invoices: accept mpp payments

channeldb/invoices.go

@@ -117,6 +117,7 @@ const (
 	resolveTimeType  tlv.Type = 11
 	expiryHeightType tlv.Type = 13
 	htlcStateType    tlv.Type = 15
+	mppTotalAmtType  tlv.Type = 17
 
 	// A set of tlv type definitions used to serialize invoice bodiees.
 	//
@@ -289,6 +290,10 @@ type InvoiceHTLC struct {
 	// Amt is the amount that is carried by this htlc.
 	Amt lnwire.MilliSatoshi
 
+	// MppTotalAmt is a field for mpp that indicates the expected total
+	// amount.
+	MppTotalAmt lnwire.MilliSatoshi
+
 	// AcceptHeight is the block height at which the invoice registry
 	// decided to accept this htlc as a payment to the invoice. At this
 	// height, the invoice cltv delay must have been met.
@@ -323,6 +328,10 @@ type HtlcAcceptDesc struct {
 	// Amt is the amount that is carried by this htlc.
 	Amt lnwire.MilliSatoshi
 
+	// MppTotalAmt is a field for mpp that indicates the expected total
+	// amount.
+	MppTotalAmt lnwire.MilliSatoshi
+
 	// Expiry is the expiry height of this htlc.
 	Expiry uint32
 
@@ -1018,6 +1027,7 @@ func serializeHtlcs(w io.Writer, htlcs map[CircuitKey]*InvoiceHTLC) error {
 		// Encode the htlc in a tlv stream.
 		chanID := key.ChanID.ToUint64()
 		amt := uint64(htlc.Amt)
+		mppTotalAmt := uint64(htlc.MppTotalAmt)
 		acceptTime := uint64(htlc.AcceptTime.UnixNano())
 		resolveTime := uint64(htlc.ResolveTime.UnixNano())
 		state := uint8(htlc.State)
@@ -1034,6 +1044,7 @@ func serializeHtlcs(w io.Writer, htlcs map[CircuitKey]*InvoiceHTLC) error {
 			tlv.MakePrimitiveRecord(resolveTimeType, &resolveTime),
 			tlv.MakePrimitiveRecord(expiryHeightType, &htlc.Expiry),
 			tlv.MakePrimitiveRecord(htlcStateType, &state),
+			tlv.MakePrimitiveRecord(mppTotalAmtType, &mppTotalAmt),
 		)
 
 		// Convert the custom records to tlv.Record types that are ready
@@ -1193,7 +1204,7 @@ func deserializeHtlcs(r io.Reader) (map[CircuitKey]*InvoiceHTLC, error) {
 			chanID                  uint64
 			state                   uint8
 			acceptTime, resolveTime uint64
-			amt                     uint64
+			amt, mppTotalAmt        uint64
 		)
 		tlvStream, err := tlv.NewStream(
 			tlv.MakePrimitiveRecord(chanIDType, &chanID),
@@ -1206,6 +1217,7 @@ func deserializeHtlcs(r io.Reader) (map[CircuitKey]*InvoiceHTLC, error) {
 			tlv.MakePrimitiveRecord(resolveTimeType, &resolveTime),
 			tlv.MakePrimitiveRecord(expiryHeightType, &htlc.Expiry),
 			tlv.MakePrimitiveRecord(htlcStateType, &state),
+			tlv.MakePrimitiveRecord(mppTotalAmtType, &mppTotalAmt),
 		)
 		if err != nil {
 			return nil, err
@@ -1221,6 +1233,7 @@ func deserializeHtlcs(r io.Reader) (map[CircuitKey]*InvoiceHTLC, error) {
 		htlc.ResolveTime = time.Unix(0, int64(resolveTime))
 		htlc.State = HtlcState(state)
 		htlc.Amt = lnwire.MilliSatoshi(amt)
+		htlc.MppTotalAmt = lnwire.MilliSatoshi(mppTotalAmt)
 
