[#969] node: Do not require shard config in relay mode

Relay storage node doesn't exec local object operations, so it doesn't need
shard configuration.

Add `required` bool parameter to `engineconfig.IterateShards`. Make it to
panic if it is `true`, and immediately return otherwise. Pass `false` if
node is configured as relay in app (it also prevents panic).

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

cmd/neofs-node/config.go

@@ -367,7 +367,9 @@ func initLocalStorage(c *cfg) {
 func initShardOptions(c *cfg) {
 	var opts [][]shard.Option
 
-	engineconfig.IterateShards(c.appCfg, func(sc *shardconfig.Config) {
+	require := !nodeconfig.Relay(c.appCfg) // relay node does not require shards
+
+	engineconfig.IterateShards(c.appCfg, require, func(sc *shardconfig.Config) {
 		var writeCacheOpts []writecache.Option
 
 		useWriteCache := sc.UseWriteCache()

cmd/neofs-node/config/engine/config.go

@@ -19,13 +19,17 @@ const (
 // of "shard" subsection of "storage" section of c, wrap them into
 // shardconfig.Config and passes to f.
 //
-// Panics if N is not a positive number.
-func IterateShards(c *config.Config, f func(*shardconfig.Config)) {
+// Panics if N is not a positive number while shards are required.
+func IterateShards(c *config.Config, required bool, f func(*shardconfig.Config)) {
 	c = c.Sub(subsection)
 
 	num := config.Uint(c, "shard_num")
 	if num == 0 {
-		panic("no shard configured")
+		if required {
+			panic("no shard configured")
+		}
+
+		return
 	}
 
 	def := c.Sub("default")

cmd/neofs-node/config/engine/config_test.go

@@ -17,9 +17,19 @@ func TestEngineSection(t *testing.T) {
 		empty := configtest.EmptyConfig()
 
 		require.Panics(t, func() {
-			engineconfig.IterateShards(empty, nil)
+			engineconfig.IterateShards(empty, true, nil)
 		})
 
+		handlerCalled := false
+
+		require.NotPanics(t, func() {
+			engineconfig.IterateShards(empty, false, func(_ *shardconfig.Config) {
+				handlerCalled = true
+			})
+		})
+
+		require.False(t, handlerCalled)
+
 		require.EqualValues(t, engineconfig.ShardPoolSizeDefault, engineconfig.ShardPoolSize(empty))
 	})
 
@@ -30,7 +40,7 @@ func TestEngineSection(t *testing.T) {
 
 		require.EqualValues(t, 15, engineconfig.ShardPoolSize(c))
 
-		engineconfig.IterateShards(c, func(sc *shardconfig.Config) {
+		engineconfig.IterateShards(c, true, func(sc *shardconfig.Config) {
 			defer func() {
 				num++
 			}()