multi: send MPP payment to blinded path

Make various sender side adjustments so that a sender is able to send an
MP payment to a single blinded path without actually including an MPP
record in the payment.

channeldb/payment_control.go

@@ -67,12 +67,12 @@ var (
 
 	// ErrValueMismatch is returned if we try to register a non-MPP attempt
 	// with an amount that doesn't match the payment amount.
-	ErrValueMismatch = errors.New("attempted value doesn't match payment" +
+	ErrValueMismatch = errors.New("attempted value doesn't match payment " +
 		"amount")
 
 	// ErrValueExceedsAmt is returned if we try to register an attempt that
 	// would take the total sent amount above the payment amount.
-	ErrValueExceedsAmt = errors.New("attempted value exceeds payment" +
+	ErrValueExceedsAmt = errors.New("attempted value exceeds payment " +
 		"amount")
 
 	// ErrNonMPPayment is returned if we try to register an MPP attempt for
@@ -83,6 +83,17 @@ var (
 	// a payment that already has an MPP attempt registered.
 	ErrMPPayment = errors.New("payment has MPP attempts")
 
+	// ErrMPPRecordInBlindedPayment is returned if we try to register an
+	// attempt with an MPP record for a payment to a blinded path.
+	ErrMPPRecordInBlindedPayment = errors.New("blinded payment cannot " +
+		"contain MPP records")
+
+	// ErrBlindedPaymentTotalAmountMismatch is returned if we try to
+	// register an HTLC shard to a blinded route where the total amount
+	// doesn't match existing shards.
+	ErrBlindedPaymentTotalAmountMismatch = errors.New("blinded path " +
+		"total amount mismatch")
+
 	// ErrMPPPaymentAddrMismatch is returned if we try to register an MPP
 	// shard where the payment address doesn't match existing shards.
 	ErrMPPPaymentAddrMismatch = errors.New("payment address mismatch")
@@ -96,7 +107,7 @@ var (
 	// attempt to a payment that has at least one of its HTLCs settled.
 	ErrPaymentPendingSettled = errors.New("payment has settled htlcs")
 
-	// ErrPaymentAlreadyFailed is returned when we try to add a new attempt
+	// ErrPaymentPendingFailed is returned when we try to add a new attempt
 	// to a payment that already has a failure reason.
 	ErrPaymentPendingFailed = errors.New("payment has failure reason")
 
@@ -334,12 +345,48 @@ func (p *PaymentControl) RegisterAttempt(paymentHash lntypes.Hash,
 			return err
 		}
 
+		// If the final hop has encrypted data, then we know this is a
+		// blinded payment. In blinded payments, MPP records are not set
+		// for split payments and the recipient is responsible for using
+		// a consistent PathID across the various encrypted data
+		// payloads that we received from them for this payment. All we
+		// need to check is that the total amount field for each HTLC
+		// in the split payment is correct.
+		isBlinded := len(attempt.Route.FinalHop().EncryptedData) != 0
+
 		// Make sure any existing shards match the new one with regards
 		// to MPP options.
 		mpp := attempt.Route.FinalHop().MPP
+
+		// MPP records should not be set for attempts to blinded paths.
+		if isBlinded && mpp != nil {
+			return ErrMPPRecordInBlindedPayment
+		}
+
 		for _, h := range payment.InFlightHTLCs() {
 			hMpp := h.Route.FinalHop().MPP
 
+			// If this is a blinded payment, then no existing HTLCs
+			// should have MPP records.
+			if isBlinded && hMpp != nil {
+				return ErrMPPRecordInBlindedPayment
+			}
+
+			// If this is a blinded payment, then we just need to
+			// check that the TotalAmtMsat field for this shard
+			// is equal to that of any other shard in the same
+			// payment.
+			if isBlinded {
+				if attempt.Route.FinalHop().TotalAmtMsat !=
+					h.Route.FinalHop().TotalAmtMsat {
+
+					//nolint:lll
+					return ErrBlindedPaymentTotalAmountMismatch
+				}
+
+				continue
+			}
+
 			switch {
 			// We tried to register a non-MPP attempt for a MPP
 			// payment.
@@ -367,9 +414,10 @@ func (p *PaymentControl) RegisterAttempt(paymentHash lntypes.Hash,
 		}
 
