discovery: replace GossipSyncer syncChanUpdates flag with SyncerType

In this commit, we introduce a new type: SyncerType. This type denotes
the type of sync a GossipSyncer is currently under. We only introduce
the two possible entry states, ActiveSync and PassiveSync. An ActiveSync
GossipSyncer will exchange channels with the remote peer and receive new
graph updates from them, while a PassiveSync GossipSyncer will not and
will only response to the remote peer's queries.

This commit does not modify the behavior and is only meant to be a
refactor.

discovery/gossiper.go

@@ -1108,16 +1108,20 @@ func (d *AuthenticatedGossiper) InitSyncState(syncPeer lnpeer.Peer,
 
 	encoding := lnwire.EncodingSortedPlain
 	syncer := newGossipSyncer(gossipSyncerCfg{
-		chainHash:       d.cfg.ChainHash,
-		peerPub:         nodeID,
-		syncChanUpdates: recvUpdates,
-		channelSeries:   d.cfg.ChanSeries,
-		encodingType:    encoding,
-		chunkSize:       encodingTypeToChunkSize[encoding],
+		chainHash:     d.cfg.ChainHash,
+		peerPub:       nodeID,
+		channelSeries: d.cfg.ChanSeries,
+		encodingType:  encoding,
+		chunkSize:     encodingTypeToChunkSize[encoding],
 		sendToPeer: func(msgs ...lnwire.Message) error {
 			return syncPeer.SendMessageLazy(false, msgs...)
 		},
 	})
+
+	if !recvUpdates {
+		syncer.syncType = uint32(PassiveSync)
+	}
+
 	d.peerSyncers[nodeID] = syncer
 
 	syncer.Start()

discovery/syncer.go

@@ -13,6 +13,39 @@ import (
 	"golang.org/x/time/rate"
 )
 
+// SyncerType encapsulates the different types of syncing mechanisms for a
+// gossip syncer.
+type SyncerType uint8
+
+const (
+	// ActiveSync denotes that a gossip syncer should exercise its default
+	// behavior. This includes reconciling the set of missing graph updates
+	// with the remote peer _and_ receiving new updates from them.
+	ActiveSync SyncerType = iota
+
+	// PassiveSync denotes that a gossip syncer:
+	//
+	//   1. Should not attempt to query the remote peer for graph updates.
+	//   2. Should respond to queries from the remote peer.
+	//   3. Should not receive new updates from the remote peer.
+	//
+	// They are started in a chansSynced state in order to accomplish their
+	// responsibilities above.
+	PassiveSync
+)
+
+// String returns a human readable string describing the target SyncerType.
+func (t SyncerType) String() string {
+	switch t {
+	case ActiveSync:
+		return "ActiveSync"
+	case PassiveSync:
+		return "PassiveSync"
+	default:
+		return fmt.Sprintf("unknown sync type %d", t)
+	}
+}
+
 // syncerState is an enum that represents the current state of the GossipSyncer.
 // As the syncer is a state machine, we'll gate our actions based off of the
 // current state and the next incoming message.
@@ -161,6 +194,17 @@ type GossipSyncer struct {
 	started sync.Once
 	stopped sync.Once
 
+	// state is the current state of the GossipSyncer.
+	//
+	// NOTE: This variable MUST be used atomically.
+	state uint32
+
+	// syncType denotes the SyncerType the gossip syncer is currently
+	// exercising.
+	//
+	// NOTE: This variable MUST be used atomically.
+	syncType uint32
+
 	// remoteUpdateHorizon is the update horizon of the remote peer. We'll
 	// use this to properly filter out any messages.
 	remoteUpdateHorizon *lnwire.GossipTimestampRange
@@ -169,11 +213,6 @@ type GossipSyncer struct {
 	// determine if we've already sent out our update.
 	localUpdateHorizon *lnwire.GossipTimestampRange
 
-	// state is the current state of the GossipSyncer.
-	//
-	// NOTE: This variable MUST be used atomically.
-	state uint32
-
 	// gossipMsgs is a channel that all messages from the target peer will
 	// be sent over.
 	gossipMsgs chan lnwire.Message
@@ -264,9 +303,11 @@ func (g *GossipSyncer) channelGraphSyncer() {
 	//  * needed if we want to sync chan state very few blocks?
 
 	for {
-		state := atomic.LoadUint32(&g.state)
-		log.Debugf("GossipSyncer(%x): state=%v", g.cfg.peerPub[:],
-			syncerState(state))
+		state := g.syncState()
+		syncType := g.SyncType()
+
+		log.Debugf("GossipSyncer(%x): state=%v, type=%v",
+			g.cfg.peerPub[:], state, syncType)
 
 		switch syncerState(state) {
 		// When we're in this state, we're trying to synchronize our
@@ -293,7 +334,7 @@ func (g *GossipSyncer) channelGraphSyncer() {
 
