discovery: convert UpdatesInHorizon to return iter.Seq2[lnwire.Message, error]

In this commit, we complete the iterator conversion work started in PR
10128 by threading the iterator pattern through to the higher-level
UpdatesInHorizon method. This change converts the method from returning
a fully materialized slice of messages to returning a lazy iterator that
yields messages on demand.

The new signature uses iter.Seq2 to allow error propagation during
iteration, eliminating the need for a separate error return value. This
approach enables callers to handle errors as they occur during iteration
rather than failing upfront.

The implementation now lazily processes channel and node updates,
yielding them as they're generated rather than accumulating them in
memory. This maintains the same ordering guarantees (channels before
nodes) while significantly reducing memory pressure when dealing with
large update sets during gossip synchronization.

discovery/chan_series.go

@@ -2,6 +2,7 @@ package discovery
 
 import (
 	"context"
+	"iter"
 	"time"
 
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
@@ -30,8 +31,8 @@ type ChannelGraphTimeSeries interface {
 	// update timestamp between the start time and end time. We'll use this
 	// to catch up a remote node to the set of channel updates that they
 	// may have missed out on within the target chain.
-	UpdatesInHorizon(chain chainhash.Hash,
-		startTime time.Time, endTime time.Time) ([]lnwire.Message, error)
+	UpdatesInHorizon(chain chainhash.Hash, startTime time.Time,
+		endTime time.Time) iter.Seq2[lnwire.Message, error]
 
 	// FilterKnownChanIDs takes a target chain, and a set of channel ID's,
 	// and returns a filtered set of chan ID's. This filtered set of chan
@@ -108,109 +109,96 @@ func (c *ChanSeries) HighestChanID(ctx context.Context,
 //
 // NOTE: This is part of the ChannelGraphTimeSeries interface.
 func (c *ChanSeries) UpdatesInHorizon(chain chainhash.Hash,
-	startTime time.Time, endTime time.Time) ([]lnwire.Message, error) {
+	startTime, endTime time.Time) iter.Seq2[lnwire.Message, error] {
 
-	var updates []lnwire.Message
-
-	// First, we'll query for all the set of channels that have an update
-	// that falls within the specified horizon.
-	chansInHorizonIter, err := c.graph.ChanUpdatesInHorizon(
+	return func(yield func(lnwire.Message, error) bool) {
+		// First, we'll query for all the set of channels that have an
+		// update that falls within the specified horizon.
+		chansInHorizon, err := c.graph.ChanUpdatesInHorizon(
 			startTime, endTime,
 		)
 		if err != nil {
-		return nil, err
+			yield(nil, err)
+			return
 		}
 
-	for channel := range chansInHorizonIter {
-		// If the channel hasn't been fully advertised yet, or is a
-		// private channel, then we'll skip it as we can't construct a
-		// full authentication proof if one is requested.
+		for channel := range chansInHorizon {
+			// If the channel hasn't been fully advertised yet, or
+			// is a private channel, then we'll skip it as we can't
+			// construct a full authentication proof if one is
+			// requested.
 			if channel.Info.AuthProof == nil {
 				continue
 			}
 
+			//nolint:ll
 			chanAnn, edge1, edge2, err := netann.CreateChanAnnouncement(
-			channel.Info.AuthProof, channel.Info, channel.Policy1,
-			channel.Policy2,
+				channel.Info.AuthProof, channel.Info,
+				channel.Policy1, channel.Policy2,
 			)
 			if err != nil {
-			return nil, err
+				if !yield(nil, err) {
+					return
 				}
 
-		// Create a slice to hold the `channel_announcement` and
-		// potentially two `channel_update` msgs.
-		//
-		// NOTE: Based on BOLT7, if a channel_announcement has no
-		// corresponding channel_updates, we must not send the
-		// channel_announcement. Thus we use this slice to decide we
-		// want to send this `channel_announcement` or not. By the end
-		// of the operation, if the len of the slice is 1, we will not
-		// send the `channel_announcement`. Otherwise, when sending the
-		// msgs, the `channel_announcement` must be sent prior to any
-		// corresponding `channel_update` or `node_annoucement`, that's
-		// why we create a slice here to maintain the order.
-		chanUpdates := make([]lnwire.Message, 0, 3)
-		chanUpdates = append(chanUpdates, chanAnn)
-
-		if edge1 != nil {
-			// We don't want to send channel updates that don't
-			// conform to the spec (anymore).
-			err := netann.ValidateChannelUpdateFields(0, edge1)
-			if err != nil {
-				log.Errorf("not sending invalid channel "+
-					"update %v: %v", edge1, err)
-			} else {
-				chanUpdates = append(chanUpdates, edge1)
-			}
-		}
-
-		if edge2 != nil {
-			err := netann.ValidateChannelUpdateFields(0, edge2)
-			if err != nil {
-				log.Errorf("not sending invalid channel "+
-					"update %v: %v", edge2, err)
-			} else {
-				chanUpdates = append(chanUpdates, edge2)
-			}
-		}
-
-		// If there's no corresponding `channel_update` to send, skip
-		// sending this `channel_announcement`.
-		if len(chanUpdates) < 2 {
 				continue
 			}
 