 		// Reconstruct the custom records fields from the parsed types
 		// map return from the tlv parser.
@@ -1324,6 +1337,7 @@ func (d *DB) updateInvoice(hash lntypes.Hash, invoices, settleIndex *bbolt.Bucke
 
 		htlc := &InvoiceHTLC{
 			Amt:           htlcUpdate.Amt,
+			MppTotalAmt:   htlcUpdate.MppTotalAmt,
 			Expiry:        htlcUpdate.Expiry,
 			AcceptHeight:  uint32(htlcUpdate.AcceptHeight),
 			AcceptTime:    now,

invoices/invoiceregistry.go

@@ -2,8 +2,10 @@ package invoices
 
 import (
 	"errors"
+	"fmt"
 	"sync"
 	"sync/atomic"
+	"time"
 
 	"github.com/davecgh/go-spew/spew"
 	"github.com/lightningnetwork/lnd/channeldb"
@@ -26,6 +28,12 @@ var (
 	ErrShuttingDown = errors.New("invoice registry shutting down")
 )
 
+const (
+	// DefaultHtlcHoldDuration defines the default for how long mpp htlcs
+	// are held while waiting for the other set members to arrive.
+	DefaultHtlcHoldDuration = 120 * time.Second
+)
+
 // HodlEvent describes how an htlc should be resolved. If HodlEvent.Preimage is
 // set, the event indicates a settle event. If Preimage is nil, it is a cancel
 // event.
@@ -49,6 +57,38 @@ type RegistryConfig struct {
 	// expiry of any invoice we create and the code effectuating this should
 	// not be hit.
 	FinalCltvRejectDelta int32
+
+	// HtlcHoldDuration defines for how long mpp htlcs are held while
+	// waiting for the other set members to arrive.
+	HtlcHoldDuration time.Duration
+
+	// Now returns the current time.
+	Now func() time.Time
+
+	// TickAfter returns a channel that is sent on after the specified
+	// duration as passed.
+	TickAfter func(duration time.Duration) <-chan time.Time
+}
+
+// htlcReleaseEvent describes an htlc auto-release event. It is used to release
+// mpp htlcs for which the complete set didn't arrive in time.
+type htlcReleaseEvent struct {
+	// hash is the payment hash of the htlc to release.
+	hash lntypes.Hash
+
+	// key is the circuit key of the htlc to release.
+	key channeldb.CircuitKey
+
+	// releaseTime is the time at which to release the htlc.
+	releaseTime time.Time
+}
+
+// Less is used to order PriorityQueueItem's by their release time such that
+// items with the older release time are at the top of the queue.
+//
+// NOTE: Part of the queue.PriorityQueueItem interface.
+func (r *htlcReleaseEvent) Less(other queue.PriorityQueueItem) bool {
+	return r.releaseTime.Before(other.(*htlcReleaseEvent).releaseTime)
 }
 
 // InvoiceRegistry is a central registry of all the outstanding invoices
@@ -82,6 +122,10 @@ type InvoiceRegistry struct {
 	// subscriber. This is used to unsubscribe from all hashes efficiently.
 	hodlReverseSubscriptions map[chan<- interface{}]map[channeldb.CircuitKey]struct{}
 
+	// htlcAutoReleaseChan contains the new htlcs that need to be
+	// auto-released.
+	htlcAutoReleaseChan chan *htlcReleaseEvent
+
 	wg   sync.WaitGroup
 	quit chan struct{}
 }
@@ -91,7 +135,6 @@ type InvoiceRegistry struct {
 // layer. The in-memory layer is in place such that debug invoices can be added
 // which are volatile yet available system wide within the daemon.
 func NewRegistry(cdb *channeldb.DB, cfg *RegistryConfig) *InvoiceRegistry {
-
 	return &InvoiceRegistry{
 		cdb:                       cdb,
 		notificationClients:       make(map[uint32]*InvoiceSubscription),
@@ -102,6 +145,7 @@ func NewRegistry(cdb *channeldb.DB, cfg *RegistryConfig) *InvoiceRegistry {
 		hodlSubscriptions:         make(map[channeldb.CircuitKey]map[chan<- interface{}]struct{}),
 		hodlReverseSubscriptions:  make(map[chan<- interface{}]map[channeldb.CircuitKey]struct{}),
 		cfg:                       cfg,
+		htlcAutoReleaseChan:       make(chan *htlcReleaseEvent),
 		quit:                      make(chan struct{}),
 	}
 }
@@ -110,7 +154,7 @@ func NewRegistry(cdb *channeldb.DB, cfg *RegistryConfig) *InvoiceRegistry {
 func (i *InvoiceRegistry) Start() error {
 	i.wg.Add(1)
 
