batchAwaitAsync

fn batchAwaitAsync(userdata: ?*anyopaque, batch: *Io.Batch) Io.Cancelable!void {
    const ev: *Evented = @ptrCast(@alignCast(userdata));
    const queue = ev.batchDrainSubmitted(batch, false) catch |err| switch (err) {
        error.ConcurrencyUnavailable => unreachable, // passed concurrency=false
        error.Canceled => |e| return e,
    } orelse return;
    if (batch.pending.head == .none) return;
    var waiter: BatchWaiter = .{
        .sleeper = .init(ev.queue, Thread.current().currentFiber()),
        .queue = queue,
    };
    if (batch.completed.head != .none) BatchWaiter.signal(&waiter);
    queue.as_object().set_context(&waiter);
    ev.yield(.{ .@"resume" = queue.as_object() });
}