routing: use nodeWithDist instead of vertex to avoid map access

The same nodeWithDist was fetched from the map for every channel it has.
This struct is not mutated so it can be fetched before and reused.

routing/pathfind.go

@@ -406,7 +406,7 @@ func findPath(g *graphParams, r *RestrictParams, cfg *PathFindingConfig,
 	// processEdge is a helper closure that will be used to make sure edges
 	// satisfy our specific requirements.
 	processEdge := func(fromVertex route.Vertex, bandwidth lnwire.MilliSatoshi,
-		edge *channeldb.ChannelEdgePolicy, toNode route.Vertex) {
+		edge *channeldb.ChannelEdgePolicy, toNodeDist *nodeWithDist) {
 
 		// If this is not a local channel and it is disabled, we will
 		// skip it.
@@ -431,17 +431,16 @@ func findPath(g *graphParams, r *RestrictParams, cfg *PathFindingConfig,
 
 		// Calculate amount that the candidate node would have to sent
 		// out.
-		toNodeDist := distance[toNode]
 		amountToSend := toNodeDist.amountToReceive
 
 		// Request the success probability for this edge.
 		edgeProbability := r.ProbabilitySource(
-			fromVertex, toNode, amountToSend,
+			fromVertex, toNodeDist.node, amountToSend,
 		)
 
 		if log.Level() <= btclog.LevelTrace {
 			log.Tracef("path finding probability: fromnode=%v, tonode=%v, "+
-				"probability=%v", fromVertex, toNode, edgeProbability)
+				"probability=%v", fromVertex, toNodeDist.node, edgeProbability)
 		}
 
 		// If the probability is zero, there is no point in trying.
@@ -588,6 +587,7 @@ func findPath(g *graphParams, r *RestrictParams, cfg *PathFindingConfig,
 		// from the heap.
 		partialPath := heap.Pop(&nodeHeap).(*nodeWithDist)
 		pivot := partialPath.node
+		pivotWithDist := distance[pivot]
 
 		// If we've reached our source (or we don't have any incoming
 		// edges), then we're done here and can exit the graph
@@ -636,7 +636,7 @@ func findPath(g *graphParams, r *RestrictParams, cfg *PathFindingConfig,
 
 			// Check if this candidate node is better than what we
 			// already have.
-			processEdge(chanSource, edgeBandwidth, inEdge, pivot)
+			processEdge(chanSource, edgeBandwidth, inEdge, pivotWithDist)
 			return nil
 		}
 
@@ -656,7 +656,7 @@ func findPath(g *graphParams, r *RestrictParams, cfg *PathFindingConfig,
 		bandWidth := distance[pivot].amountToReceive
 		for _, reverseEdge := range additionalEdgesWithSrc[pivot] {
 			processEdge(reverseEdge.sourceNode, bandWidth,
-				reverseEdge.edge, pivot)
+				reverseEdge.edge, pivotWithDist)
 		}
 	}