discovery: add async timestamp range queue to prevent blocking

In this commit, we introduce an asynchronous processing queue for
GossipTimestampRange messages in the GossipSyncer. This change addresses
a critical issue where the gossiper could block indefinitely when
processing timestamp range messages during periods of high load.

Previously, when a peer sent a GossipTimestampRange message, the
gossiper would synchronously call ApplyGossipFilter, which could block
on semaphore acquisition, database queries, and rate limiting. This
synchronous processing created a bottleneck where the entire peer
message processing pipeline would stall, potentially causing timeouts
and disconnections.

The new design adds a timestampRangeQueue channel with a capacity of 1
message and a dedicated goroutine for processing these messages
asynchronously. This follows the established pattern used for other
message types in the syncer. When the queue is full, we drop messages
and log a warning rather than blocking indefinitely, providing graceful
degradation under extreme load conditions.

discovery/syncer.go

@@ -54,6 +54,12 @@ const (
 	PinnedSync
 )
 
+const (
+	// defaultTimestampQueueSize is the size of the timestamp range queue
+	// used.
+	defaultTimestampQueueSize = 1
+)
+
 // String returns a human readable string describing the target SyncerType.
 func (t SyncerType) String() string {
 	switch t {
@@ -285,6 +291,10 @@ type gossipSyncerCfg struct {
 	// updates for a channel and returns true if the channel should be
 	// considered a zombie based on these timestamps.
 	isStillZombieChannel func(time.Time, time.Time) bool
+
+	// timestampQueueSize is the size of the timestamp range queue. If not
+	// set, defaults to the global timestampQueueSize constant.
+	timestampQueueSize int
 }
 
 // GossipSyncer is a struct that handles synchronizing the channel graph state
@@ -381,6 +391,11 @@ type GossipSyncer struct {
 	// respond to gossip timestamp range messages.
 	syncerSema chan struct{}
 
+	// timestampRangeQueue is a buffered channel for queuing timestamp range
+	// messages that need to be processed asynchronously. This prevents the
+	// gossiper from blocking when ApplyGossipFilter is called.
+	timestampRangeQueue chan *lnwire.GossipTimestampRange
+
 	sync.Mutex
 
 	// cg is a helper that encapsulates a wait group and quit channel and
@@ -392,14 +407,23 @@ type GossipSyncer struct {
 // newGossipSyncer returns a new instance of the GossipSyncer populated using
 // the passed config.
 func newGossipSyncer(cfg gossipSyncerCfg, sema chan struct{}) *GossipSyncer {
+	// Use the configured queue size if set, otherwise use the default.
+	queueSize := cfg.timestampQueueSize
+	if queueSize == 0 {
+		queueSize = defaultTimestampQueueSize
+	}
+
 	return &GossipSyncer{
 		cfg:                cfg,
 		syncTransitionReqs: make(chan *syncTransitionReq),
 		historicalSyncReqs: make(chan *historicalSyncReq),
 		gossipMsgs:         make(chan lnwire.Message, syncerBufferSize),
 		queryMsgs:          make(chan lnwire.Message, syncerBufferSize),
-		syncerSema:         sema,
-		cg:                 fn.NewContextGuard(),
+		timestampRangeQueue: make(
+			chan *lnwire.GossipTimestampRange, queueSize,
+		),
+		syncerSema: sema,
+		cg:         fn.NewContextGuard(),
 	}
 }
 
@@ -422,6 +446,13 @@ func (g *GossipSyncer) Start() {
 			g.cg.WgAdd(1)
 			go g.replyHandler(ctx)
 		}
+
+		// Start the timestamp range queue processor to handle gossip
+		// filter applications asynchronously.
+		if !g.cfg.noTimestampQueryOption {
+			g.cg.WgAdd(1)
+			go g.processTimestampRangeQueue(ctx)
+		}
 	})
 }
 
@@ -672,6 +703,63 @@ func (g *GossipSyncer) replyHandler(ctx context.Context) {
 	}
 }
 
+// processTimestampRangeQueue handles timestamp range messages from the queue
+// asynchronously. This prevents blocking the gossiper when rate limiting is
+// active and multiple peers are trying to apply gossip filters.
+func (g *GossipSyncer) processTimestampRangeQueue(ctx context.Context) {
+	defer g.cg.WgDone()
+
+	for {
+		select {
+		case msg := <-g.timestampRangeQueue:
+			// Process the timestamp range message. If we hit an
+			// error, log it but continue processing to avoid
+			// blocking the queue.
+			err := g.ApplyGossipFilter(ctx, msg)
+			switch {
+			case errors.Is(err, ErrGossipSyncerExiting):
+				return
+
+			case errors.Is(err, lnpeer.ErrPeerExiting):
+				return
+
+			case err != nil:
+				log.Errorf("Unable to apply gossip filter: %v",
+					err)
+			}
+
+		case <-g.cg.Done():
+			return
+
+		case <-ctx.Done():
+			return
+		}
+	}
+}
+
+// QueueTimestampRange attempts to queue a timestamp range message for
+// asynchronous processing. If the queue is full, it returns false to indicate
+// the message was dropped.
+func (g *GossipSyncer) QueueTimestampRange(
+	msg *lnwire.GossipTimestampRange) bool {
+
+	// If timestamp queries are disabled, don't queue the message.
+	if g.cfg.noTimestampQueryOption {
+		return false
+	}
+
+	select {
+	case g.timestampRangeQueue <- msg:
+		return true
+
+	// Queue is full, drop the message to prevent blocking.
+	default:
+		log.Warnf("Timestamp range queue full for peer %x, "+
+			"dropping message", g.cfg.peerPub[:])
+		return false
+	}
+}
+
 // sendGossipTimestampRange constructs and sets a GossipTimestampRange for the
 // syncer and sends it to the remote peer.
 func (g *GossipSyncer) sendGossipTimestampRange(ctx context.Context,