multi: return error from MarkEdgeLive if not found

Let MarkEdgLive return a new ErrNotZombieEdge error if an entry with the
given channel ID cannot be found. In processZombieUpdate, we then
check for this error and log accordingly.

channeldb/error.go

@@ -75,6 +75,10 @@ var (
 	// but it is marked as a zombie within the zombie index.
 	ErrZombieEdge = errors.New("edge marked as zombie")
 
+	// ErrZombieEdgeNotFound is an error returned when we attempt to find an
+	// edge in the zombie index which is not there.
+	ErrZombieEdgeNotFound = errors.New("edge not found in zombie index")
+
 	// ErrEdgeAlreadyExist is returned when edge with specific
 	// channel id can't be added because it already exist.
 	ErrEdgeAlreadyExist = fmt.Errorf("edge already exist")

channeldb/graph.go

@@ -3512,6 +3512,11 @@ func (c *ChannelGraph) MarkEdgeLive(chanID uint64) error {
 
 		var k [8]byte
 		byteOrder.PutUint64(k[:], chanID)
+
+		if len(zombieIndex.Get(k[:])) == 0 {
+			return ErrZombieEdgeNotFound
+		}
+
 		return zombieIndex.Delete(k[:])
 	}, func() {})
 	if err != nil {

channeldb/graph_test.go

@@ -3089,6 +3089,12 @@ func TestGraphZombieIndex(t *testing.T) {
 	// it within the index.
 	require.NoError(t, graph.MarkEdgeLive(edge.ChannelID))
 
+	// Attempting to mark the edge as live again now that it is no longer
+	// in the zombie index should fail.
+	require.ErrorIs(
+		t, graph.MarkEdgeLive(edge.ChannelID), ErrZombieEdgeNotFound,
+	)
+
 	isZombie, _, _ = graph.IsZombieEdge(edge.ChannelID)
 	require.False(t, isZombie)
 

discovery/gossiper.go

@@ -2057,10 +2057,19 @@ func (d *AuthenticatedGossiper) processZombieUpdate(
 	// come through again.
 	baseScid := lnwire.NewShortChanIDFromInt(chanInfo.ChannelID)
 	err = d.cfg.Router.MarkEdgeLive(baseScid)
-	if err != nil {
+	switch {
+	case errors.Is(err, channeldb.ErrZombieEdgeNotFound):
+		log.Errorf("edge with chan_id=%v was not found in the "+
+			"zombie index: %v", err)
+
+		return nil
+
+	case err != nil:
 		return fmt.Errorf("unable to remove edge with "+
 			"chan_id=%v from zombie index: %v",
 			msg.ShortChannelID, err)
+
+	default:
 	}
 
 	log.Debugf("Removed edge with chan_id=%v from zombie "+