graph/db: convert ChanUpdatesInHorizon to use iterators

diff --git a/discovery/chan_series.go b/discovery/chan_series.go
index 1fcb13d..6c75635 100644
--- a/discovery/chan_series.go
+++ b/discovery/chan_series.go
@@ -5,6 +5,7 @@ import (
 	"time"
 
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
+	"github.com/lightningnetwork/lnd/fn/v2"
 	graphdb "github.com/lightningnetwork/lnd/graph/db"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/netann"
@@ -114,12 +115,13 @@ func (c *ChanSeries) UpdatesInHorizon(chain chainhash.Hash,
 
 	// 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(
+	chansInHorizonIter, err := c.graph.ChanUpdatesInHorizon(
 		startTime, endTime,
 	)
 	if err != nil {
 		return nil, err
 	}
+	chansInHorizon := fn.Collect(chansInHorizonIter)
 
 	// nodesFromChan records the nodes seen from the channels.
 	nodesFromChan := make(map[[33]byte]struct{}, len(chansInHorizon)*2)
diff --git a/graph/builder.go b/graph/builder.go
index 5940936..c77d0fe 100644
--- a/graph/builder.go
+++ b/graph/builder.go
@@ -593,13 +593,13 @@ func (b *Builder) pruneZombieChans() error {
 
 	startTime := time.Unix(0, 0)
 	endTime := time.Now().Add(-1 * chanExpiry)
-	oldEdges, err := b.cfg.Graph.ChanUpdatesInHorizon(startTime, endTime)
+	oldEdgesIter, err := b.cfg.Graph.ChanUpdatesInHorizon(startTime, endTime)
 	if err != nil {
 		return fmt.Errorf("unable to fetch expired channel updates "+
 			"chans: %v", err)
 	}
 
-	for _, u := range oldEdges {
+	for u := range oldEdgesIter {
 		err = filterPruneChans(u.Info, u.Policy1, u.Policy2)
 		if err != nil {
 			return fmt.Errorf("error filtering channels to "+
diff --git a/graph/db/graph_test.go b/graph/db/graph_test.go
index 9a25f22..730fd83 100644
--- a/graph/db/graph_test.go
+++ b/graph/db/graph_test.go
@@ -2028,10 +2028,11 @@ func TestChanUpdatesInHorizon(t *testing.T) {
 
 	// If we issue an arbitrary query before any channel updates are
 	// inserted in the database, we should get zero results.
-	chanUpdates, err := graph.ChanUpdatesInHorizon(
+	chanUpdatesIter, err := graph.ChanUpdatesInHorizon(
 		time.Unix(999, 0), time.Unix(9999, 0),
 	)
 	require.NoError(t, err, "unable to updates for updates")
+	chanUpdates := fn.Collect(chanUpdatesIter)
 	if len(chanUpdates) != 0 {
 		t.Fatalf("expected 0 chan updates, instead got %v",
 			len(chanUpdates))
@@ -2144,12 +2145,13 @@ func TestChanUpdatesInHorizon(t *testing.T) {
 		},
 	}
 	for _, queryCase := range queryCases {
-		resp, err := graph.ChanUpdatesInHorizon(
+		respIter, err := graph.ChanUpdatesInHorizon(
 			queryCase.start, queryCase.end,
 		)
 		if err != nil {
 			t.Fatalf("unable to query for updates: %v", err)
 		}
+		resp := fn.Collect(respIter)
 
 		if len(resp) != len(queryCase.resp) {
 			t.Fatalf("expected %v chans, got %v chans",
diff --git a/graph/db/interfaces.go b/graph/db/interfaces.go
index fb77120..c97b26c 100644
--- a/graph/db/interfaces.go
+++ b/graph/db/interfaces.go
@@ -220,8 +220,8 @@ type V1Store interface { //nolint:interfacebloat
 	// ChanUpdatesInHorizon returns all the known channel edges which have
 	// at least one edge that has an update timestamp within the specified
 	// horizon.
-	ChanUpdatesInHorizon(startTime, endTime time.Time) ([]ChannelEdge,
-		error)
+	ChanUpdatesInHorizon(startTime, endTime time.Time,
+		opts ...IteratorOption) (iter.Seq[ChannelEdge], error)
 
