routing: return structured error for send to route

routing/payment_lifecycle.go

@@ -12,6 +12,20 @@ import (
 	"github.com/lightningnetwork/lnd/routing/route"
 )
 
+// errNoRoute is returned when all routes from the payment session have been
+// attempted.
+type errNoRoute struct {
+	// lastError is the error encountered during the last payment attempt,
+	// if at least one attempt has been made.
+	lastError *htlcswitch.ForwardingError
+}
+
+// Error returns a string representation of the error.
+func (e errNoRoute) Error() string {
+	return fmt.Sprintf("unable to route payment to destination: %v",
+		e.lastError)
+}
+
 // paymentLifecycle holds all information about the current state of a payment
 // needed to resume if from any point.
 type paymentLifecycle struct {
@@ -23,7 +37,7 @@ type paymentLifecycle struct {
 	finalCLTVDelta uint16
 	attempt        *channeldb.PaymentAttemptInfo
 	circuit        *sphinx.Circuit
-	lastError      error
+	lastError      *htlcswitch.ForwardingError
 }
 
 // resumePayment resumes the paymentLifecycle from the current state.
@@ -218,10 +232,8 @@ func (p *paymentLifecycle) createNewPaymentAttempt() (lnwire.ShortChannelID,
 		// payment, we'll return that.
 		if p.lastError != nil {
 			return lnwire.ShortChannelID{}, nil,
-				fmt.Errorf("unable to route payment to "+
-					"destination: %v", p.lastError)
+				errNoRoute{lastError: p.lastError}
 		}
-
 		// Terminal state, return.
 		return lnwire.ShortChannelID{}, nil, err
 	}
@@ -326,9 +338,21 @@ func (p *paymentLifecycle) sendPaymentAttempt(firstHop lnwire.ShortChannelID,
 // handleSendError inspects the given error from the Switch and determines
 // whether we should make another payment attempt.
 func (p *paymentLifecycle) handleSendError(sendErr error) error {
-	finalOutcome := p.router.processSendError(
-		p.paySession, &p.attempt.Route, sendErr,
-	)
+	var finalOutcome bool
+
+	// If an internal, non-forwarding error occurred, we can stop trying.
+	fErr, ok := sendErr.(*htlcswitch.ForwardingError)
+	if !ok {
+		finalOutcome = true
+	} else {
+		finalOutcome = p.router.processSendError(
+			p.paySession, &p.attempt.Route, fErr,
+		)
+
+		// Save the forwarding error so it can be returned if this turns
+		// out to be the last attempt.
+		p.lastError = fErr
+	}
 
 	if finalOutcome {
 		log.Errorf("Payment %x failed with final outcome: %v",
@@ -348,7 +372,5 @@ func (p *paymentLifecycle) handleSendError(sendErr error) error {
 		return sendErr
 	}
 
-	// We get ready to make another payment attempt.
-	p.lastError = sendErr
 	return nil
 }

routing/router.go

@@ -1658,7 +1658,23 @@ func (r *ChannelRouter) SendToRoute(hash lntypes.Hash, route *route.Route) (
 	// Since this is the first time this payment is being made, we pass nil
 	// for the existing attempt.
 	preimage, _, err := r.sendPayment(nil, payment, paySession)
-	return preimage, err
+	if err != nil {
+		// SendToRoute should return a structured error. In case the
+		// provided route fails, payment lifecycle will return a
+		// noRouteError with the structured error embedded.
+		if noRouteError, ok := err.(errNoRoute); ok {
+			if noRouteError.lastError == nil {
+				return lntypes.Preimage{},
+					errors.New("failure message missing")
+			}
+
+			return lntypes.Preimage{}, noRouteError.lastError
+		}
+
+		return lntypes.Preimage{}, err
+	}
+
+	return preimage, nil
 }
 
