Function process_historic 

pub fn process_historic(
    conn: &mut Connection,
    room: &str,
    events: &[Value],
    keys: &PublicKeyMap,
    now: &str,
) -> Result<usize, MatrixError>

Events of the past from a peer (get_missing_events, backfill, an auth chain): each is checked (signature, hash or skeleton form, id, its own auth events, the state before it), stored as a non-extremity, and its timeline row is written when this server has none. They never change the current state or the forward extremities; the live event that needed them does that.