 	// FilterKnownChanIDs takes a set of channel IDs and return the subset
 	// of chan ID's that we don't know and are not known zombies of the
diff --git a/graph/db/kv_store.go b/graph/db/kv_store.go
index 080fd38..7bd9588 100644
--- a/graph/db/kv_store.go
+++ b/graph/db/kv_store.go
@@ -2040,107 +2040,202 @@ type ChannelEdge struct {
 	Node2 *models.Node
 }
 
-// ChanUpdatesInHorizon returns all the known channel edges which have at least
-// one edge that has an update timestamp within the specified horizon.
-func (c *KVStore) ChanUpdatesInHorizon(startTime,
-	endTime time.Time) ([]ChannelEdge, error) {
-
-	// To ensure we don't return duplicate ChannelEdges, we'll use an
-	// additional map to keep track of the edges already seen to prevent
-	// re-adding it.
-	var edgesSeen map[uint64]struct{}
-	var edgesToCache map[uint64]ChannelEdge
-	var edgesInHorizon []ChannelEdge
+// updateChanCacheBatch updates the channel cache with multiple edges at once.
+// This method acquires the cache lock only once for the entire batch.
+func (c *KVStore) updateChanCacheBatch(edgesToCache map[uint64]ChannelEdge) {
+	if len(edgesToCache) == 0 {
+		return
+	}
 
 	c.cacheMu.Lock()
 	defer c.cacheMu.Unlock()
 
-	var hits int
+	for cid, edge := range edgesToCache {
+		c.chanCache.insert(cid, edge)
+	}
+}
+
+// isEmptyGraphError returns true if the error indicates the graph database
+// is empty (no edges or nodes exist). These errors are expected when the
+// graph is first created or has no data.
+func isEmptyGraphError(err error) bool {
+	return errors.Is(err, ErrGraphNoEdgesFound) ||
+		errors.Is(err, ErrGraphNodesNotFound)
+}
+
+// chanUpdatesIterator holds the state for chunked channel update iteration.
+type chanUpdatesIterator struct {
+	// batchSize is the amount of channel updates to read at a single time.
+	batchSize int
+
+	// startTime is the start time of the iteration request.
+	startTime time.Time
+
+	// endTime is the end time of the iteration request.
+	endTime time.Time
+
+	// edgesSeen is used to dedup edges.
+	edgesSeen map[uint64]struct{}
+
+	// edgesToCache houses all the edges that we read from the disk which
+	// aren't yet cached. This is used to update the cache after a batch
+	// chunk.
+	edgesToCache map[uint64]ChannelEdge
+
+	// lastSeenKey is the last index key seen. This is used to resume
+	// iteration.
+	lastSeenKey []byte
+
+	// hits is the number of cache hits.
+	hits int
+
+	// total is the total number of edges requested.
+	total int
+}
+
+// newChanUpdatesIterator makes a new chan updates iterator.
+func newChanUpdatesIterator(batchSize int,
+	startTime, endTime time.Time) *chanUpdatesIterator {
+
+	return &chanUpdatesIterator{
+		batchSize:    batchSize,
+		startTime:    startTime,
+		endTime:      endTime,
+		edgesSeen:    make(map[uint64]struct{}),
+		edgesToCache: make(map[uint64]ChannelEdge),
+		lastSeenKey:  nil,
+	}
+}
+
+// fetchNextChanUpdateBatch retrieves the next batch of channel edges within the
+// horizon. Returns the batch, whether there are more edges, and any error.
+func (c *KVStore) fetchNextChanUpdateBatch(
+	state *chanUpdatesIterator) ([]ChannelEdge, bool, error) {
+
+	var (
+		batch   []ChannelEdge
+		hasMore bool
+	)
 	err := kvdb.View(c.db, func(tx kvdb.RTx) error {
 		edges := tx.ReadBucket(edgeBucket)
 		if edges == nil {
 			return ErrGraphNoEdgesFound
 		}
+
 		edgeIndex := edges.NestedReadBucket(edgeIndexBucket)
 		if edgeIndex == nil {
 			return ErrGraphNoEdgesFound
 		}
+
 		edgeUpdateIndex := edges.NestedReadBucket(edgeUpdateIndexBucket)
 		if edgeUpdateIndex == nil {
 			return ErrGraphNoEdgesFound
 		}
-
 		nodes := tx.ReadBucket(nodeBucket)
 		if nodes == nil {
 			return ErrGraphNodesNotFound
 		}
 