-	go i.invoiceEventNotifier()
+	go i.invoiceEventLoop()
 
 	return nil
 }
@@ -130,13 +174,31 @@ type invoiceEvent struct {
 	invoice *channeldb.Invoice
 }
 
-// invoiceEventNotifier is the dedicated goroutine responsible for accepting
+// tickAt returns a channel that ticks at the specified time. If the time has
+// already passed, it will tick immediately.
+func (i *InvoiceRegistry) tickAt(t time.Time) <-chan time.Time {
+	now := i.cfg.Now()
+	return i.cfg.TickAfter(t.Sub(now))
+}
+
+// invoiceEventLoop is the dedicated goroutine responsible for accepting
 // new notification subscriptions, cancelling old subscriptions, and
 // dispatching new invoice events.
-func (i *InvoiceRegistry) invoiceEventNotifier() {
+func (i *InvoiceRegistry) invoiceEventLoop() {
 	defer i.wg.Done()
 
+	// Set up a heap for htlc auto-releases.
+	autoReleaseHeap := &queue.PriorityQueue{}
+
 	for {
+		// If there is something to release, set up a release tick
+		// channel.
+		var nextReleaseTick <-chan time.Time
+		if autoReleaseHeap.Len() > 0 {
+			head := autoReleaseHeap.Top().(*htlcReleaseEvent)
+			nextReleaseTick = i.tickAt(head.releaseTime)
+		}
+
 		select {
 		// A new invoice subscription for all invoices has just arrived!
 		// We'll query for any backlog notifications, then add it to the
@@ -202,6 +264,29 @@ func (i *InvoiceRegistry) invoiceEventNotifier() {
 				i.singleNotificationClients[e.id] = e
 			}
 
+		// A new htlc came in for auto-release.
+		case event := <-i.htlcAutoReleaseChan:
+			log.Debugf("Scheduling auto-release for htlc: "+
+				"hash=%v, key=%v at %v",
+				event.hash, event.key, event.releaseTime)
+
+			// We use an independent timer for every htlc rather
+			// than a set timer that is reset with every htlc coming
+			// in. Otherwise the sender could keep resetting the
+			// timer until the broadcast window is entered and our
+			// channel is force closed.
+			autoReleaseHeap.Push(event)
+
+		// The htlc at the top of the heap needs to be auto-released.
+		case <-nextReleaseTick:
+			event := autoReleaseHeap.Pop().(*htlcReleaseEvent)
+			err := i.cancelSingleHtlc(
+				event.hash, event.key,
+			)
+			if err != nil {
+				log.Errorf("HTLC timer: %v", err)
+			}
+
 		case <-i.quit:
 			return
 		}
@@ -418,6 +503,114 @@ func (i *InvoiceRegistry) LookupInvoice(rHash lntypes.Hash) (channeldb.Invoice,
 	return i.cdb.LookupInvoice(rHash)
 }
 
