Ensure chain notifier is started before accessed.

The use case comes from the RPC layer that is ready before the
chain notifier which is used in the sub server.

lnrpc/chainrpc/chainnotifier_server.go

@@ -63,6 +63,11 @@ var (
 	// has been shut down.
 	ErrChainNotifierServerShuttingDown = errors.New("chain notifier RPC " +
 		"subserver shutting down")
+
+	// ErrChainNotifierServerNotActive indicates that the chain notifier hasn't
+	// finished the startup process.
+	ErrChainNotifierServerNotActive = errors.New("chain notifier RPC is" +
+		"still in the process of starting")
 )
 
 // fileExists reports whether the named file or directory exists.
@@ -196,6 +201,10 @@ func (s *Server) RegisterWithRootServer(grpcServer *grpc.Server) error {
 func (s *Server) RegisterConfirmationsNtfn(in *ConfRequest,
 	confStream ChainNotifier_RegisterConfirmationsNtfnServer) error {
 
+	if !s.cfg.ChainNotifier.Started() {
+		return ErrChainNotifierServerNotActive
+	}
+
 	// We'll start by reconstructing the RPC request into what the
 	// underlying ChainNotifier expects.
 	var txid chainhash.Hash
@@ -292,6 +301,10 @@ func (s *Server) RegisterConfirmationsNtfn(in *ConfRequest,
 func (s *Server) RegisterSpendNtfn(in *SpendRequest,
 	spendStream ChainNotifier_RegisterSpendNtfnServer) error {
 
+	if !s.cfg.ChainNotifier.Started() {
+		return ErrChainNotifierServerNotActive
+	}
+
 	// We'll start by reconstructing the RPC request into what the
 	// underlying ChainNotifier expects.
 	var op *wire.OutPoint
@@ -399,6 +412,10 @@ func (s *Server) RegisterSpendNtfn(in *SpendRequest,
 func (s *Server) RegisterBlockEpochNtfn(in *BlockEpoch,
 	epochStream ChainNotifier_RegisterBlockEpochNtfnServer) error {
 
+	if !s.cfg.ChainNotifier.Started() {
+		return ErrChainNotifierServerNotActive
+	}
+
 	// We'll start by reconstructing the RPC request into what the
 	// underlying ChainNotifier expects.
 	var hash chainhash.Hash