signal: handle shutdown properly

This addresses issues related to the mobile lnd builds, where
calling `stopDaemon` and subsequently calling LndMobile's `Start`
results in crash.

lnd.go

@@ -190,7 +190,7 @@ type ListenerCfg struct {
 // 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.
-func Main(cfg *Config, lisCfg ListenerCfg, shutdownChan <-chan struct{}) error {
+func Main(cfg *Config, lisCfg ListenerCfg, interceptor signal.Interceptor) error {
 	defer func() {
 		ltndLog.Info("Shutdown complete\n")
 		err := cfg.LogWriter.Close()
@@ -378,6 +378,7 @@ func Main(cfg *Config, lisCfg ListenerCfg, shutdownChan <-chan struct{}) error {
 	rpcServer := newRPCServer(
 		cfg, interceptorChain, lisCfg.ExternalRPCSubserverCfg,
 		lisCfg.ExternalRestRegistrar,
+		interceptor,
 	)
 
 	err = rpcServer.RegisterWithGrpcServer(grpcServer)
@@ -408,7 +409,7 @@ func Main(cfg *Config, lisCfg ListenerCfg, shutdownChan <-chan struct{}) error {
 	// started with the --noseedbackup flag, we use the default password
 	// for wallet encryption.
 	if !cfg.NoSeedBackup {
-		params, err := waitForWalletPassword(cfg, pwService)
+		params, err := waitForWalletPassword(cfg, pwService, interceptor.ShutdownChannel())
 		if err != nil {
 			err := fmt.Errorf("unable to set up wallet password "+
 				"listeners: %v", err)
@@ -793,7 +794,7 @@ func Main(cfg *Config, lisCfg ListenerCfg, shutdownChan <-chan struct{}) error {
 			"start_height=%v", bestHeight)
 
 		for {
-			if !signal.Alive() {
+			if !interceptor.Alive() {
 				return nil
 			}
 
@@ -856,7 +857,7 @@ func Main(cfg *Config, lisCfg ListenerCfg, shutdownChan <-chan struct{}) error {
 
 	// Wait for shutdown signal from either a graceful server stop or from
 	// the interrupt handler.
-	<-shutdownChan
+	<-interceptor.ShutdownChannel()
 	return nil
 }
 
@@ -1328,7 +1329,8 @@ func startRestProxy(cfg *Config, rpcServer *rpcServer, restDialOpts []grpc.DialO
 // waitForWalletPassword blocks until a password is provided by the user to
 // this RPC server.
 func waitForWalletPassword(cfg *Config,
-	pwService *walletunlocker.UnlockerService) (*WalletUnlockParams, error) {
+	pwService *walletunlocker.UnlockerService,
+	shutdownChan <-chan struct{}) (*WalletUnlockParams, error) {
 
 	chainConfig := cfg.Bitcoin
 	if cfg.registeredChains.PrimaryChain() == chainreg.LitecoinChain {
@@ -1430,7 +1432,8 @@ func waitForWalletPassword(cfg *Config,
 			MacResponseChan: pwService.MacResponseChan,
 		}, nil
 
-	case <-signal.ShutdownChannel():
+	// If we got a shutdown signal we just return with an error immediately
+	case <-shutdownChan:
 		return nil, fmt.Errorf("shutting down")
 	}
 }