Function mark_showing 

fn mark_showing(
    attached: &HashMap<SessionSlot, usize>,
    seen_failed: &HashMap<SessionSlot, SystemTime>,
    slot: &SessionSlot,
    at: SystemTime,
) -> bool

Whether a failure at at is still the reader’s to see.

One predicate for its two readers - the home’s rows, and the attach that spends the mark - so the test the home draws by and the test that decides whether a refresh is owed cannot drift apart.