await

    fn await(group: Group, ev: *Evented, awaiter: *Fiber) bool {
        group.lock(ev);
        defer group.unlock(ev);
        if (@atomicLoad(List, group.listPtr(), .monotonic).fibers.unpack()) |_| {
            if (group.registerAwaiter(awaiter) and awaiter.cancel_protection.check() == .unblocked) {
                // The awaiter already had an unacknowledged cancelation request before
                // attempting to await a group, so propagate the cancelation to the group.
                assert(!group.cancelLocked(ev, null));
            }
            return false;
        }
        return true;
    }