Function exchange 

pub fn exchange(
    sharing: &Sharing<'_>,
    stores: &[Store],
    now_ms: i64,
) -> Result<Outcome, String>

One exchange: the changes found here go out, the others’ come in. One at a time on this computer, whatever runs it (exchange_lock): another running, a quick one waits for it; a whole one does nothing, and says so (share-busy).