Function mark_superseded 

pub async fn mark_superseded(
    conn: &mut PgConnection,
    id: Uuid,
    superseded_by_id: Uuid,
) -> ModelResult<()>

Points an old attempt at the newer one that replaced it. The old row keeps its state and terminal_at: it really was registered.

superseded_by_id may name a row that does not exist yet, as long as it is inserted before the caller’s transaction commits: the foreign key is deferred, which is what lets the successor take the completion’s one live slot without the old attempt ever pointing at itself.