 			// With the message sent successfully, we'll transition
 			// into the next state where we wait for their reply.
-			atomic.StoreUint32(&g.state, uint32(waitingQueryRangeReply))
+			g.setSyncState(waitingQueryRangeReply)
 
 		// In this state, we've sent out our initial channel range
 		// query and are waiting for the final response from the remote
@@ -350,13 +391,13 @@ func (g *GossipSyncer) channelGraphSyncer() {
 			// If this wasn't our last query, then we'll need to
 			// transition to our waiting state.
 			if !done {
-				atomic.StoreUint32(&g.state, uint32(waitingQueryChanReply))
+				g.setSyncState(waitingQueryChanReply)
 				continue
 			}
 
 			// If we're fully synchronized, then we can transition
 			// to our terminal state.
-			atomic.StoreUint32(&g.state, uint32(chansSynced))
+			g.setSyncState(chansSynced)
 
 		// In this state, we've just sent off a new query for channels
 		// that we don't yet know of. We'll remain in this state until
@@ -373,7 +414,7 @@ func (g *GossipSyncer) channelGraphSyncer() {
 				// state to send of the remaining query chunks.
 				_, ok := msg.(*lnwire.ReplyShortChanIDsEnd)
 				if ok {
-					atomic.StoreUint32(&g.state, uint32(queryNewChannels))
+					g.setSyncState(queryNewChannels)
 					continue
 				}
 
@@ -395,7 +436,7 @@ func (g *GossipSyncer) channelGraphSyncer() {
 			// If we haven't yet sent out our update horizon, and
 			// we want to receive real-time channel updates, we'll
 			// do so now.
-			if g.localUpdateHorizon == nil && g.cfg.syncChanUpdates {
+			if g.localUpdateHorizon == nil && syncType == ActiveSync {
 				// TODO(roasbeef): query DB for most recent
 				// update?
 
@@ -524,14 +565,14 @@ func (g *GossipSyncer) processChanRangeReply(msg *lnwire.ReplyChannelRange) erro
 		log.Infof("GossipSyncer(%x): remote peer has no new chans",
 			g.cfg.peerPub[:])
 
-		atomic.StoreUint32(&g.state, uint32(chansSynced))
+		g.setSyncState(chansSynced)
 		return nil
 	}
 
 	// Otherwise, we'll set the set of channels that we need to query for
 	// the next state, and also transition our state.
 	g.newChansToQuery = newChans
-	atomic.StoreUint32(&g.state, uint32(queryNewChannels))
+	g.setSyncState(queryNewChannels)
 
 	log.Infof("GossipSyncer(%x): starting query for %v new chans",
 		g.cfg.peerPub[:], len(newChans))
@@ -933,7 +974,22 @@ func (g *GossipSyncer) ProcessQueryMsg(msg lnwire.Message, peerQuit <-chan struc
 	}
 }
 
+// setSyncState sets the gossip syncer's state to the given state.
+func (g *GossipSyncer) setSyncState(state syncerState) {
+	atomic.StoreUint32(&g.state, uint32(state))
+}
+
 // syncState returns the current syncerState of the target GossipSyncer.
 func (g *GossipSyncer) syncState() syncerState {
 	return syncerState(atomic.LoadUint32(&g.state))
 }
+
+// setSyncType sets the gossip syncer's sync type to the given type.
+func (g *GossipSyncer) setSyncType(syncType SyncerType) {
+	atomic.StoreUint32(&g.syncType, uint32(syncType))
+}
+
+// SyncType returns the current SyncerType of the target GossipSyncer.
+func (g *GossipSyncer) SyncType() SyncerType {
+	return SyncerType(atomic.LoadUint32(&g.syncType))
+}

discovery/syncer_test.go

@@ -120,10 +120,9 @@ func newTestSyncer(hID lnwire.ShortChannelID,
 
 	msgChan := make(chan []lnwire.Message, 20)
 	cfg := gossipSyncerCfg{
-		syncChanUpdates: true,
-		channelSeries:   newMockChannelGraphTimeSeries(hID),
-		encodingType:    encodingType,
-		chunkSize:       chunkSize,
+		channelSeries: newMockChannelGraphTimeSeries(hID),
+		encodingType:  encodingType,
+		chunkSize:     chunkSize,
 		sendToPeer: func(msgs ...lnwire.Message) error {
 			msgChan <- msgs
 			return nil