graph/db: refactor source node fetching logic

Small refactor to introduce a re-usable getSourceNode function that we
can then make use of in the upcoming kvdb->SQL migration logic.

Author: ellemouton

graph/db/kv_store.go

@@ -881,19 +881,23 @@ func (c *KVStore) ForEachNodeCacheable(cb func(route.Vertex,
 // as the center node within a star-graph. This method may be used to kick off
 // a path finding algorithm in order to explore the reachability of another
 // node based off the source node.
-func (c *KVStore) SourceNode(_ context.Context) (*models.LightningNode,
-	error) {
+func (c *KVStore) SourceNode(_ context.Context) (*models.LightningNode, error) {
+	return sourceNode(c.db)
+}
 
+// sourceNode fetches the source node of the graph. The source node is treated
+// as the center node within a star-graph.
+func sourceNode(db kvdb.Backend) (*models.LightningNode, error) {
 	var source *models.LightningNode
-	err := kvdb.View(c.db, func(tx kvdb.RTx) error {
+	err := kvdb.View(db, func(tx kvdb.RTx) error {
 		// First grab the nodes bucket which stores the mapping from
 		// pubKey to node information.
 		nodes := tx.ReadBucket(nodeBucket)
 		if nodes == nil {
 			return ErrGraphNotFound
 		}
 
-		node, err := c.sourceNode(nodes)
+		node, err := sourceNodeWithTx(nodes)
 		if err != nil {
 			return err
 		}
@@ -910,13 +914,11 @@ func (c *KVStore) SourceNode(_ context.Context) (*models.LightningNode,
 	return source, nil
 }
 
-// sourceNode uses an existing database transaction and returns the source node
-// of the graph. The source node is treated as the center node within a
+// sourceNodeWithTx uses an existing database transaction and returns the source
+// node of the graph. The source node is treated as the center node within a
 // star-graph. This method may be used to kick off a path finding algorithm in
 // order to explore the reachability of another node based off the source node.
-func (c *KVStore) sourceNode(nodes kvdb.RBucket) (*models.LightningNode,
-	error) {
-
+func sourceNodeWithTx(nodes kvdb.RBucket) (*models.LightningNode, error) {
 	selfPub := nodes.Get(sourceKey)
 	if selfPub == nil {
 		return nil, ErrSourceNodeNotSet
@@ -1569,7 +1571,7 @@ func (c *KVStore) pruneGraphNodes(nodes kvdb.RwBucket,
 
 	// We'll retrieve the graph's source node to ensure we don't remove it
 	// even if it no longer has any open channels.
-	sourceNode, err := c.sourceNode(nodes)
+	sourceNode, err := sourceNodeWithTx(nodes)
 	if err != nil {
 		return nil, err
 	}
@@ -3255,7 +3257,7 @@ func (c *KVStore) ForEachSourceNodeChannel(cb func(chanPoint wire.OutPoint,
 			return ErrGraphNotFound
 		}
 
-		node, err := c.sourceNode(nodes)
+		node, err := sourceNodeWithTx(nodes)
 		if err != nil {
 			return err
 		}

graph/db/sql_store.go

@@ -3699,7 +3699,7 @@ type srcNodeInfo struct {
 	pub route.Vertex
 }
 
-// getSourceNode returns the DB node ID and pub key of the source node for the
+// sourceNode returns the DB node ID and pub key of the source node for the
 // specified protocol version.
 func (s *SQLStore) getSourceNode(ctx context.Context, db SQLQueries,
 	version ProtocolVersion) (int64, route.Vertex, error) {