peernotifier: Add peer notifier package for peer online/offline events

This commit adds a peer notifier package which provides clients with
a subscription to peer online and offline events.

server.go

@@ -44,6 +44,7 @@ import (
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/nat"
 	"github.com/lightningnetwork/lnd/netann"
+	"github.com/lightningnetwork/lnd/peernotifier"
 	"github.com/lightningnetwork/lnd/pool"
 	"github.com/lightningnetwork/lnd/routing"
 	"github.com/lightningnetwork/lnd/routing/route"
@@ -188,6 +189,8 @@ type server struct {
 
 	channelNotifier *channelnotifier.ChannelNotifier
 
+	peerNotifier *peernotifier.PeerNotifier
+
 	witnessBeacon contractcourt.WitnessBeacon
 
 	breachArbiter *breachArbiter
@@ -1081,6 +1084,10 @@ func newServer(listenAddrs []net.Addr, chanDB *channeldb.DB,
 		return nil, err
 	}
 
+	// Assemble a peer notifier which will provide clients with subscriptions
+	// to peer online and offline events.
+	s.peerNotifier = peernotifier.New()
+
 	if cfg.WtClient.Active {
 		policy := wtpolicy.DefaultPolicy()
 
@@ -1183,6 +1190,10 @@ func (s *server) Start() error {
 			startErr = err
 			return
 		}
+		if err := s.peerNotifier.Start(); err != nil {
+			startErr = err
+			return
+		}
 		if err := s.sphinx.Start(); err != nil {
 			startErr = err
 			return
@@ -1341,6 +1352,7 @@ func (s *server) Stop() error {
 		s.chainArb.Stop()
 		s.sweeper.Stop()
 		s.channelNotifier.Stop()
+		s.peerNotifier.Stop()
 		s.cc.wallet.Shutdown()
 		s.cc.chainView.Stop()
 		s.connMgr.Stop()
@@ -2713,7 +2725,8 @@ func (s *server) addPeer(p *peer) {
 	// TODO(roasbeef): pipe all requests through to the
 	// queryHandler/peerManager
 
-	pubStr := string(p.addr.IdentityKey.SerializeCompressed())
+	pubSer := p.addr.IdentityKey.SerializeCompressed()
+	pubStr := string(pubSer)
 
 	s.peersByPub[pubStr] = p
 
@@ -2722,6 +2735,13 @@ func (s *server) addPeer(p *peer) {
 	} else {
 		s.outboundPeers[pubStr] = p
 	}
+
+	// Inform the peer notifier of a peer online event so that it can be reported
+	// to clients listening for peer events.
+	var pubKey [33]byte
+	copy(pubKey[:], pubSer)
+
+	s.peerNotifier.NotifyPeerOnline(pubKey)
 }
 
 // peerInitializer asynchronously starts a newly connected peer after it has
@@ -2963,7 +2983,8 @@ func (s *server) removePeer(p *peer) {
 		return
 	}
 
-	pubStr := string(p.addr.IdentityKey.SerializeCompressed())
+	pubSer := p.addr.IdentityKey.SerializeCompressed()
+	pubStr := string(pubSer)
 
 	delete(s.peersByPub, pubStr)
 
@@ -2972,6 +2993,13 @@ func (s *server) removePeer(p *peer) {
 	} else {
 		delete(s.outboundPeers, pubStr)
 	}
+
+	// Inform the peer notifier of a peer offline event so that it can be
+	// reported to clients listening for peer events.
+	var pubKey [33]byte
+	copy(pubKey[:], pubSer)
+
+	s.peerNotifier.NotifyPeerOffline(pubKey)
 }
 
 // openChanReq is a message sent to the server in order to request the