-		// Append the all the msgs to the slice.
-		updates = append(updates, chanUpdates...)
+			if !yield(chanAnn, nil) {
+				return
 			}
 
-	// Next, we'll send out all the node announcements that have an update
-	// within the horizon as well. We send these second to ensure that they
-	// follow any active channels they have.
+			// We don't want to send channel updates that don't
+			// conform to the spec (anymore), so check to make sure
+			// that these channel updates are valid before yielding
+			// them.
+			if edge1 != nil {
+				err := netann.ValidateChannelUpdateFields(
+					0, edge1,
+				)
+				if err != nil {
+					log.Errorf("not sending invalid "+
+						"channel update %v: %v",
+						edge1, err)
+				} else if !yield(edge1, nil) {
+					return
+				}
+			}
+			if edge2 != nil {
+				err := netann.ValidateChannelUpdateFields(
+					0, edge2,
+				)
+				if err != nil {
+					log.Errorf("not sending invalid "+
+						"channel update %v: %v", edge2,
+						err)
+				} else if !yield(edge2, nil) {
+					return
+				}
+			}
+		}
+
+		// Next, we'll send out all the node announcements that have an
+		// update within the horizon as well. We send these second to
+		// ensure that they follow any active channels they have.
 		nodeAnnsInHorizon, err := c.graph.NodeUpdatesInHorizon(
 			startTime, endTime, graphdb.WithIterPublicNodesOnly(),
 		)
-	if err != nil {
-		return nil, err
-	}
-
 		for nodeAnn := range nodeAnnsInHorizon {
 			nodeUpdate, err := nodeAnn.NodeAnnouncement(true)
 			if err != nil {
-			return nil, err
+				if !yield(nil, err) {
+					return
 				}
 
-		if err := netann.ValidateNodeAnnFields(nodeUpdate); err != nil {
-			log.Debugf("Skipping forwarding invalid node "+
-				"announcement %x: %v", nodeAnn.PubKeyBytes, err)
-
 				continue
 			}
 
-		updates = append(updates, nodeUpdate)
+			if !yield(nodeUpdate, nil) {
+				return
+			}
+		}
 	}
-
-	return updates, nil
 }
 
 // FilterKnownChanIDs takes a target chain, and a set of channel ID's, and

discovery/syncer.go

@@ -1442,23 +1442,12 @@ func (g *GossipSyncer) ApplyGossipFilter(ctx context.Context,
 
 	// Now that the remote peer has applied their filter, we'll query the
 	// database for all the messages that are beyond this filter.
-	newUpdatestoSend, err := g.cfg.channelSeries.UpdatesInHorizon(
+	newUpdatestoSend := g.cfg.channelSeries.UpdatesInHorizon(
 		g.cfg.chainHash, startTime, endTime,
 	)
-	if err != nil {
-		returnSema()
-		return err
-	}
 
 	log.Infof("GossipSyncer(%x): applying new remote update horizon: "+
-		"start=%v, end=%v, backlog_size=%v", g.cfg.peerPub[:],
-		startTime, endTime, len(newUpdatestoSend))
-
-	// If we don't have any to send, then we can return early.
-	if len(newUpdatestoSend) == 0 {
-		returnSema()
-		return nil
-	}
+		"start=%v, end=%v", g.cfg.peerPub[:], startTime, endTime)
 
 	// Set the atomic flag to indicate we're starting to send the backlog.
 	// If the swap fails, it means another goroutine is already active, so
@@ -1478,7 +1467,7 @@ func (g *GossipSyncer) ApplyGossipFilter(ctx context.Context,
 		defer returnSema()
 		defer g.isSendingBacklog.Store(false)
 
-		for _, msg := range newUpdatestoSend {
+		for msg := range newUpdatestoSend {
 			err := g.sendToPeerSync(ctx, msg)
 			switch {
 			case err == ErrGossipSyncerExiting:

discovery/syncer_test.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"iter"
 	"math"
 	"reflect"
 	"sort"
@@ -86,13 +87,20 @@ func (m *mockChannelGraphTimeSeries) HighestChanID(_ context.Context,
 }
 
 func (m *mockChannelGraphTimeSeries) UpdatesInHorizon(chain chainhash.Hash,
-	startTime time.Time, endTime time.Time) ([]lnwire.Message, error) {
+	startTime, endTime time.Time) iter.Seq2[lnwire.Message, error] {
 
+	return func(yield func(lnwire.Message, error) bool) {
 		m.horizonReq <- horizonQuery{
 			chain, startTime, endTime,
 		}
 
-	return <-m.horizonResp, nil
+		msgs := <-m.horizonResp
+		for _, msg := range msgs {
+			if !yield(msg, nil) {
+				return
+			}
+		}
+	}
 }
 
 func (m *mockChannelGraphTimeSeries) FilterKnownChanIDs(chain chainhash.Hash,