routing: implement route failure fallback in SendPayment

This commit adds payment route failure fallback to SendPayment. By
this, we mean that we now take all the possible routes found during
path finding and try them in series. Either a route fails and we move
onto the next one, or the route is successful and we terminate early.

With this commit, sending payments using lnd is now much more robust as
if there exists an eligible route with sufficient capacity, it will be
utilized.

routing/router_test.go

@@ -146,3 +146,67 @@ func TestFindRoutesFeeSorting(t *testing.T) {
 	}
 }
 
+// TestSendPaymentRouteFailureFallback tests that when sending a payment, if
+// one of the target routes is seen as unavailable, then the next route in the
+// queue is used instead. This process should continue until either a payment
+// succeeds, or all routes have been exhausted.
+func TestSendPaymentRouteFailureFallback(t *testing.T) {
+	const startingBlockHeight = 101
+	ctx, cleanUp, err := createTestCtx(startingBlockHeight, basicGraphFilePath)
+	defer cleanUp()
+	if err != nil {
+		t.Fatalf("unable to create router: %v", err)
+	}
+
+	// Craft a LightningPayment struct that'll send a payment from roasbeef
+	// to luo ji for 100 satoshis.
+	var payHash [32]byte
+	payment := LightningPayment{
+		Target:      ctx.aliases["luoji"],
+		Amount:      btcutil.Amount(1000),
+		PaymentHash: payHash,
+	}
+
+	var preImage [32]byte
+	copy(preImage[:], bytes.Repeat([]byte{9}, 32))
+
+	// We'll modify the SendToSwitch method that's been set within the
+	// router's configuration to ignore the path that has luo ji as the
+	// first hop. This should force the router to instead take the
+	// available two hop path (through satoshi).
+	ctx.router.cfg.SendToSwitch = func(n *btcec.PublicKey,
+		_ *lnwire.UpdateAddHTLC) ([32]byte, error) {
+
+		if ctx.aliases["luoji"].IsEqual(n) {
+			return [32]byte{}, errors.New("send error")
+		}
+
+		return preImage, nil
+	}
+
+	// Send off the payment request to the router, route through satoshi
+	// should've been selected as a fall back and succeeded correctly.
+	paymentPreImage, route, err := ctx.router.SendPayment(&payment)
+	if err != nil {
+		t.Fatalf("unable to send payment: %v", err)
+	}
+
+	// The route selected should have two hops
+	if len(route.Hops) != 2 {
+		t.Fatalf("incorrect route length: expected %v got %v", 2,
+			len(route.Hops))
+	}
+
+	// The preimage should match up with the once created above.
+	if !bytes.Equal(paymentPreImage[:], preImage[:]) {
+		t.Fatalf("incorrect preimage used: expected %x got %x",
+			preImage[:], paymentPreImage[:])
+	}
+
+	// The route should have satoshi as the first hop.
+	if route.Hops[0].Channel.Node.Alias != "satoshi" {
+		t.Fatalf("route should go through satoshi as first hop, "+
+			"instead passes through: %v",
+			route.Hops[0].Channel.Node.Alias)
+	}
+}