lnd+cmd: move interrupt into cmd

If the main package is used as a library, we don't want it to
register interrupt signals itself. Rather we want to pass in the
shutdown channel manually. We do this in the cmd now.

cmd/lnd/main.go

@@ -4,8 +4,9 @@ import (
 	"fmt"
 	"os"
 
-	flags "github.com/jessevdk/go-flags"
+	"github.com/jessevdk/go-flags"
 	"github.com/lightningnetwork/lnd"
+	"github.com/lightningnetwork/lnd/signal"
 )
 
 func main() {
@@ -17,9 +18,15 @@ func main() {
 		os.Exit(1)
 	}
 
+	// Hook interceptor for os signals.
+	signal.Intercept()
+
 	// Call the "real" main in a nested manner so the defers will properly
 	// be executed in the case of a graceful shutdown.
-	if err := lnd.Main(loadedConfig, lnd.ListenerCfg{}); err != nil {
+	err = lnd.Main(
+		loadedConfig, lnd.ListenerCfg{}, signal.ShutdownChannel(),
+	)
+	if err != nil {
 		if e, ok := err.(*flags.Error); ok && e.Type == flags.ErrHelp {
 		} else {
 			_, _ = fmt.Fprintln(os.Stderr, err)

lnd.go

@@ -147,13 +147,11 @@ type ListenerCfg struct {
 // listeners.
 type rpcListeners func() ([]*ListenerWithSignal, func(), error)
 
-// Main is the true entry point for lnd. This function is required since defers
-// created in the top-level scope of a main method aren't executed if os.Exit()
-// is called.
-func Main(config *Config, lisCfg ListenerCfg) error {
-	// Hook interceptor for os signals.
-	signal.Intercept()
-
+// Main is the true entry point for lnd. It accepts a fully populated and
+// 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(config *Config, lisCfg ListenerCfg, shutdownChan <-chan struct{}) error {
 	cfg = config
 	defer func() {
 		ltndLog.Info("Shutdown complete")
@@ -728,7 +726,7 @@ func Main(config *Config, lisCfg ListenerCfg) error {
 
 	// Wait for shutdown signal from either a graceful server stop or from
 	// the interrupt handler.
-	<-signal.ShutdownChannel()
+	<-shutdownChan
 	return nil
 }