multi: send to a blinded path in an invoice

Update the SendPayment flow so that it is able to send to an invoice
containing a blinded path.

routing/payment_session.go

@@ -5,6 +5,7 @@ import (
 
 	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/btcsuite/btclog"
+	sphinx "github.com/lightningnetwork/lightning-onion"
 	"github.com/lightningnetwork/lnd/build"
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/channeldb/models"
@@ -205,6 +206,18 @@ func newPaymentSession(p *LightningPayment, selfNode route.Vertex,
 		return nil, err
 	}
 
+	if p.BlindedPayment != nil {
+		if len(edges) != 0 {
+			return nil, fmt.Errorf("cannot have both route hints " +
+				"and blinded path")
+		}
+
+		edges, err = p.BlindedPayment.toRouteHints()
+		if err != nil {
+			return nil, err
+		}
+	}
+
 	logPrefix := fmt.Sprintf("PaymentSession(%x):", p.Identifier())
 
 	return &paymentSession{
@@ -389,6 +402,11 @@ func (p *paymentSession) RequestRoute(maxAmt, feeLimit lnwire.MilliSatoshi,
 			return nil, err
 		}
 
+		var blindedPath *sphinx.BlindedPath
+		if p.payment.BlindedPayment != nil {
+			blindedPath = p.payment.BlindedPayment.BlindedPath
+		}
+
 		// With the next candidate path found, we'll attempt to turn
 		// this into a route by applying the time-lock and fee
 		// requirements.
@@ -401,7 +419,7 @@ func (p *paymentSession) RequestRoute(maxAmt, feeLimit lnwire.MilliSatoshi,
 				records:     p.payment.DestCustomRecords,
 				paymentAddr: p.payment.PaymentAddr,
 				metadata:    p.payment.Metadata,
-			}, nil,
+			}, blindedPath,
 		)
 		if err != nil {
 			return nil, err

routing/router.go

@@ -922,9 +922,19 @@ type LightningPayment struct {
 	// NOTE: This is optional unless required by the payment. When providing
 	// multiple routes, ensure the hop hints within each route are chained
 	// together and sorted in forward order in order to reach the
-	// destination successfully.
+	// destination successfully. This is mutually exclusive to the
+	// BlindedPayment field.
 	RouteHints [][]zpay32.HopHint
 
+	// BlindedPayment holds the information about a blinded path to the
+	// payment recipient. This is mutually exclusive to the RouteHints
+	// field.
+	//
+	// NOTE: a recipient may provide multiple blinded payment paths in the
+	// same invoice. Currently, LND will only attempt to use the first one.
+	// A future PR will handle multiple blinded payment paths.
+	BlindedPayment *BlindedPayment
+
 	// OutgoingChannelIDs is the list of channels that are allowed for the
 	// first hop. If nil, any channel may be used.
 	OutgoingChannelIDs []uint64