events can fall outside the state’s chain, and a peer validating the event needs them too. The two are walked separately and merged, so requests at the same state can share its cached walk.