[#155] sdk/object: Add CutPayload method to RawObject

Add CutPayload method that returns RawObject's copy with untethered empty
payload. Changing the result payload doesn't affect source object. Changing
the other fields affects source object.

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

pkg/object/raw.go

@@ -120,3 +120,16 @@ func (o *RawObject) SetSessionToken(v *token.SessionToken) {
 func (o *RawObject) SetType(v Type) {
 	o.setType(v)
 }
+
+// CutPayload returns RawObject w/ empty payload.
+//
+// Changes of non-payload fields affect source object.
+func (o *RawObject) CutPayload() *RawObject {
+	if o != nil {
+		return &RawObject{
+			rwObject: o.rwObject.cutPayload(),
+		}
+	}
+
+	return nil
+}

pkg/object/raw_test.go

@@ -224,3 +224,30 @@ func TestRawObject_SetType(t *testing.T) {
 
 	require.Equal(t, typ, obj.GetType())
 }
+
+func TestRawObject_CutPayload(t *testing.T) {
+	o1 := NewRaw()
+
+	p1 := []byte{12, 3}
+	o1.SetPayload(p1)
+
+	sz := uint64(13)
+	o1.SetPayloadSize(sz)
+
+	o2 := o1.CutPayload()
+
+	require.Equal(t, sz, o2.GetPayloadSize())
+	require.Empty(t, o2.GetPayload())
+
+	sz++
+	o1.SetPayloadSize(sz)
+
+	require.Equal(t, sz, o1.GetPayloadSize())
+	require.Equal(t, sz, o2.GetPayloadSize())
+
+	p2 := []byte{4, 5, 6}
+	o2.SetPayload(p2)
+
+	require.Equal(t, p2, o2.GetPayload())
+	require.Equal(t, p1, o1.GetPayload())
+}

pkg/object/rw.go

@@ -303,3 +303,11 @@ func (o *rwObject) setType(t Type) {
 		h.SetObjectType(t.ToV2())
 	})
 }
+
+func (o *rwObject) cutPayload() *rwObject {
+	ov2 := new(object.Object)
+	*ov2 = *(*object.Object)(o)
+	ov2.SetPayload(nil)
+
+	return (*rwObject)(ov2)
+}