+// startHtlcTimer starts a new timer via the invoice registry main loop that
+// cancels a single htlc on an invoice when the htlc hold duration has passed.
+func (i *InvoiceRegistry) startHtlcTimer(hash lntypes.Hash,
+	key channeldb.CircuitKey, acceptTime time.Time) error {
+
+	releaseTime := acceptTime.Add(i.cfg.HtlcHoldDuration)
+	event := &htlcReleaseEvent{
+		hash:        hash,
+		key:         key,
+		releaseTime: releaseTime,
+	}
+
+	select {
+	case i.htlcAutoReleaseChan <- event:
+		return nil
+
+	case <-i.quit:
+		return ErrShuttingDown
+	}
+}
+
+// cancelSingleHtlc cancels a single accepted htlc on an invoice.
+func (i *InvoiceRegistry) cancelSingleHtlc(hash lntypes.Hash,
+	key channeldb.CircuitKey) error {
+
+	i.Lock()
+	defer i.Unlock()
+
+	updateInvoice := func(invoice *channeldb.Invoice) (
+		*channeldb.InvoiceUpdateDesc, error) {
+
+		// Only allow individual htlc cancelation on open invoices.
+		if invoice.State != channeldb.ContractOpen {
+			log.Debugf("cancelSingleHtlc: invoice %v no longer "+
+				"open", hash)
+
+			return nil, nil
+		}
+
+		// Lookup the current status of the htlc in the database.
+		htlc, ok := invoice.Htlcs[key]
+		if !ok {
+			return nil, fmt.Errorf("htlc %v not found", key)
+		}
+
+		// Cancelation is only possible if the htlc wasn't already
+		// resolved.
+		if htlc.State != channeldb.HtlcStateAccepted {
+			log.Debugf("cancelSingleHtlc: htlc %v on invoice %v "+
+				"is already resolved", key, hash)
+
+			return nil, nil
+		}
+
+		log.Debugf("cancelSingleHtlc: cancelling htlc %v on invoice %v",
+			key, hash)
+
+		// Return an update descriptor that cancels htlc and keeps
+		// invoice open.
+		canceledHtlcs := map[channeldb.CircuitKey]struct{}{
+			key: {},
+		}
+
+		return &channeldb.InvoiceUpdateDesc{
+			CancelHtlcs: canceledHtlcs,
+		}, nil
+	}
+
+	// Try to mark the specified htlc as canceled in the invoice database.
+	// Intercept the update descriptor to set the local updated variable. If
+	// no invoice update is performed, we can return early.
+	var updated bool
+	invoice, err := i.cdb.UpdateInvoice(hash,
+		func(invoice *channeldb.Invoice) (
+			*channeldb.InvoiceUpdateDesc, error) {
+
+			updateDesc, err := updateInvoice(invoice)
+			if err != nil {
+				return nil, err
+			}
+			updated = updateDesc != nil
+
+			return updateDesc, err
+		},
+	)
+	if err != nil {
+		return err
+	}
+	if !updated {
+		return nil
+	}
+
+	// The invoice has been updated. Notify subscribers of the htlc
+	// resolution.
+	htlc, ok := invoice.Htlcs[key]
+	if !ok {
+		return fmt.Errorf("htlc %v not found", key)
+	}
+	if htlc.State == channeldb.HtlcStateCanceled {
+		i.notifyHodlSubscribers(HodlEvent{
+			CircuitKey:   key,
+			AcceptHeight: int32(htlc.AcceptHeight),
+			Preimage:     nil,
+		})
+	}
+	return nil
+}
+
 // NotifyExitHopHtlc attempts to mark an invoice as settled. The return value
 // describes how the htlc should be resolved.
 //
@@ -428,6 +621,11 @@ func (i *InvoiceRegistry) LookupInvoice(rHash lntypes.Hash) (channeldb.Invoice,
 // to be taken on the htlc (settle or cancel). The caller needs to ensure that
 // the channel is either buffered or received on from another goroutine to
 // prevent deadlock.
+//
+// In the case that the htlc is part of a larger set of htlcs that pay to the
+// same invoice (multi-path payment), the htlc is held until the set is
+// complete. If the set doesn't fully arrive in time, a timer will cancel the
+// held htlc.
 func (i *InvoiceRegistry) NotifyExitHopHtlc(rHash lntypes.Hash,
 	amtPaid lnwire.MilliSatoshi, expiry uint32, currentHeight int32,
 	circuitKey channeldb.CircuitKey, hodlChan chan<- interface{},
@@ -436,9 +634,11 @@ func (i *InvoiceRegistry) NotifyExitHopHtlc(rHash lntypes.Hash,
 	i.Lock()
 	defer i.Unlock()
 
+	mpp := payload.MultiPath()
+
 	debugLog := func(s string) {
-		log.Debugf("Invoice(%x): %v, amt=%v, expiry=%v, circuit=%v",
+		log.Debugf("Invoice(%x): %v, amt=%v, expiry=%v, circuit=%v, "+
-			rHash[:], s, amtPaid, expiry, circuitKey)
+			"mpp=%v", rHash[:], s, amtPaid, expiry, circuitKey, mpp)
 	}
 
