wait

    pub fn wait(event: *Event, io: Io) Cancelable!void {
        if (@cmpxchgStrong(Event, event, .unset, .waiting, .acquire, .acquire)) |prev| switch (prev) {
            .unset => unreachable,
            .waiting => {},
            .is_set => return,
        };
        errdefer {
            // Ideally we would restore the event back to `.unset` instead of `.waiting`, but there
            // might be other threads waiting on the event. In theory we could track the *number* of
            // waiting threads in the unused bits of the `Event`, but that has its own problem: the
            // waiters would wake up when a *new waiter* was added. So it's easiest to just leave
            // the state at `.waiting`---at worst it causes one redundant call to `futexWake`.
        }
        while (true) {
            try io.futexWait(Event, event, .waiting);
            switch (@atomicLoad(Event, event, .acquire)) {
                .unset => unreachable, // `reset` called before pending `wait` returned
                .waiting => continue,
                .is_set => return,
            }
        }
    }