-		// We'll now obtain a cursor to perform a range query within
-		// the index to find all channels within the horizon.
+		// With all the relevant buckets read, we'll now create a fresh
+		// read cursor.
 		updateCursor := edgeUpdateIndex.ReadCursor()
 
+		// We'll now use the start and end time to create the keys that
+		// we'll use to seek.
 		var startTimeBytes, endTimeBytes [8 + 8]byte
 		byteOrder.PutUint64(
-			startTimeBytes[:8], uint64(startTime.Unix()),
+			startTimeBytes[:8], uint64(state.startTime.Unix()),
 		)
 		byteOrder.PutUint64(
-			endTimeBytes[:8], uint64(endTime.Unix()),
+			endTimeBytes[:8], uint64(state.endTime.Unix()),
 		)
 
-		// With our start and end times constructed, we'll step through
-		// the index collecting the info and policy of each update of
-		// each channel that has a last update within the time range.
-		//
-		//nolint:ll
-		for indexKey, _ := updateCursor.Seek(startTimeBytes[:]); indexKey != nil &&
-			bytes.Compare(indexKey, endTimeBytes[:]) <= 0; indexKey, _ = updateCursor.Next() {
-			// We have a new eligible entry, so we'll slice of the
-			// chan ID so we can query it in the DB.
-			chanID := indexKey[8:]
+		var indexKey []byte
+
+		// If we left off earlier, then we'll use that key as the
+		// starting point.
+		switch {
+		case state.lastSeenKey != nil:
+			// Seek to the last seen key, moving to the key right
+			// after it.
+			indexKey, _ = updateCursor.Seek(state.lastSeenKey)
+
+			if bytes.Equal(indexKey, state.lastSeenKey) {
+				indexKey, _ = updateCursor.Next()
+			}
+
+		// Otherwise, we'll move to the very start of the time range.
+		default:
+			indexKey, _ = updateCursor.Seek(startTimeBytes[:])
+		}
+
+		// TODO(roasbeef): iterate the channel graph cache instead w/ a
+		// treap ordering?
+
+		// Now we'll read items up to the batch size, exiting early if
+		// we exceed the ending time.
+		for len(batch) < state.batchSize && indexKey != nil {
+			// If we're at the end, then we'll break out now.
+			if bytes.Compare(indexKey, endTimeBytes[:]) > 0 {
+				break
+			}
 
-			// If we've already retrieved the info and policies for
-			// this edge, then we can skip it as we don't need to do
-			// so again.
+			chanID := indexKey[8:]
 			chanIDInt := byteOrder.Uint64(chanID)
-			if _, ok := edgesSeen[chanIDInt]; ok {
+
+			if state.lastSeenKey == nil {
+				state.lastSeenKey = make([]byte, len(indexKey))
+			}
+			copy(state.lastSeenKey, indexKey)
+
+			// If we've seen this channel ID already, then we'll
+			// skip it.
+			if _, ok := state.edgesSeen[chanIDInt]; ok {
+				indexKey, _ = updateCursor.Next()
 				continue
 			}
 
+			// Before we read the edge info, we'll see if this
+			// element is already in the cache or not.
+			c.cacheMu.RLock()
 			if channel, ok := c.chanCache.get(chanIDInt); ok {
-				hits++
-				edgesSeen[chanIDInt] = struct{}{}
-				edgesInHorizon = append(edgesInHorizon, channel)
+				state.edgesSeen[chanIDInt] = struct{}{}
+
+				batch = append(batch, channel)
+
+				state.hits++
+				state.total++
+
+				indexKey, _ = updateCursor.Next()
+
+				c.cacheMu.RUnlock()
 
 				continue
 			}
+			c.cacheMu.RUnlock()
 