 // sendPayment attempts to send a payment as described within the passed
@@ -1740,12 +1756,7 @@ func (r *ChannelRouter) sendPayment(
 // to continue with an alternative route. This is indicated by the boolean
 // return value.
 func (r *ChannelRouter) processSendError(paySession PaymentSession,
-	rt *route.Route, err error) bool {
-
-	fErr, ok := err.(*htlcswitch.ForwardingError)
-	if !ok {
-		return true
-	}
+	rt *route.Route, fErr *htlcswitch.ForwardingError) bool {
 
 	errSource := fErr.ErrorSource
 	errVertex := route.NewVertex(errSource)

routing/router_test.go

@@ -3136,3 +3136,107 @@ func TestRouterPaymentStateMachine(t *testing.T) {
 		}
 	}
 }
+
+// TestSendToRouteStructuredError asserts that SendToRoute returns a structured
+// error.
+func TestSendToRouteStructuredError(t *testing.T) {
+	t.Parallel()
+
+	// Setup a three node network.
+	chanCapSat := btcutil.Amount(100000)
+	testChannels := []*testChannel{
+		symmetricTestChannel("a", "b", chanCapSat, &testChannelPolicy{
+			Expiry:  144,
+			FeeRate: 400,
+			MinHTLC: 1,
+			MaxHTLC: lnwire.NewMSatFromSatoshis(chanCapSat),
+		}, 1),
+		symmetricTestChannel("b", "c", chanCapSat, &testChannelPolicy{
+			Expiry:  144,
+			FeeRate: 400,
+			MinHTLC: 1,
+			MaxHTLC: lnwire.NewMSatFromSatoshis(chanCapSat),
+		}, 2),
+	}
+
+	testGraph, err := createTestGraphFromChannels(testChannels)
+	if err != nil {
+		t.Fatalf("unable to create graph: %v", err)
+	}
+	defer testGraph.cleanUp()
+
+	const startingBlockHeight = 101
+
+	ctx, cleanUp, err := createTestCtxFromGraphInstance(
+		startingBlockHeight, testGraph,
+	)
+	if err != nil {
+		t.Fatalf("unable to create router: %v", err)
+	}
+	defer cleanUp()
+
+	// Setup a route from source a to destination c. The route will be used
+	// in a call to SendToRoute. SendToRoute also applies channel updates,
+	// but it saves us from including RequestRoute in the test scope too.
+	hop1 := ctx.aliases["b"]
+	hop2 := ctx.aliases["c"]
+
+	hops := []*route.Hop{
+		{
+			ChannelID:   1,
+			PubKeyBytes: hop1,
+		},
+		{
+			ChannelID:   2,
+			PubKeyBytes: hop2,
+		},
+	}
+
+	rt, err := route.NewRouteFromHops(
+		lnwire.MilliSatoshi(10000), 100,
+		ctx.aliases["a"], hops,
+	)
+	if err != nil {
+		t.Fatalf("unable to create route: %v", err)
+	}
+
+	// We'll modify the SendToSwitch method so that it simulates a failed
+	// payment with an error originating from the first hop of the route.
+	// The unsigned channel update is attached to the failure message.
+	ctx.router.cfg.Payer.(*mockPaymentAttemptDispatcher).setPaymentResult(
+		func(firstHop lnwire.ShortChannelID) ([32]byte, error) {
+
+			v := ctx.aliases["b"]
+			source, err := btcec.ParsePubKey(
+				v[:], btcec.S256(),
+			)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			return [32]byte{}, &htlcswitch.ForwardingError{
+				ErrorSource: source,
+				FailureMessage: &lnwire.FailFeeInsufficient{
+					Update: lnwire.ChannelUpdate{},
+				},
+			}
+		})
+
+	// The payment parameter is mostly redundant in SendToRoute. Can be left
+	// empty for this test.
+	var payment lntypes.Hash
+
+	// Send off the payment request to the router. The specified route
+	// should be attempted and the channel update should be received by
+	// router and ignored because it is missing a valid signature.
+	_, err = ctx.router.SendToRoute(payment, rt)
+
+	fErr, ok := err.(*htlcswitch.ForwardingError)
+	if !ok {
+		t.Fatalf("expected forwarding error")
+	}
+
+	if _, ok := fErr.FailureMessage.(*lnwire.FailFeeInsufficient); !ok {
+		t.Fatalf("expected fee insufficient error")
+	}
+}