[#64] transformer: Simplify interface

Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>

object/transformer/transformer.go

@@ -20,6 +20,7 @@ type payloadSizeLimiter struct {
 	written, writtenCurrent uint64
 
 	current, parent *object.Object
+	payload         []byte
 
 	currentHashers, parentHashers []payloadChecksumHasher
 
@@ -29,12 +30,12 @@ type payloadSizeLimiter struct {
 
 	parAttrs []object.Attribute
 
-	nextTarget ObjectTarget
+	nextTarget ObjectWriter
 }
 
 type Params struct {
 	Key                    *ecdsa.PrivateKey
-	NextTargetInit         func() ObjectTarget
+	NextTargetInit         TargetInitializer
 	SessionToken           *session.Object
 	NetworkState           EpochSource
 	MaxSize                uint64
@@ -48,7 +49,7 @@ type Params struct {
 // is false.
 //
 // Objects w/ payload size less or equal than max size remain untouched.
-func NewPayloadSizeLimiter(p Params) ObjectTarget {
+func NewPayloadSizeLimiter(p Params) ChunkedObjectWriter {
 	return &payloadSizeLimiter{
 		Params:  p,
 		splitID: object.NewSplitID(),
@@ -120,6 +121,7 @@ func (s *payloadSizeLimiter) initializeCurrent() {
 	s.nextTarget = s.NextTargetInit()
 	s.writtenCurrent = 0
 	s.initPayloadHashers()
+	s.payload = make([]byte, 0)
 }
 
 func (s *payloadSizeLimiter) initPayloadHashers() {
@@ -160,12 +162,9 @@ func (s *payloadSizeLimiter) release(ctx context.Context, finalize bool) (*Acces
 		return nil, fmt.Errorf("fillHeader: %w", err)
 	}
 
-	if err := s.nextTarget.WriteHeader(ctx, s.current); err != nil {
-		return nil, fmt.Errorf("could not write header to next target: %w", err)
-	}
-
-	if _, err := s.nextTarget.Close(ctx); err != nil {
-		return nil, fmt.Errorf("could not close next target: %w", err)
+	s.current.SetPayload(s.payload)
+	if err := s.nextTarget.WriteObject(ctx, s.current); err != nil {
+		return nil, fmt.Errorf("could not write to next target: %w", err)
 	}
 
 	// save identifier of the released object
@@ -262,7 +261,7 @@ func (s *payloadSizeLimiter) writeChunk(ctx context.Context, chunk []byte) error
 			cut = leftToEdge
 		}
 
-		if err := s.writeHashes(ctx, chunk[:cut]); err != nil {
+		if err := s.writeHashes(chunk[:cut]); err != nil {
 			return fmt.Errorf("could not write chunk to target: %w", err)
 		}
 
@@ -278,11 +277,8 @@ func (s *payloadSizeLimiter) writeChunk(ctx context.Context, chunk []byte) error
 	}
 }
 
-func (s *payloadSizeLimiter) writeHashes(ctx context.Context, chunk []byte) error {
-	_, err := s.nextTarget.Write(ctx, chunk)
-	if err != nil {
-		return err
-	}
+func (s *payloadSizeLimiter) writeHashes(chunk []byte) error {
+	s.payload = append(s.payload, chunk...)
 
 	// The `Write` method of `hash.Hash` never returns an error.
 	for i := range s.currentHashers {