-			// First, we'll fetch the static edge information.
+			// The edge wasn't in the cache, so we'll fetch it along
+			// w/ the edge policies and nodes.
 			edgeInfo, err := fetchChanEdgeInfo(edgeIndex, chanID)
 			if err != nil {
-				chanID := byteOrder.Uint64(chanID)
-				return fmt.Errorf("unable to fetch info for "+
-					"edge with chan_id=%v: %v", chanID, err)
+				return fmt.Errorf("unable to fetch info "+
+					"for edge with chan_id=%v: %v",
+					chanIDInt, err)
 			}
-
-			// With the static information obtained, we'll now
-			// fetch the dynamic policy info.
 			edge1, edge2, err := fetchChanEdgePolicies(
 				edgeIndex, edges, chanID,
 			)
 			if err != nil {
-				chanID := byteOrder.Uint64(chanID)
-				return fmt.Errorf("unable to fetch policies "+
-					"for edge with chan_id=%v: %v", chanID,
-					err)
+				return fmt.Errorf("unable to fetch "+
+					"policies for edge with chan_id=%v: %v",
+					chanIDInt, err)
 			}
-
 			node1, err := fetchLightningNode(
 				nodes, edgeInfo.NodeKey1Bytes[:],
 			)
 			if err != nil {
 				return err
 			}
-
 			node2, err := fetchLightningNode(
 				nodes, edgeInfo.NodeKey2Bytes[:],
 			)
@@ -2148,9 +2243,8 @@ func (c *KVStore) ChanUpdatesInHorizon(startTime,
 				return err
 			}
 
-			// Finally, we'll collate this edge with the rest of
-			// edges to be returned.
-			edgesSeen[chanIDInt] = struct{}{}
+			// Now we have all the information we need to build the
+			// channel edge.
 			channel := ChannelEdge{
 				Info:    &edgeInfo,
 				Policy1: edge1,
@@ -2158,41 +2252,102 @@ func (c *KVStore) ChanUpdatesInHorizon(startTime,
 				Node1:   &node1,
 				Node2:   &node2,
 			}
-			edgesInHorizon = append(edgesInHorizon, channel)
-			edgesToCache[chanIDInt] = channel
+
+			state.edgesSeen[chanIDInt] = struct{}{}
+			state.edgesToCache[chanIDInt] = channel
+
+			batch = append(batch, channel)
+
+			state.total++
+
+			// Advance the iterator to the next entry.
+			indexKey, _ = updateCursor.Next()
+		}
+
+		// If we haven't yet crossed the endTimeBytes, then we still
+		// have more entries to deliver.
+		if indexKey != nil &&
+			bytes.Compare(indexKey, endTimeBytes[:]) <= 0 {
+
+			hasMore = true
 		}
 
 		return nil
 	}, func() {
-		edgesSeen = make(map[uint64]struct{})
-		edgesToCache = make(map[uint64]ChannelEdge)
-		edgesInHorizon = nil
+		batch = nil
+		hasMore = false
 	})
-	switch {
-	case errors.Is(err, ErrGraphNoEdgesFound):
-		fallthrough
-	case errors.Is(err, ErrGraphNodesNotFound):
-		break
-
-	case err != nil:
-		return nil, err
+	if err != nil {
+		return nil, false, err
 	}
 
-	// Insert any edges loaded from disk into the cache.
-	for chanid, channel := range edgesToCache {
-		c.chanCache.insert(chanid, channel)
-	}
+	return batch, hasMore, nil
+}
 
-	if len(edgesInHorizon) > 0 {
-		log.Debugf("ChanUpdatesInHorizon hit percentage: %.2f (%d/%d)",
-			float64(hits)*100/float64(len(edgesInHorizon)), hits,
-			len(edgesInHorizon))
-	} else {
-		log.Debugf("ChanUpdatesInHorizon returned no edges in "+
-			"horizon (%s, %s)", startTime, endTime)
+// ChanUpdatesInHorizon returns all the known channel edges which have at least
+// one edge that has an update timestamp within the specified horizon.
+func (c *KVStore) ChanUpdatesInHorizon(startTime, endTime time.Time,
+	opts ...IteratorOption) (iter.Seq[ChannelEdge], error) {
+
+	cfg := defaultIteratorConfig()
+	for _, opt := range opts {
+		opt(cfg)
 	}
 
