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.go

@@ -164,7 +164,7 @@ type PaymentSession interface {
 type paymentSession struct {
 	additionalEdges map[route.Vertex][]*channeldb.CachedEdgePolicy
 
-	getBandwidthHints func() (bandwidthHints, error)
+	getBandwidthHints func(routingGraph) (bandwidthHints, error)
 
 	payment *LightningPayment
 
@@ -172,7 +172,7 @@ type paymentSession struct {
 
 	pathFinder pathFinder
 
-	routingGraph routingGraph
+	getRoutingGraph func() (routingGraph, func(), error)
 
 	// pathFindingConfig defines global parameters that control the
 	// trade-off in path finding between fees and probabiity.
@@ -192,8 +192,8 @@ type paymentSession struct {
 
 // newPaymentSession instantiates a new payment session.
 func newPaymentSession(p *LightningPayment,
-	getBandwidthHints func() (bandwidthHints, error),
-	routingGraph routingGraph,
+	getBandwidthHints func(routingGraph) (bandwidthHints, error),
+	getRoutingGraph func() (routingGraph, func(), error),
 	missionControl MissionController, pathFindingConfig PathFindingConfig) (
 	*paymentSession, error) {
 
@@ -209,7 +209,7 @@ func newPaymentSession(p *LightningPayment,
 		getBandwidthHints: getBandwidthHints,
 		payment:           p,
 		pathFinder:        findPath,
-		routingGraph:      routingGraph,
+		getRoutingGraph:   getRoutingGraph,
 		pathFindingConfig: pathFindingConfig,
 		missionControl:    missionControl,
 		minShardAmt:       DefaultShardMinAmt,
@@ -274,33 +274,42 @@ func (p *paymentSession) RequestRoute(maxAmt, feeLimit lnwire.MilliSatoshi,
 	}
 
 	for {
+		// Get a routing graph.
+		routingGraph, cleanup, err := p.getRoutingGraph()
+		if err != nil {
+			return nil, err
+		}
+
 		// We'll also obtain a set of bandwidthHints from the lower
 		// layer for each of our outbound channels. This will allow the
 		// path finding to skip any links that aren't active or just
 		// don't have enough bandwidth to carry the payment. New
 		// bandwidth hints are queried for every new path finding
 		// attempt, because concurrent payments may change balances.
-		bandwidthHints, err := p.getBandwidthHints()
+		bandwidthHints, err := p.getBandwidthHints(routingGraph)
 		if err != nil {
 			return nil, err
 		}
 
 		p.log.Debugf("pathfinding for amt=%v", maxAmt)
 
-		sourceVertex := p.routingGraph.sourceNode()
+		sourceVertex := routingGraph.sourceNode()
 
 		// Find a route for the current amount.
 		path, err := p.pathFinder(
 			&graphParams{
 				additionalEdges: p.additionalEdges,
 				bandwidthHints:  bandwidthHints,
-				graph:           p.routingGraph,
+				graph:           routingGraph,
 			},
 			restrictions, &p.pathFindingConfig,
 			sourceVertex, p.payment.Target,
 			maxAmt, finalHtlcExpiry,
 		)
 
+		// Close routing graph.
+		cleanup()
+
 		switch {
 		case err == errNoPathFound:
 			// Don't split if this is a legacy payment without mpp