[#293] object: Fix payload size limiter

* Sending empty chunks by `writeChunk` should not release new
  objects as this doesn't change `payloadSizeLimiter` internal
  state.
* This also fixes the bug with patcher when an offset of a patch
  equals to `MaxSize` - `payloadSizeLimiter` releases object again
  although state is the same. This led to error because EC-encoder
  receieved empty payload and couldn't not process it.

Signed-off-by: Airat Arifullin <a.arifullin@yadro.com>

object/transformer/transformer.go

@@ -261,7 +261,7 @@ func (s *payloadSizeLimiter) initializeLinking(parHdr *object.Object) {
 func (s *payloadSizeLimiter) writeChunk(ctx context.Context, chunk []byte) error {
 	for {
 		// statement is true if the previous write of bytes reached exactly the boundary.
-		if s.written > 0 && s.written%s.MaxSize == 0 {
+		if len(chunk) > 0 && s.written > 0 && s.written%s.MaxSize == 0 {
 			if s.written == s.MaxSize {
 				s.prepareFirstChild()
 			}

object/transformer/transformer_test.go

@@ -15,6 +15,41 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+func TestTransformerNonEffectiveWrites(t *testing.T) {
+	const maxSize = 16
+
+	tt := new(testTarget)
+	target, pk := newPayloadSizeLimiter(maxSize, 0, func() ObjectWriter { return tt })
+	cnr := cidtest.ID()
+	hdr := newObject(cnr)
+	var owner user.ID
+	user.IDFromKey(&owner, pk.PrivateKey.PublicKey)
+	hdr.SetOwnerID(owner)
+	payload := make([]byte, 3*maxSize)
+	_, _ = rand.Read(payload)
+
+	ctx := context.Background()
+	require.NoError(t, target.WriteHeader(ctx, hdr))
+
+	_, err := target.Write(ctx, payload)
+	require.NoError(t, err)
+
+	for range 5 {
+		// run onto unchanged target state
+		_, err = target.Write(ctx, []byte{})
+		require.NoError(t, err)
+	}
+
+	_, err = target.Close(ctx)
+	require.NoError(t, err)
+
+	require.Equal(t, 4, len(tt.objects))
+	require.Equal(t, payload[:maxSize], tt.objects[0].Payload())
+	require.Equal(t, payload[maxSize:2*maxSize], tt.objects[1].Payload())
+	require.Equal(t, payload[2*maxSize:], tt.objects[2].Payload())
+	require.Equal(t, 3, len(tt.objects[3].Children()))
+}
+
 func TestTransformer(t *testing.T) {
 	const maxSize = 100