graph/db: cache source node info

Save future round trips by caching source node info since this info will
never change.

graph/db/sql_store.go

@@ -119,6 +119,9 @@ type SQLStore struct {
 	chanScheduler batch.Scheduler[SQLQueries]
 	nodeScheduler batch.Scheduler[SQLQueries]
 
+	srcNodes  map[ProtocolVersion]*srcNodeInfo
+	srcNodeMu sync.Mutex
+
 	// Temporary fall-back to the KVStore so that we can implement the
 	// interface incrementally.
 	*KVStore
@@ -156,6 +159,7 @@ func NewSQLStore(cfg *SQLStoreConfig, db BatchedSQLQueries, kvStore *KVStore,
 		KVStore:     kvStore,
 		rejectCache: newRejectCache(opts.RejectCacheSize),
 		chanCache:   newChannelCache(opts.ChannelCacheSize),
+		srcNodes:    make(map[ProtocolVersion]*srcNodeInfo),
 	}
 
 	s.chanScheduler = batch.NewTimeScheduler(
@@ -382,7 +386,7 @@ func (s *SQLStore) SourceNode() (*models.LightningNode, error) {
 
 	var node *models.LightningNode
 	err := s.db.ExecTx(ctx, sqldb.ReadTxOpt(), func(db SQLQueries) error {
-		_, nodePub, err := getSourceNode(ctx, db, ProtocolV1)
+		_, nodePub, err := s.getSourceNode(ctx, db, ProtocolV1)
 		if err != nil {
 			return fmt.Errorf("unable to fetch V1 source node: %w",
 				err)
@@ -416,7 +420,7 @@ func (s *SQLStore) SetSourceNode(node *models.LightningNode) error {
 
 		// Make sure that if a source node for this version is already
 		// set, then the ID is the same as the one we are about to set.
-		dbSourceNodeID, _, err := getSourceNode(ctx, db, ProtocolV1)
+		dbSourceNodeID, _, err := s.getSourceNode(ctx, db, ProtocolV1)
 		if err != nil && !errors.Is(err, ErrSourceNodeNotSet) {
 			return fmt.Errorf("unable to fetch source node: %w",
 				err)
@@ -1254,11 +1258,29 @@ func upsertNodeExtraSignedFields(ctx context.Context, db SQLQueries,
 	return nil
 }
 
+// srcNodeInfo holds the information about the source node of the graph.
+type srcNodeInfo struct {
+	// id is the DB level ID of the source node entry in the "nodes" table.
+	id int64
+
+	// pub is the public key of the source node.
+	pub route.Vertex
+}
+
 // getSourceNode returns the DB node ID and pub key of the source node for the
 // specified protocol version.
-func getSourceNode(ctx context.Context, db SQLQueries,
+func (s *SQLStore) getSourceNode(ctx context.Context, db SQLQueries,
 	version ProtocolVersion) (int64, route.Vertex, error) {
 
+	s.srcNodeMu.Lock()
+	defer s.srcNodeMu.Unlock()
+
+	// If we already have the source node ID and pub key cached, then
+	// return them.
+	if info, ok := s.srcNodes[version]; ok {
+		return info.id, info.pub, nil
+	}
+
 	var pubKey route.Vertex
 
 	nodes, err := db.GetSourceNodesByVersion(ctx, int16(version))
@@ -1276,6 +1298,11 @@ func getSourceNode(ctx context.Context, db SQLQueries,
 
 	copy(pubKey[:], nodes[0].PubKey)
 
+	s.srcNodes[version] = &srcNodeInfo{
+		id:  nodes[0].NodeID,
+		pub: pubKey,
+	}
+
 	return nodes[0].NodeID, pubKey, nil
 }