Lotus chain nodes relay indexer pubsub messages

Content providers announce the availability of indexer data using gossip pubsub.  The content providers are not connected directly to indexers, so the pubsub messages are relayed to indexers via chain nodes. This PR makes chain nodes relay gossip pubsub messages, on the /indexer/ingest/<netname> topic.

node/modules/services.go

@@ -198,6 +198,36 @@ func HandleIncomingMessages(mctx helpers.MetricsCtx, lc fx.Lifecycle, ps *pubsub
 	waitForSync(stmgr, pubsubMsgsSyncEpochs, subscribe)
 }
 
+func RelayIndexerMessages(lc fx.Lifecycle, ps *pubsub.PubSub, nn dtypes.NetworkName, h host.Host) error {
+	topicName := build.IngestTopic(nn)
+
+	v := sub.NewIndexerMessageValidator(h.ID())
+
+	if err := ps.RegisterTopicValidator(topicName, v.Validate); err != nil {
+		panic(err)
+	}
+
+	topicHandle, err := ps.Join(topicName)
+	if err != nil {
+		return xerrors.Errorf("failed to join pubsub topic %s: %w", topicName, err)
+	}
+	cancelFunc, err := topicHandle.Relay()
+	if err != nil {
+		return xerrors.Errorf("failed to relay to pubsub messages for topic %s: %w", topicName, err)
+	}
+
+	// Cancel message relay on shutdown.
+	lc.Append(fx.Hook{
+		OnStop: func(_ context.Context) error {
+			cancelFunc()
+			return nil
+		},
+	})
+
+	log.Infof("relaying messages for pubsub topic %s", topicName)
+	return nil
+}
+
 func NewLocalDiscovery(lc fx.Lifecycle, ds dtypes.MetadataDS) (*discoveryimpl.Local, error) {
 	local, err := discoveryimpl.NewLocal(namespace.Wrap(ds, datastore.NewKey("/deals/local")))
 	if err != nil {