[#35] Implement netmap source interface in morph wrapper

Netmap source interface used by object service to build
placement based on current or previous network map.

Signed-off-by: Alex Vanin <alexey@nspcc.ru>

pkg/morph/client/netmap/wrapper/netmap.go

@@ -1,12 +1,43 @@
 package wrapper
 
-// NetMap represents the NeoFS network map.
-// FIXME: correct the definition.
-type NetMap struct{}
+import (
+	"github.com/nspcc-dev/neofs-api-go/pkg/netmap"
+	v2netmap "github.com/nspcc-dev/neofs-api-go/v2/netmap"
+	grpcNetmap "github.com/nspcc-dev/neofs-api-go/v2/netmap/grpc"
+	client "github.com/nspcc-dev/neofs-node/pkg/morph/client/netmap"
+	"github.com/pkg/errors"
+)
 
 // GetNetMap receives information list about storage nodes
 // through the Netmap contract call, composes network map
-// from them and returns it.
-func (w *Wrapper) GetNetMap() (*NetMap, error) {
-	panic("implement me")
+// from them and returns it. With diff == 0 returns current
+// network map, else return snapshot of previous network map.
+func (w Wrapper) GetNetMap(diff uint64) (*netmap.Netmap, error) {
+	args := client.GetSnapshotArgs{}
+	args.SetDiff(diff)
+
+	peers, err := w.client.Snapshot(args)
+	if err != nil {
+		return nil, err
+	}
+
+	rawPeers := peers.Peers() // slice of serialized node infos
+	infos := make([]v2netmap.NodeInfo, 0, len(rawPeers))
+
+	for _, peer := range rawPeers {
+		grpcNodeInfo := new(grpcNetmap.NodeInfo) // transport representation of struct
+		err = grpcNodeInfo.Unmarshal(peer)
+		if err != nil {
+			// consider unmarshalling into different major versions
+			// of NodeInfo structure, if there any
+			return nil, errors.Wrap(err, "can't unmarshal peer info")
+		}
+
+		v2 := v2netmap.NodeInfoFromGRPCMessage(grpcNodeInfo)
+		infos = append(infos, *v2)
+	}
+
+	nodes := netmap.NodesFromV2(infos)
+
+	return netmap.NewNetmap(nodes)
 }