-	return edgesInHorizon, nil
+	return func(yield func(ChannelEdge) bool) {
+		iterState := newChanUpdatesIterator(
+			cfg.chanUpdateIterBatchSize, startTime, endTime,
+		)
+
+		for {
+			// At the top of the loop, we'll read the next batch
+			// chunk from disk. We'll also determine if we have any
+			// more entries after this or not.
+			batch, hasMore, err := c.fetchNextChanUpdateBatch(
+				iterState,
+			)
+			// TODO(roasbeef): yield error here?
+			if err != nil {
+				// These errors just mean the graph is empty,
+				// which is OK.
+				if !isEmptyGraphError(err) {
+
+					log.Errorf("ChanUpdatesInHorizon "+
+						"batch error: %v", err)
+					return
+				}
+				// Continue with empty batch
+			}
+
+			// We'll now yield each edge that we just read. If yield
+			// returns false, then that means that we'll exit early.
+			for _, edge := range batch {
+				if !yield(edge) {
+					return
+				}
+			}
+
+			// Update cache after successful batch yield.
+			c.updateChanCacheBatch(iterState.edgesToCache)
+			iterState.edgesToCache = make(map[uint64]ChannelEdge)
+
+			// If we we're done, then we can just break out here
+			// now.
+			if !hasMore || len(batch) == 0 {
+				break
+			}
+		}
+
+		if iterState.total > 0 {
+			log.Tracef("ChanUpdatesInHorizon hit percentage: "+
+				"%.2f (%d/%d)", float64(iterState.hits)*100/
+				float64(iterState.total), iterState.hits,
+				iterState.total)
+		} else {
+			log.Tracef("ChanUpdatesInHorizon returned no edges "+
+				"in horizon (%s, %s)", startTime, endTime)
+		}
+	}, nil
 }
 
 // nodeUpdatesIterator maintains state for iterating through node updates.
diff --git a/graph/db/sql_store.go b/graph/db/sql_store.go
index b617aa3..38485f6 100644
--- a/graph/db/sql_store.go
+++ b/graph/db/sql_store.go
@@ -933,7 +933,8 @@ func (s *SQLStore) ForEachNodeChannel(ctx context.Context, nodePub route.Vertex,
 //
 // NOTE: This is part of the V1Store interface.
 func (s *SQLStore) ChanUpdatesInHorizon(startTime,
-	endTime time.Time) ([]ChannelEdge, error) {
+	endTime time.Time,
+	opts ...IteratorOption) (iter.Seq[ChannelEdge], error) {
 
 	s.cacheMu.Lock()
 	defer s.cacheMu.Unlock()
@@ -1033,7 +1034,13 @@ func (s *SQLStore) ChanUpdatesInHorizon(startTime,
 			"horizon (%s, %s)", startTime, endTime)
 	}
 
-	return edges, nil
+	return func(yield func(ChannelEdge) bool) {
+		for _, edge := range edges {
+			if !yield(edge) {
+				return
+			}
+		}
+	}, nil
 }
 
 // ForEachNodeCached is similar to forEachNode, but it returns DirectedChannel
diff --git a/graph/interfaces.go b/graph/interfaces.go
index ec4f42b..cf660ec 100644
--- a/graph/interfaces.go
+++ b/graph/interfaces.go
@@ -2,6 +2,7 @@ package graph
 
 import (
 	"context"
+	"iter"
 	"time"
 
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
@@ -152,8 +153,9 @@ type DB interface {
 	// ChanUpdatesInHorizon returns all the known channel edges which have
 	// at least one edge that has an update timestamp within the specified
 	// horizon.
-	ChanUpdatesInHorizon(startTime, endTime time.Time) (
-		[]graphdb.ChannelEdge, error)
+	ChanUpdatesInHorizon(startTime, endTime time.Time,
+		opts ...graphdb.IteratorOption) (
+		iter.Seq[graphdb.ChannelEdge], error)
 
 	// DeleteChannelEdges removes edges with the given channel IDs from the
 	// database and marks them as zombies. This ensures that we're unable to