lnd: introduce the Providers interface

This is an interface that the LND can provide to other callers systems
in the same process.

cmd/lnd/main.go

@@ -37,6 +37,7 @@ func main() {
 	// be executed in the case of a graceful shutdown.
 	if err = lnd.Main(
 		loadedConfig, lnd.ListenerCfg{}, implCfg, shutdownInterceptor,
+		nil,
 	); err != nil {
 		_, _ = fmt.Fprintln(os.Stderr, err)
 		os.Exit(1)

cmd/multinode/graph_mux.go

@@ -0,0 +1 @@
+package multinode

cmd/multinode/main.go

@@ -0,0 +1 @@
+package multinode

interfaces.go

@@ -46,3 +46,21 @@ type GraphSource interface {
 	// found.
 	LookupAlias(pub *btcec.PublicKey) (string, error)
 }
+
+// Providers is an interface that LND itself can satisfy.
+type Providers interface {
+	// GraphSource can be used to obtain the graph source that this LND node
+	// provides.
+	GraphSource() (GraphSource, error)
+}
+
+// A compile-time check to ensure that LND's main server struct satisfies the
+// Provider interface.
+var _ Providers = (*server)(nil)
+
+// GraphSource returns this LND nodes graph DB as a GraphSource.
+//
+// NOTE: this method is part of the Providers interface.
+func (s *server) GraphSource() (GraphSource, error) {
+	return s.graphDB, nil
+}

lnd.go

@@ -144,8 +144,10 @@ var errStreamIsolationWithProxySkip = errors.New(
 // validated main configuration struct and an optional listener config struct.
 // This function starts all main system components then blocks until a signal
 // is received on the shutdownChan at which point everything is shut down again.
+// If a providers channel is provided, LND will send back a result on it once
+// all the necessary providers are ready.
 func Main(cfg *Config, lisCfg ListenerCfg, implCfg *ImplementationCfg,
-	interceptor signal.Interceptor) error {
+	interceptor signal.Interceptor, providers chan<- Providers) error {
 
 	defer func() {
 		ltndLog.Info("Shutdown complete\n")
@@ -612,6 +614,11 @@ func Main(cfg *Config, lisCfg ListenerCfg, implCfg *ImplementationCfg,
 		return mkErr("unable to create server: %v", err)
 	}
 
+	select {
+	case providers <- server:
+	default:
+	}
+
 	// Set up an autopilot manager from the current config. This will be
 	// used to manage the underlying autopilot agent, starting and stopping
 	// it at will.

mobile/bindings.go

@@ -110,7 +110,7 @@ func Start(extraArgs string, rpcReady Callback) {
 		defer close(quit)
 
 		if err := lnd.Main(
-			loadedConfig, cfg, implCfg, shutdownInterceptor,
+			loadedConfig, cfg, implCfg, shutdownInterceptor, nil,
 		); err != nil {
 			if e, ok := err.(*flags.Error); ok &&
 				e.Type == flags.ErrHelp {