routing: check loop conditions at end

This prepares for routing to self. When checking the condition at the
start, the loop would terminate immediately because the source is equal
to the target.

routing/pathfind.go

@@ -534,7 +534,7 @@ func findPath(g *graphParams, r *RestrictParams, cfg *PathFindingConfig,
 	// heap.
 	heap.Push(&nodeHeap, distance[target])
 
-	for nodeHeap.Len() != 0 {
+	for {
 		nodesVisited++
 
 		// Fetch the node within the smallest distance from our source
@@ -585,13 +585,17 @@ func findPath(g *graphParams, r *RestrictParams, cfg *PathFindingConfig,
 			// already have.
 			processEdge(fromNode, policy, partialPath)
 		}
+
+		if nodeHeap.Len() == 0 {
+			break
+		}
 	}
 
 	// Use the distance map to unravel the forward path from source to
 	// target.
 	var pathEdges []*channeldb.ChannelEdgePolicy
 	currentNode := source
-	for currentNode != target { // TODO(roasbeef): assumes no cycles
+	for {
 		// Determine the next hop forward using the next map.
 		currentNodeWithDist, ok := distance[currentNode]
 		if !ok {
@@ -605,6 +609,10 @@ func findPath(g *graphParams, r *RestrictParams, cfg *PathFindingConfig,
 
 		// Advance current node.
 		currentNode = currentNodeWithDist.nextHop.Node.PubKeyBytes
+
+		if currentNode == target {
+			break
+		}
 	}
 
 	// The route is invalid if it spans more than 20 hops. The current