signal: ensure interceptor is closed on startup error paths

docs/release-notes/release-notes-0.21.0.md

@@ -75,6 +75,11 @@
   transitions during startup, avoiding lost unlocks during slow database
   initialization.
 
+* [Ensure that the interceptor is properly closed on failure paths in `lnd.Start`](https://github.com/lightningnetwork/lnd/pull/10587).
+  This is mostly relevant for mobile and lnd-as-a-lib use-cases where subsequent
+  attempts to start lnd would otherwise fail with "intercept already started"
+  errors as the state stays in-process.
+
 # New Features
 
 - Basic Support for [onion messaging forwarding](https://github.com/lightningnetwork/lnd/pull/9868) 
@@ -191,6 +196,7 @@
 * Boris Nagaev
 * Elle Mouton
 * Erick Cestari
+* Hampus Sjöberg
 * hieblmi
 * Matt Morehouse
 * Mohamed Awnallah

lnd.go

@@ -146,7 +146,7 @@ var errStreamIsolationWithProxySkip = errors.New(
 // 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, implCfg *ImplementationCfg,
-	interceptor signal.Interceptor) error {
+	interceptor signal.Interceptor) (mainErr error) {
 
 	defer func() {
 		ltndLog.Info("Shutdown complete")
@@ -156,6 +156,17 @@ func Main(cfg *Config, lisCfg ListenerCfg, implCfg *ImplementationCfg,
 		}
 	}()
 
+	// Make sure the signal interceptor is always fully stopped on any
+	// startup/runtime error so callers can safely retry startup.
+	defer func() {
+		if mainErr == nil {
+			return
+		}
+
+		interceptor.RequestShutdown()
+		<-interceptor.ShutdownChannel()
+	}()
+
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
 

lnd_main_test.go

@@ -0,0 +1,33 @@
+package lnd
+
+import (
+	"net"
+	"testing"
+	"time"
+
+	"github.com/lightningnetwork/lnd/lncfg"
+	"github.com/lightningnetwork/lnd/signal"
+	"github.com/stretchr/testify/require"
+)
+
+// TestMainErrorShutsDownInterceptor asserts that Main error paths always close
+// the signal interceptor so startup can be retried in-process.
+func TestMainErrorShutsDownInterceptor(t *testing.T) {
+	interceptor, err := signal.Intercept()
+	require.NoError(t, err)
+
+	cfg := DefaultConfig()
+	cfg.Pprof = &lncfg.Pprof{}
+	cfg.RPCListeners = []net.Addr{
+		&net.UnixAddr{Net: "invalid-network"},
+	}
+	mainErr := Main(&cfg, ListenerCfg{}, nil, interceptor)
+	require.Error(t, mainErr)
+
+	select {
+	case <-interceptor.ShutdownChannel():
+	case <-time.After(5 * time.Second):
+		t.Fatalf("interceptor wasn't shut down after Main returned " +
+			"error")
+	}
+}