[#50] object/head: Add right child to Head operation result

Head service receives right child of the processing object in some cases.
Add right child to Head result in order to use it as needed.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/services/object/head/res.go

@@ -5,9 +5,13 @@ import (
 )
 
 type Response struct {
-	hdr *object.Object
+	hdr, rightChild *object.Object
 }
 
 func (r *Response) Header() *object.Object {
 	return r.hdr
 }
+
+func (r *Response) RightChild() *object.Object {
+	return r.rightChild
+}

pkg/services/object/head/service.go

@@ -77,15 +77,18 @@ func (s *Service) Head(ctx context.Context, prm *Prm) (*Response, error) {
 	addr.SetContainerID(prm.addr.GetContainerID())
 	addr.SetObjectID(rightChildID)
 
-	rightChild, err := s.Head(ctx, new(Prm).WithAddress(addr))
+	r, err = s.Head(ctx, new(Prm).WithAddress(addr))
 	if err != nil {
 		return nil, errors.Wrapf(err, "(%T) could not get right child header", s)
 	}
 
+	rightChild := r.Header()
+
 	// TODO: check if received parent has requested address
 
 	return &Response{
-		hdr: object.NewFromSDK(rightChild.Header().GetParent()),
+		hdr:        object.NewFromSDK(rightChild.GetParent()),
+		rightChild: rightChild,
 	}, nil
 }