lnd+graph: add GraphBootstrapper to the GraphSource interface

So that LND can use a different GraphSource for network bootstrapping
and does not need to rely on its local graph db.

graph/sources/chan_graph.go

@@ -8,6 +8,8 @@ import (
 
 	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/btcsuite/btcd/wire"
+	"github.com/lightningnetwork/lnd/autopilot"
+	"github.com/lightningnetwork/lnd/discovery"
 	graphdb "github.com/lightningnetwork/lnd/graph/db"
 	"github.com/lightningnetwork/lnd/graph/db/models"
 	"github.com/lightningnetwork/lnd/graph/session"
@@ -217,6 +219,18 @@ func (s *DBSource) FetchLightningNode(_ context.Context,
 	return s.db.FetchLightningNode(nodePub)
 }
 
+// GraphBootstrapper returns a NetworkPeerBootstrapper instance backed by the
+// ChannelGraph instance.
+//
+// NOTE: this is part of the GraphSource interface.
+func (s *DBSource) GraphBootstrapper(_ context.Context) (
+	discovery.NetworkPeerBootstrapper, error) {
+
+	chanGraph := autopilot.ChannelGraphFromDatabase(s.db)
+
+	return discovery.NewGraphBootstrapper(chanGraph)
+}
+
 // kvdbRTx is an implementation of graphdb.RTx backed by a KVDB database read
 // transaction.
 type kvdbRTx struct {

graph/sources/interfaces.go

@@ -6,6 +6,7 @@ import (
 
 	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/lightningnetwork/lnd/channeldb"
+	"github.com/lightningnetwork/lnd/discovery"
 	"github.com/lightningnetwork/lnd/graph/db/models"
 	"github.com/lightningnetwork/lnd/graph/session"
 	"github.com/lightningnetwork/lnd/lnrpc/invoicesrpc"
@@ -68,4 +69,9 @@ type GraphSource interface {
 	// graphdb.ErrGraphNodeNotFound is returned.
 	FetchLightningNode(ctx context.Context, nodePub route.Vertex) (
 		*models.LightningNode, error)
+
+	// GraphBootstrapper returns a network peer bootstrapper that can be
+	// used to discover new peers to connect to.
+	GraphBootstrapper(ctx context.Context) (
+		discovery.NetworkPeerBootstrapper, error)
 }

server.go

@@ -2817,6 +2817,7 @@ out:
 // based on the server, and currently active bootstrap mechanisms as defined
 // within the current configuration.
 func initNetworkBootstrappers(s *server) ([]discovery.NetworkPeerBootstrapper, error) {
+	ctx := context.TODO()
 	srvrLog.Infof("Initializing peer network bootstrappers!")
 
 	var bootStrappers []discovery.NetworkPeerBootstrapper
@@ -2824,8 +2825,7 @@ func initNetworkBootstrappers(s *server) ([]discovery.NetworkPeerBootstrapper, e
 	// First, we'll create an instance of the ChannelGraphBootstrapper as
 	// this can be used by default if we've already partially seeded the
 	// network.
-	chanGraph := autopilot.ChannelGraphFromDatabase(s.graphDB)
+	graphBootstrapper, err := s.graphSource.GraphBootstrapper(ctx)
-	graphBootstrapper, err := discovery.NewGraphBootstrapper(chanGraph)
 	if err != nil {
 		return nil, err
 	}