func EnumerateChildrenAsync(ctx context.Context, ds DAGService, root *Node, set key.KeySet) error {
	toprocess := make(chan []key.Key, 8)
	nodes := make(chan *NodeOption, 8)

	ctx, cancel := context.WithCancel(ctx)
	defer cancel()
	defer close(toprocess)

	go fetchNodes(ctx, ds, toprocess, nodes)

	nodes <- &NodeOption{Node: root}
	live := 1

	for {
		select {
		case opt, ok := <-nodes:
			if !ok {
				return nil
			}

			if opt.Err != nil {
				return opt.Err
			}

			nd := opt.Node

			// a node has been fetched
			live--

			var keys []key.Key
			for _, lnk := range nd.Links {
				k := key.Key(lnk.Hash)
				if !set.Has(k) {
					set.Add(k)
					live++
					keys = append(keys, k)
				}
			}

			if live == 0 {
				return nil
			}

			if len(keys) > 0 {
				select {
				case toprocess <- keys:
				case <-ctx.Done():
					return ctx.Err()
				}
			}
		case <-ctx.Done():
			return ctx.Err()
		}
	}
}

func Descendants(ctx context.Context, ds dag.DAGService, set key.KeySet, roots []key.Key, bestEffort bool) error {
	for _, k := range roots {
		set.Add(k)
		nd, err := ds.Get(ctx, k)
		if err != nil {
			return err
		}

		// EnumerateChildren recursively walks the dag and adds the keys to the given set
		err = dag.EnumerateChildren(ctx, ds, nd, set, bestEffort)
		if err != nil {
			return err
		}
	}

	return nil
}

func Descendants(ds dag.DAGService, set key.KeySet, roots []key.Key) error {
	for _, k := range roots {
		set.Add(k)
		nd, err := ds.Get(context.Background(), k)
		if err != nil {
			return err
		}

		// EnumerateChildren recursively walks the dag and adds the keys to the given set
		err = dag.EnumerateChildren(context.Background(), ds, nd, set)
		if err != nil {
			return err
		}
	}

	return nil
}

// EnumerateChildren will walk the dag below the given root node and add all
// unseen children to the passed in set.
// TODO: parallelize to avoid disk latency perf hits?
func EnumerateChildren(ctx context.Context, ds DAGService, root *Node, set key.KeySet) error {
	for _, lnk := range root.Links {
		k := key.Key(lnk.Hash)
		if !set.Has(k) {
			set.Add(k)
			child, err := ds.Get(ctx, k)
			if err != nil {
				return err
			}
			err = EnumerateChildren(ctx, ds, child, set)
			if err != nil {
				return err
			}
		}
	}
	return nil
}

// EnumerateChildren will walk the dag below the given root node and add all
// unseen children to the passed in set.
// TODO: parallelize to avoid disk latency perf hits?
func EnumerateChildren(ctx context.Context, ds DAGService, root *Node, set key.KeySet, bestEffort bool) error {
	for _, lnk := range root.Links {
		k := key.Key(lnk.Hash)
		if !set.Has(k) {
			set.Add(k)
			child, err := ds.Get(ctx, k)
			if err != nil {
				if bestEffort && err == ErrNotFound {
					continue
				} else {
					return err
				}
			}
			err = EnumerateChildren(ctx, ds, child, set, bestEffort)
			if err != nil {
				return err
			}
		}
	}
	return nil
}