 		// If this is a non-MPP attempt, it must match the total amount
-		// exactly.
+		// exactly. Note that a blinded payment is considered an MPP
+		// attempt.
 		amt := attempt.Route.ReceiverAmt()
-		if mpp == nil && amt != payment.Info.Value {
+		if !isBlinded && mpp == nil && amt != payment.Info.Value {
 			return ErrValueMismatch
 		}
 

itest/list_on_test.go

@@ -586,6 +586,10 @@ var allTestCases = []*lntest.TestCase{
 		Name:     "on chain to blinded",
 		TestFunc: testErrorHandlingOnChainFailure,
 	},
+	{
+		Name:     "mpp to single blinded path",
+		TestFunc: testMPPToSingleBlindedPath,
+	},
 	{
 		Name:     "removetx",
 		TestFunc: testRemoveTx,

itest/lnd_route_blinding_test.go

@@ -878,3 +878,176 @@ func testErrorHandlingOnChainFailure(ht *lntest.HarnessTest) {
 	ht.CloseChannel(testCase.carol, testCase.channels[2])
 	testCase.cancel()
 }
+
+// testMPPToSingleBlindedPath tests that a two-shard MPP payment can be sent
+// over a single blinded path.
+// The following graph is created where Dave is the destination node, and he
+// will choose Carol as the introduction node. The channel capacities are set in
+// such a way that Alice will have to split the payment to dave over both the
+// A->B->C-D and A->E->C->D routes.
+//
+//		    ---- Bob ---
+//	          /		\
+//	       Alice		Carol --- Dave
+//		   \		 /
+//		    ---- Eve ---
+func testMPPToSingleBlindedPath(ht *lntest.HarnessTest) {
+	// Create a five-node context consisting of Alice, Bob and three new
+	// nodes.
+	alice, bob := ht.Alice, ht.Bob
+
+	// Restrict Dave so that he only ever chooses the Carol->Dave path for
+	// a blinded route.
+	dave := ht.NewNode("dave", []string{
+		"--invoices.blinding.min-num-real-hops=1",
+		"--invoices.blinding.num-hops=1",
+	})
+	carol := ht.NewNode("carol", nil)
+	eve := ht.NewNode("eve", nil)
+
+	// Connect nodes to ensure propagation of channels.
+	ht.EnsureConnected(alice, bob)
+	ht.EnsureConnected(alice, eve)
+	ht.EnsureConnected(carol, bob)
+	ht.EnsureConnected(carol, eve)
+	ht.EnsureConnected(carol, dave)
+
+	// Send coins to the nodes and mine 1 blocks to confirm them.
+	for i := 0; i < 2; i++ {
+		ht.FundCoinsUnconfirmed(btcutil.SatoshiPerBitcoin, carol)
+		ht.FundCoinsUnconfirmed(btcutil.SatoshiPerBitcoin, dave)
+		ht.FundCoinsUnconfirmed(btcutil.SatoshiPerBitcoin, eve)
+		ht.MineBlocksAndAssertNumTxes(1, 3)
+	}
+
+	const paymentAmt = btcutil.Amount(300000)
+
+	nodes := []*node.HarnessNode{alice, bob, carol, dave, eve}
+	reqs := []*lntest.OpenChannelRequest{
+		{
+			Local:  alice,
+			Remote: bob,
+			Param: lntest.OpenChannelParams{
+				Amt: paymentAmt * 2 / 3,
+			},
+		},
+		{
+			Local:  alice,
+			Remote: eve,
+			Param: lntest.OpenChannelParams{
+				Amt: paymentAmt * 2 / 3,
+			},
+		},
+		{
+			Local:  bob,
+			Remote: carol,
+			Param: lntest.OpenChannelParams{
+				Amt: paymentAmt * 2,
+			},
+		},
+		{
+			Local:  eve,
+			Remote: carol,
+			Param: lntest.OpenChannelParams{
+				Amt: paymentAmt * 2,
+			},
+		},
+		{
+			Local:  carol,
+			Remote: dave,
+			Param: lntest.OpenChannelParams{
+				Amt: paymentAmt * 2,
+			},
+		},
+	}
+
+	channelPoints := ht.OpenMultiChannelsAsync(reqs)
+
+	// Make sure every node has heard about every channel.
+	for _, hn := range nodes {
+		for _, cp := range channelPoints {
+			ht.AssertTopologyChannelOpen(hn, cp)
+		}
+
+		// Each node should have exactly 5 edges.
+		ht.AssertNumEdges(hn, len(channelPoints), false)
+	}
+
+	// Make Dave create an invoice with a blinded path for Alice to pay.
