batchAwaitAsync

fn batchAwaitAsync(userdata: ?*anyopaque, batch: *Io.Batch) Io.Cancelable!void {
    const ev: *Evented = @ptrCast(@alignCast(userdata));
    var maybe_sync: CancelRegion.Sync.Maybe = .{ .cancel_region = .init() };
    defer maybe_sync.deinit(ev);
    ev.batchDrainSubmitted(&maybe_sync, batch, false) catch |err| switch (err) {
        error.ConcurrencyUnavailable => unreachable, // passed concurrency=false
        error.Canceled => |e| return e,
    };
    maybe_sync.leaveSync(ev);
    while (true) {
        batchDrainReady(batch) catch |err| switch (err) {
            error.Timeout => unreachable, // no timeout
        };
        if (batch.completed.head != .none or batch.pending.head == .none) return;
        ev.yield(null, .{ .batch_await = batch });
    }
}