 	// Create the update context containing the relevant details of the
@@ -450,6 +650,7 @@ func (i *InvoiceRegistry) NotifyExitHopHtlc(rHash lntypes.Hash,
 		currentHeight:        currentHeight,
 		finalCltvRejectDelta: i.cfg.FinalCltvRejectDelta,
 		customRecords:        payload.CustomRecords(),
+		mpp:                  mpp,
 	}
 
 	// We'll attempt to settle an invoice matching this rHash on disk (if
@@ -514,6 +715,21 @@ func (i *InvoiceRegistry) NotifyExitHopHtlc(rHash lntypes.Hash,
 		}, nil
 
 	case channeldb.HtlcStateSettled:
+		// Also settle any previously accepted htlcs. The invoice state
+		// is leading. If an htlc is marked as settled, we should follow
+		// now and settle the htlc with our peer.
+		for key, htlc := range invoice.Htlcs {
+			if htlc.State != channeldb.HtlcStateSettled {
+				continue
+			}
+
+			i.notifyHodlSubscribers(HodlEvent{
+				CircuitKey:   key,
+				Preimage:     &invoice.Terms.PaymentPreimage,
+				AcceptHeight: int32(htlc.AcceptHeight),
+			})
+		}
+
 		return &HodlEvent{
 			CircuitKey:   circuitKey,
 			Preimage:     &invoice.Terms.PaymentPreimage,
@@ -521,6 +737,19 @@ func (i *InvoiceRegistry) NotifyExitHopHtlc(rHash lntypes.Hash,
 		}, nil
 
 	case channeldb.HtlcStateAccepted:
+		// (Re)start the htlc timer if the invoice is still open. It can
+		// only happen for mpp payments that there are htlcs in state
+		// Accepted while the invoice is Open.
+		if invoice.State == channeldb.ContractOpen {
+			err := i.startHtlcTimer(
+				rHash, circuitKey,
+				invoiceHtlc.AcceptTime,
+			)
+			if err != nil {
+				return nil, err
+			}
+		}
+
 		i.hodlSubscribe(hodlChan, circuitKey)
 		return nil, nil
 

invoices/invoiceregistry_test.go

@@ -16,6 +16,8 @@ import (
 var (
 	testTimeout = 5 * time.Second
 
+	testTime = time.Date(2018, time.February, 2, 14, 0, 0, 0, time.UTC)
+
 	preimage = lntypes.Preimage{
 		0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
 		0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1,
@@ -59,20 +61,27 @@ var (
 
 type testContext struct {
 	registry *InvoiceRegistry
+	clock    *testClock
 
 	cleanup func()
 	t       *testing.T
 }
 
 func newTestContext(t *testing.T) *testContext {
+	clock := newTestClock(testTime)
+
 	cdb, cleanup, err := newDB()
 	if err != nil {
 		t.Fatal(err)
 	}
+	cdb.Now = clock.now
 
 	// Instantiate and start the invoice ctx.registry.
 	cfg := RegistryConfig{
 		FinalCltvRejectDelta: testFinalCltvRejectDelta,
+		HtlcHoldDuration:     30 * time.Second,
+		Now:                  clock.now,
+		TickAfter:            clock.tickAfter,
 	}
 	registry := NewRegistry(cdb, &cfg)
 