+	invoice := &lnrpc.Invoice{
+		Memo:  "test",
+		Value: int64(paymentAmt),
+		Blind: true,
+	}
+	invoiceResp := dave.RPC.AddInvoice(invoice)
+
+	sendReq := &routerrpc.SendPaymentRequest{
+		PaymentRequest: invoiceResp.PaymentRequest,
+		MaxParts:       10,
+		TimeoutSeconds: 60,
+		FeeLimitMsat:   noFeeLimitMsat,
+	}
+	payment := ht.SendPaymentAssertSettled(alice, sendReq)
+
+	preimageBytes, err := hex.DecodeString(payment.PaymentPreimage)
+	require.NoError(ht, err)
+
+	preimage, err := lntypes.MakePreimage(preimageBytes)
+	require.NoError(ht, err)
+
+	hash, err := lntypes.MakeHash(invoiceResp.RHash)
+	require.NoError(ht, err)
+
+	// Make sure we got the preimage.
+	require.True(ht, preimage.Matches(hash), "preimage doesn't match")
+
+	// Check that Alice split the payment in at least two shards. Because
+	// the hand-off of the htlc to the link is asynchronous (via a mailbox),
+	// there is some non-determinism in the process. Depending on whether
+	// the new pathfinding round is started before or after the htlc is
+	// locked into the channel, different sharding may occur. Therefore, we
+	// can only check if the number of shards isn't below the theoretical
+	// minimum.
+	succeeded := 0
+	for _, htlc := range payment.Htlcs {
+		if htlc.Status == lnrpc.HTLCAttempt_SUCCEEDED {
+			succeeded++
+		}
+	}
+
+	const minExpectedShards = 2
+	require.GreaterOrEqual(ht, succeeded, minExpectedShards,
+		"expected shards not reached")
+
+	// Make sure Dave show the invoice as settled for the full amount.
+	inv := dave.RPC.LookupInvoice(invoiceResp.RHash)
+
+	require.EqualValues(ht, paymentAmt, inv.AmtPaidSat,
+		"incorrect payment amt")
+
+	require.Equal(ht, lnrpc.Invoice_SETTLED, inv.State,
+		"Invoice not settled")
+
+	settled := 0
+	for _, htlc := range inv.Htlcs {
+		if htlc.State == lnrpc.InvoiceHTLCState_SETTLED {
+			settled++
+		}
+	}
+	require.Equal(ht, succeeded, settled, "num of HTLCs wrong")
+
+	// Close all channels without mining the closing transactions.
+	ht.CloseChannelAssertPending(alice, channelPoints[0], false)
+	ht.CloseChannelAssertPending(alice, channelPoints[1], false)
+	ht.CloseChannelAssertPending(bob, channelPoints[2], false)
+	ht.CloseChannelAssertPending(eve, channelPoints[3], false)
+	ht.CloseChannelAssertPending(carol, channelPoints[4], false)
+
+	// Now mine a block to include all the closing transactions.
+	ht.MineBlocksAndAssertNumTxes(1, 5)
+
+	// Assert that the channels are closed.
+	for _, hn := range nodes {
+		ht.AssertNumWaitingClose(hn, 0)
+	}
+}

routing/payment_session.go

@@ -338,8 +338,12 @@ func (p *paymentSession) RequestRoute(maxAmt, feeLimit lnwire.MilliSatoshi,
 		switch {
 		case err == errNoPathFound:
 			// Don't split if this is a legacy payment without mpp
-			// record.
-			if p.payment.PaymentAddr == nil {
+			// record. If it has a blinded path though, then we
+			// can split. Split payments to blinded paths won't have
+			// MPP records.
+			if p.payment.PaymentAddr == nil &&
+				p.payment.BlindedPayment == nil {
+
 				p.log.Debugf("not splitting because payment " +
 					"address is unspecified")
 
@@ -357,7 +361,8 @@ func (p *paymentSession) RequestRoute(maxAmt, feeLimit lnwire.MilliSatoshi,
 				!destFeatures.HasFeature(lnwire.AMPOptional) {
 
 				p.log.Debug("not splitting because " +
-					"destination doesn't declare MPP or AMP")
+					"destination doesn't declare MPP or " +
+					"AMP")
 
 				return nil, errNoPathFound
 			}