[#1793] node/netmap: Change interface of the latest network map reader

Replace `ProcessCurrentNetMap` method of `NodeState` interface with
`ReadCurrentNetMap` one with two changes:
 * Replace network map type from NeoFS SDK package with the
   protocol-generated message. This replaces all the business logic to
   the application layer.
 * Support error return. This allows to cover problem node states.

Return an error from `NodeState.ReadCurrentNetMap` method implemeted
through `atomic.Value` if `Store` method has not been called yet.

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

cmd/neofs-node/config.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"net"
 	"path/filepath"
@@ -140,8 +141,19 @@ type cfg struct {
 	netMap atomicstd.Value // type netmap.NetMap
 }
 
-func (c *cfg) ProcessCurrentNetMap(f func(netmap.NetMap)) {
+// ReadCurrentNetMap reads network map which has been cached at the
-	f(c.netMap.Load().(netmap.NetMap))
+// latest epoch. Returns an error if value has not been cached yet.
+//
+// Provides interface for NetmapService server.
+func (c *cfg) ReadCurrentNetMap(msg *netmapV2.NetMap) error {
+	val := c.netMap.Load()
+	if val == nil {
+		return errors.New("missing local network map")
+	}
+
+	val.(netmap.NetMap).WriteToV2(msg)
+
+	return nil
 }
 
 type cfgGRPC struct {

pkg/services/netmap/executor.go

@@ -27,9 +27,10 @@ type NodeState interface {
 	// in NeoFS API v2 NodeInfo structure.
 	LocalNodeInfo() (*netmap.NodeInfo, error)
 
-	// ProcessCurrentNetMap passes current local network map of the storage node
+	// ReadCurrentNetMap reads current local network map of the storage node
-	// into the given handler.
+	// into the given parameter. Returns any error encountered which prevented
-	ProcessCurrentNetMap(func(netmapSDK.NetMap))
+	// the network map to be read.
+	ReadCurrentNetMap(*netmap.NetMap) error
 }
 
 // NetworkInfo encapsulates source of the
@@ -129,14 +130,15 @@ func (s *executorSvc) NetworkInfo(
 }
 
 func (s *executorSvc) Snapshot(_ context.Context, req *netmap.SnapshotRequest) (*netmap.SnapshotResponse, error) {
-	var nmV2 netmap.NetMap
+	var nm netmap.NetMap
 
-	s.state.ProcessCurrentNetMap(func(netMap netmapSDK.NetMap) {
+	err := s.state.ReadCurrentNetMap(&nm)
-		netMap.WriteToV2(&nmV2)
+	if err != nil {
-	})
+		return nil, fmt.Errorf("read current local network map: %w", err)
+	}
 
 	body := new(netmap.SnapshotResponseBody)
-	body.SetNetMap(&nmV2)
+	body.SetNetMap(&nm)
 
 	resp := new(netmap.SnapshotResponse)
 	resp.SetBody(body)