beacon: add constructor

server.go

@@ -595,10 +595,7 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 		quit:       make(chan struct{}),
 	}
 
-	s.witnessBeacon = &preimageBeacon{
+	s.witnessBeacon = newPreimageBeacon(dbs.ChanStateDB.NewWitnessCache())
-		wCache:      dbs.ChanStateDB.NewWitnessCache(),
-		subscribers: make(map[uint64]*preimageSubscriber),
-	}
 
 	currentHash, currentHeight, err := s.cc.ChainIO.GetBestBlock()
 	if err != nil {

witness_beacon.go

@@ -28,6 +28,13 @@ type preimageBeacon struct {
 	subscribers   map[uint64]*preimageSubscriber
 }
 
+func newPreimageBeacon(wCache *channeldb.WitnessCache) *preimageBeacon {
+	return &preimageBeacon{
+		wCache:      wCache,
+		subscribers: make(map[uint64]*preimageSubscriber),
+	}
+}
+
 // SubscribeUpdates returns a channel that will be sent upon *each* time a new
 // preimage is discovered.
 func (p *preimageBeacon) SubscribeUpdates() *contractcourt.WitnessSubscription {