Function apply_join 

pub fn apply_join(
    conn: &mut Connection,
    room_id: &str,
    caller_user_id: i64,
    caller_mxid: &str,
    displayname: &str,
    now: &str,
    origin_ts: i64,
) -> Result<HashSet<i64>, MatrixError>

POST /join’s write for an already-invited or newly-public-eligible caller (idempotent-join and ban checks happen in the handler, before the policy hook — see [join_room]). Returns the post-join wake set. The join event carries the caller’s effective label as displayname, resolved by the caller BEFORE it takes the messenger connection this writes through.