routing/router: resume payment state machine at startup

On startup the router will fetch the in-flight payments from the control
tower, and resume their execution.

routing/missioncontrol.go

@@ -242,6 +242,19 @@ func (m *missionControl) NewPaymentSessionForRoute(preBuiltRoute *route.Route) *
 	}
 }
 
+// NewPaymentSessionEmpty creates a new paymentSession instance that is empty,
+// and will be exhausted immediately. Used for failure reporting to
+// missioncontrol for resumed payment we don't want to make more attempts for.
+func (m *missionControl) NewPaymentSessionEmpty() *paymentSession {
+	return &paymentSession{
+		pruneViewSnapshot:    m.GraphPruneView(),
+		errFailedPolicyChans: make(map[EdgeLocator]struct{}),
+		mc:                   m,
+		preBuiltRoute:        &route.Route{},
+		preBuiltRouteTried:   true,
+	}
+}
+
 // generateBandwidthHints is a helper function that's utilized the main
 // findPath function in order to obtain hints from the lower layer w.r.t to the
 // available bandwidth of edges on the network. Currently, we'll only obtain

routing/router.go

@@ -492,6 +492,40 @@ func (r *ChannelRouter) Start() error {
 		}
 	}
 
+	// If any payments are still in flight, we resume, to make sure their
+	// results are properly handled.
+	payments, err := r.cfg.Control.FetchInFlightPayments()
+	if err != nil {
+		return err
+	}
+
+	for _, payment := range payments {
+		log.Infof("Resuming payment with hash %v", payment.Info.PaymentHash)
+		r.wg.Add(1)
+		go func(payment *channeldb.InFlightPayment) {
+			defer r.wg.Done()
+
+			// We create a dummy, empty payment session such that
+			// we won't make another payment attempt when the
+			// result for the in-flight attempt is received.
+			paySession := r.missionControl.NewPaymentSessionEmpty()
+
+			lPayment := &LightningPayment{
+				PaymentHash: payment.Info.PaymentHash,
+			}
+
+			_, _, err = r.sendPayment(payment.Attempt, lPayment, paySession)
+			if err != nil {
+				log.Errorf("Resuming payment with hash %v "+
+					"failed: %v.", payment.Info.PaymentHash, err)
+				return
+			}
+
+			log.Infof("Resumed payment with hash %v completed.",
+				payment.Info.PaymentHash)
+		}(payment)
+	}
+
 	r.wg.Add(1)
 	go r.networkHandler()