[#212] pool: Control sub tree nodes order

Signed-off-by: Alex Vanin <a.vanin@yadro.com>

pool/tree/pool.go

@@ -26,6 +26,16 @@ const (
 	defaultStreamTimeout      = 10 * time.Second
 )
 
+// SubTreeSort defines an order of nodes returned from GetSubTree RPC.
+type SubTreeSort int32
+
+const (
+	// NoneOrder does not specify order of nodes returned in GetSubTree RPC.
+	NoneOrder SubTreeSort = iota
+	// AscendingOrder specifies ascending alphabetical order of nodes based on FilePath attribute.
+	AscendingOrder
+)
+
 var (
 	// ErrNodeNotFound is returned from Tree service in case of not found error.
 	ErrNodeNotFound = errors.New("not found")
@@ -119,6 +129,7 @@ type GetSubTreeParams struct {
 	RootID      uint64
 	Depth       uint32
 	BearerToken []byte
+	Order       SubTreeSort
 }
 
 // AddNodeParams groups parameters of Pool.AddNode operation.
@@ -388,12 +399,17 @@ func (p *Pool) GetSubTree(ctx context.Context, prm GetSubTreeParams) (*SubTreeRe
 			RootId:      prm.RootID,
 			Depth:       prm.Depth,
 			BearerToken: prm.BearerToken,
-			OrderBy: &grpcService.GetSubTreeRequest_Body_Order{
+			OrderBy:     new(grpcService.GetSubTreeRequest_Body_Order),
-				Direction: grpcService.GetSubTreeRequest_Body_Order_Asc,
-			},
 		},
 	}
 
+	switch prm.Order {
+	case AscendingOrder:
+		request.Body.OrderBy.Direction = grpcService.GetSubTreeRequest_Body_Order_Asc
+	default:
+		request.Body.OrderBy.Direction = grpcService.GetSubTreeRequest_Body_Order_None
+	}
+
 	if err := p.signRequest(request.Body, func(key, sign []byte) {
 		request.Signature = &grpcService.Signature{
 			Key:  key,