Function held 

fn held<'a>(
    slot: &'a mut Option<Connection>,
    address: Option<&str>,
) -> Option<&'a Connection>

The connection kept in slot to the bus at address: what came on it read first (drained: late answers, and the bus gone), then the one kept while the bus keeps it, else a new one (crate::bus::connect, bus::SETUP at most). None without such a bus, or while it cannot be reached (tried again at the next reading).