[#50] services/object: Implement GetRange service

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

pkg/services/object/range/local.go

@@ -0,0 +1,36 @@
+package rangesvc
+
+import (
+	"io"
+
+	"github.com/nspcc-dev/neofs-api-go/pkg/object"
+	"github.com/nspcc-dev/neofs-node/pkg/local_object_storage/localstore"
+	"github.com/pkg/errors"
+)
+
+type localRangeWriter struct {
+	addr *object.Address
+
+	rng *object.Range
+
+	storage *localstore.Storage
+}
+
+func (l *localRangeWriter) WriteTo(w io.Writer) (int64, error) {
+	obj, err := l.storage.Get(l.addr)
+	if err != nil {
+		return 0, errors.Wrapf(err, "(%T) could not get object from local storage", l)
+	}
+
+	payload := obj.GetPayload()
+	left := l.rng.GetOffset()
+	right := left + l.rng.GetLength()
+
+	if ln := uint64(len(payload)); ln < right {
+		return 0, errors.Errorf("(%T) object range is out-of-boundaries (size %d, range [%d:%d]", l, ln, left, right)
+	}
+
+	n, err := w.Write(payload[left:right])
+
+	return int64(n), err
+}