waitTimeout

    pub fn waitTimeout(event: *Event, io: Io, timeout: Timeout) WaitTimeoutError!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`.
        }
        try io.futexWaitTimeout(Event, event, .waiting, timeout);
        switch (@atomicLoad(Event, event, .acquire)) {
            .unset => unreachable, // `reset` called before pending `wait` returned
            .waiting => return error.Timeout,
            .is_set => return,
        }
    }