[#1451] placement: Return copy of slice from container nodes cache

Nodes from cache could be changed by traverser, if no objectID specified.
So it is required to return copy of cache's slice.

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

pkg/services/object_manager/placement/cache.go

@@ -44,7 +44,7 @@ func (c *ContainerNodesCache) ContainerNodes(nm *netmapSDK.NetMap, cnr cid.ID, p
 		raw, ok := c.containerCache.Get(cnr)
 		c.mtx.Unlock()
 		if ok {
-			return raw, nil
+			return c.cloneResult(raw), nil
 		}
 	} else {
 		c.lastEpoch = nm.Epoch()
@@ -65,5 +65,14 @@ func (c *ContainerNodesCache) ContainerNodes(nm *netmapSDK.NetMap, cnr cid.ID, p
 		c.containerCache.Add(cnr, cn)
 	}
 	c.mtx.Unlock()
-	return cn, nil
+	return c.cloneResult(cn), nil
+}
+
+func (c *ContainerNodesCache) cloneResult(nodes [][]netmapSDK.NodeInfo) [][]netmapSDK.NodeInfo {
+	result := make([][]netmapSDK.NodeInfo, len(nodes))
+	for repIdx := range nodes {
+		result[repIdx] = make([]netmapSDK.NodeInfo, 0, len(nodes[repIdx]))
+		result[repIdx] = append(result[repIdx], nodes[repIdx]...)
+	}
+	return result
 }