multi: use single read transaction for path finding

This commit partially reverts bf27d05a.
To avoid creating multiple database transactions during a single path
finding operation, we create an explicit transaction when the cached
graph is instantiated.
We cache the source node to avoid needing to look that up for every path
finding session.
The database transaction will be nil in case of the in-memory graph.

routing/payment_session_source.go

@@ -17,7 +17,10 @@ var _ PaymentSessionSource = (*SessionSource)(nil)
 type SessionSource struct {
 	// Graph is the channel graph that will be used to gather metrics from
 	// and also to carry out path finding queries.
-	Graph routingGraph
+	Graph *channeldb.ChannelGraph
+
+	// SourceNode is the graph's source node.
+	SourceNode *channeldb.LightningNode
 
 	// GetLink is a method that allows querying the lower link layer
 	// to determine the up to date available bandwidth at a prospective link
@@ -40,6 +43,21 @@ type SessionSource struct {
 	PathFindingConfig PathFindingConfig
 }
 
+// getRoutingGraph returns a routing graph and a clean-up function for
+// pathfinding.
+func (m *SessionSource) getRoutingGraph() (routingGraph, func(), error) {
+	routingTx, err := NewCachedGraph(m.SourceNode, m.Graph)
+	if err != nil {
+		return nil, nil, err
+	}
+	return routingTx, func() {
+		err := routingTx.close()
+		if err != nil {
+			log.Errorf("Error closing db tx: %v", err)
+		}
+	}, nil
+}
+
 // NewPaymentSession creates a new payment session backed by the latest prune
 // view from Mission Control. An optional set of routing hints can be provided
 // in order to populate additional edges to explore when finding a path to the
@@ -47,14 +65,14 @@ type SessionSource struct {
 func (m *SessionSource) NewPaymentSession(p *LightningPayment) (
 	PaymentSession, error) {
 
-	sourceNode := m.Graph.sourceNode()
-
-	getBandwidthHints := func() (bandwidthHints, error) {
-		return newBandwidthManager(m.Graph, sourceNode, m.GetLink)
+	getBandwidthHints := func(graph routingGraph) (bandwidthHints, error) {
+		return newBandwidthManager(
+			graph, m.SourceNode.PubKeyBytes, m.GetLink,
+		)
 	}
 
 	session, err := newPaymentSession(
-		p, getBandwidthHints, m.Graph,
+		p, getBandwidthHints, m.getRoutingGraph,
 		m.MissionControl, m.PathFindingConfig,
 	)
 	if err != nil {