routerrpc: report route success probability

The intention of adding this field is to make probability based path
finding more transparent and help understand why a route is returned.

lnrpc/routerrpc/router_backend.go

@@ -231,13 +231,41 @@ func (r *RouterBackend) QueryRoutes(ctx context.Context,
 		return nil, err
 	}
 
+	// Calculate route success probability. Do not rely on a probability
+	// that could have been returned from path finding, because mission
+	// control may have been disabled in the provided ProbabilitySource.
+	successProb := r.getSuccessProbability(route)
+
 	routeResp := &lnrpc.QueryRoutesResponse{
-		Routes: []*lnrpc.Route{rpcRoute},
+		Routes:      []*lnrpc.Route{rpcRoute},
+		SuccessProb: successProb,
 	}
 
 	return routeResp, nil
 }
 
+// getSuccessProbability returns the success probability for the given route
+// based on the current state of mission control.
+func (r *RouterBackend) getSuccessProbability(rt *route.Route) float64 {
+	fromNode := rt.SourcePubKey
+	amtToFwd := rt.TotalAmount
+	successProb := 1.0
+	for _, hop := range rt.Hops {
+		toNode := hop.PubKeyBytes
+
+		probability := r.MissionControl.GetProbability(
+			fromNode, toNode, amtToFwd,
+		)
+
+		successProb *= probability
+
+		amtToFwd = hop.AmtToForward
+		fromNode = toNode
+	}
+
+	return successProb
+}
+
 // rpcEdgeToPair looks up the provided channel and returns the channel endpoints
 // as a directed pair.
 func (r *RouterBackend) rpcEdgeToPair(e *lnrpc.EdgeLocator) (

lnrpc/rpc.proto

@@ -1704,7 +1704,17 @@ message EdgeLocator {
 }
 
 message QueryRoutesResponse {
+    /**
+    The route that results from the path finding operation. This is still a
+    repeated field to retain backwards compatibility.
+    */
     repeated Route routes = 1 [json_name = "routes"];
+
+    /**
+    The success probability of the returned route based on the current mission
+    control state. [EXPERIMENTAL]
+    */
+    double success_prob = 2 [json_name = "success_prob"];
 }
 
 message Hop {

lnrpc/rpc.swagger.json

@@ -3163,7 +3163,13 @@
           "type": "array",
           "items": {
             "$ref": "#/definitions/lnrpcRoute"
-          }
+          },
+          "description": "*\nThe route that results from the path finding operation. This is still a\nrepeated field to retain backwards compatibility."
+        },
+        "success_prob": {
+          "type": "number",
+          "format": "double",
+          "title": "*\nThe success probability of the returned route based on the current mission\ncontrol state. [EXPERIMENTAL]"
         }
       }
     },