lnd+graph: add GraphSource interface and implementation

This commit adds a new GraphSources interface that LND requires for
graph related read-only queries. As of this commit, the interface is
empty but it will be populated over the next couple of commits. We add
an implementation of this interface backed by a pointer to a
graphdb.ChannelGraph.

The infrustructure is put into place so that the GraphSoure provided to
LND can be overridden by a caller of the lnd.Main function. By default,
LND will satisfy the interface itself via the new `graphsource.DBSource`
struct.

config.go

@@ -1802,6 +1802,7 @@ func (c *Config) ImplementationConfig(
 			),
 			WalletConfigBuilder: rpcImpl,
 			ChainControlBuilder: rpcImpl,
+			GraphProvider:       rpcImpl,
 		}
 	}
 
@@ -1813,6 +1814,7 @@ func (c *Config) ImplementationConfig(
 		DatabaseBuilder:     NewDefaultDatabaseBuilder(c, ltndLog),
 		WalletConfigBuilder: defaultImpl,
 		ChainControlBuilder: defaultImpl,
+		GraphProvider:       defaultImpl,
 	}
 }
 

config_builder.go

@@ -36,6 +36,7 @@ import (
 	"github.com/lightningnetwork/lnd/fn"
 	"github.com/lightningnetwork/lnd/funding"
 	graphdb "github.com/lightningnetwork/lnd/graph/db"
+	"github.com/lightningnetwork/lnd/graph/sources"
 	"github.com/lightningnetwork/lnd/invoices"
 	"github.com/lightningnetwork/lnd/keychain"
 	"github.com/lightningnetwork/lnd/kvdb"
@@ -125,6 +126,14 @@ type ChainControlBuilder interface {
 		*btcwallet.Config) (*chainreg.ChainControl, func(), error)
 }
 
+// GraphProvider is an interface that must be satisfied by any external system
+// that wants to provide LND with graph information.
+type GraphProvider interface {
+	// Graph returns the GraphSource that LND will use for read-only graph
+	// related queries.
+	Graph(context.Context, *DatabaseInstances) (sources.GraphSource, error)
+}
+
 // ImplementationCfg is a struct that holds all configuration items for
 // components that can be implemented outside lnd itself.
 type ImplementationCfg struct {
@@ -155,6 +164,10 @@ type ImplementationCfg struct {
 	// AuxComponents is a set of auxiliary components that can be used by
 	// lnd for certain custom channel types.
 	AuxComponents
+
+	// GraphProvider is a type that can provide a custom GraphSource for LND
+	// to use for read-only graph calls.
+	GraphProvider
 }
 
 // AuxComponents is a set of auxiliary components that can be used by lnd for
@@ -249,6 +262,17 @@ func (d *DefaultWalletImpl) RegisterGrpcSubserver(s *grpc.Server) error {
 	return nil
 }
 
+// Graph returns the GraphSource that LND will use for read-only graph related
+// queries. By default, the GraphSource implementation is this LND node's
+// backing graphdb.ChannelGraph.
+//
+// NOTE: this is part of the GraphProvider interface.
+func (d *DefaultWalletImpl) Graph(_ context.Context,
+	dbs *DatabaseInstances) (sources.GraphSource, error) {
+
+	return sources.NewDBGSource(dbs.GraphDB), nil
+}
+
 // ValidateMacaroon extracts the macaroon from the context's gRPC metadata,
 // checks its signature, makes sure all specified permissions for the called
 // method are contained within and finally ensures all caveat conditions are

graph/sources/chan_graph.go

@@ -0,0 +1,21 @@
+package sources
+
+import graphdb "github.com/lightningnetwork/lnd/graph/db"
+
+// DBSource is an implementation of the GraphSource interface backed by a local
+// persistence layer holding graph related data.
+type DBSource struct {
+	db *graphdb.ChannelGraph
+}
+
+// A compile-time check to ensure that sources.DBSource implements the
+// GraphSource interface.
+var _ GraphSource = (*DBSource)(nil)
+
+// NewDBGSource returns a new instance of the DBSource backed by a
+// graphdb.ChannelGraph instance.
+func NewDBGSource(db *graphdb.ChannelGraph) *DBSource {
+	return &DBSource{
+		db: db,
+	}
+}

graph/sources/interfaces.go

@@ -0,0 +1,6 @@
+package sources
+
+// GraphSource defines the read-only graph interface required by LND for graph
+// related queries.
+type GraphSource interface {
+}

lnd.go

@@ -485,6 +485,11 @@ func Main(cfg *Config, lisCfg ListenerCfg, implCfg *ImplementationCfg,
 		return mkErr("error deriving node key: %v", err)
 	}
 
+	graphSource, err := implCfg.Graph(ctx, dbs)
+	if err != nil {
+		return mkErr("error obtaining graph source: %v", err)
+	}
+
 	if cfg.Tor.StreamIsolation && cfg.Tor.SkipProxyForClearNetTargets {
 		return errStreamIsolationWithProxySkip
 	}
@@ -601,7 +606,7 @@ func Main(cfg *Config, lisCfg ListenerCfg, implCfg *ImplementationCfg,
 		cfg, cfg.Listeners, dbs, activeChainControl, &idKeyDesc,
 		activeChainControl.Cfg.WalletUnlockParams.ChansToRestore,
 		multiAcceptor, torController, tlsManager, leaderElector,
-		implCfg,
+		graphSource, implCfg,
 	)
 	if err != nil {
 		return mkErr("unable to create server: %v", err)

server.go

@@ -45,6 +45,7 @@ import (
 	graphdb "github.com/lightningnetwork/lnd/graph/db"
 	"github.com/lightningnetwork/lnd/graph/db/models"
 	graphsession "github.com/lightningnetwork/lnd/graph/session"
+	"github.com/lightningnetwork/lnd/graph/sources"
 	"github.com/lightningnetwork/lnd/healthcheck"
 	"github.com/lightningnetwork/lnd/htlcswitch"
 	"github.com/lightningnetwork/lnd/htlcswitch/hop"
@@ -261,6 +262,10 @@ type server struct {
 
 	graphDB *graphdb.ChannelGraph
 
+	// graphSource can be used for any read only graph queries. This may be
+	// implemented by this LND node or some other external source.
+	graphSource sources.GraphSource
+
 	chanStateDB *channeldb.ChannelStateDB
 
 	addrSource channeldb.AddrSource
@@ -507,7 +512,7 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 	chansToRestore walletunlocker.ChannelsToRecover,
 	chanPredicate chanacceptor.ChannelAcceptor,
 	torController *tor.Controller, tlsManager *TLSManager,
-	leaderElector cluster.LeaderElector,
+	leaderElector cluster.LeaderElector, graphSource sources.GraphSource,
 	implCfg *ImplementationCfg) (*server, error) {
 
 	var (
@@ -613,6 +618,7 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 		cfg:            cfg,
 		implCfg:        implCfg,
 		graphDB:        dbs.GraphDB,
+		graphSource:    graphSource,
 		chanStateDB:    dbs.ChanStateDB.ChannelStateDB(),
 		addrSource:     addrSource,
 		miscDB:         dbs.ChanStateDB,