@@ -84,6 +93,7 @@ func newTestContext(t *testing.T) *testContext {
 
 	ctx := testContext{
 		registry: registry,
+		clock:    clock,
 		t:        t,
 		cleanup: func() {
 			registry.Stop()
@@ -683,3 +693,85 @@ func (p *mockPayload) MultiPath() *record.MPP {
 func (p *mockPayload) CustomRecords() hop.CustomRecordSet {
 	return make(hop.CustomRecordSet)
 }
+
+// TestSettleMpp tests settling of an invoice with multiple partial payments.
+func TestSettleMpp(t *testing.T) {
+	defer timeout(t)()
+
+	ctx := newTestContext(t)
+	defer ctx.cleanup()
+
+	// Add the invoice.
+	_, err := ctx.registry.AddInvoice(testInvoice, hash)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	mppPayload := &mockPayload{
+		mpp: record.NewMPP(testInvoiceAmt, [32]byte{}),
+	}
+
+	// Send htlc 1.
+	hodlChan1 := make(chan interface{}, 1)
+	event, err := ctx.registry.NotifyExitHopHtlc(
+		hash, testInvoice.Terms.Value/2,
+		testHtlcExpiry,
+		testCurrentHeight, getCircuitKey(10), hodlChan1, mppPayload,
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if event != nil {
+		t.Fatal("expected no direct resolution")
+	}
+
+	// Simulate mpp timeout releasing htlc 1.
+	ctx.clock.setTime(testTime.Add(30 * time.Second))
+
+	hodlEvent := (<-hodlChan1).(HodlEvent)
+	if hodlEvent.Preimage != nil {
+		t.Fatal("expected cancel event")
+	}
+
+	// Send htlc 2.
+	hodlChan2 := make(chan interface{}, 1)
+	event, err = ctx.registry.NotifyExitHopHtlc(
+		hash, testInvoice.Terms.Value/2,
+		testHtlcExpiry,
+		testCurrentHeight, getCircuitKey(11), hodlChan2, mppPayload,
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if event != nil {
+		t.Fatal("expected no direct resolution")
+	}
+
+	// Send htlc 3.
+	hodlChan3 := make(chan interface{}, 1)
+	event, err = ctx.registry.NotifyExitHopHtlc(
+		hash, testInvoice.Terms.Value/2,
+		testHtlcExpiry,
+		testCurrentHeight, getCircuitKey(12), hodlChan3, mppPayload,
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if event == nil {
+		t.Fatal("expected a settle event")
+	}
+
+	// Check that settled amount is equal to the sum of values of the htlcs
+	// 0 and 1.
+	inv, err := ctx.registry.LookupInvoice(hash)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if inv.State != channeldb.ContractSettled {
+		t.Fatal("expected invoice to be settled")
+	}
+	if inv.AmtPaid != testInvoice.Terms.Value {
+		t.Fatalf("amount incorrect, expected %v but got %v",
+			testInvoice.Terms.Value, inv.AmtPaid)
+	}
+}

invoices/update.go

@@ -7,6 +7,7 @@ import (
 
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/lnwire"
+	"github.com/lightningnetwork/lnd/record"
 )
 
 // updateResult is the result of the invoice update call.
@@ -24,6 +25,13 @@ const (
 	resultDuplicateToSettled
 	resultAccepted
 	resultSettled
+	resultInvoiceNotOpen
+	resultPartialAccepted
+	resultMppInProgress
+	resultAddressMismatch
+	resultHtlcSetTotalMismatch
+	resultHtlcSetTotalTooLow
+	resultHtlcSetOverpayment
 )
 
 // String returns a human-readable representation of the invoice update result.
@@ -63,6 +71,27 @@ func (u updateResult) String() string {
 	case resultSettled:
 		return "settled"
 
+	case resultInvoiceNotOpen:
+		return "invoice no longer open"
+
+	case resultPartialAccepted:
+		return "partial payment accepted"
+
+	case resultMppInProgress:
+		return "mpp reception in progress"
+
+	case resultAddressMismatch:
+		return "payment address mismatch"
+
+	case resultHtlcSetTotalMismatch:
+		return "htlc total amt doesn't match set total"
+
+	case resultHtlcSetTotalTooLow:
+		return "set total too low for invoice"
+
+	case resultHtlcSetOverpayment:
+		return "mpp is overpaying set total"
+
 	default:
 		return "unknown"
 	}
@@ -77,6 +106,7 @@ type invoiceUpdateCtx struct {
 	currentHeight        int32
 	finalCltvRejectDelta int32
 	customRecords        hop.CustomRecordSet
+	mpp                  *record.MPP
 }
 
 // updateInvoice is a callback for DB.UpdateInvoice that contains the invoice
@@ -102,8 +132,125 @@ func updateInvoice(ctx *invoiceUpdateCtx, inv *channeldb.Invoice) (
 		}
 	}
 
-	// If the invoice is already canceled, there is no further checking to
+	if ctx.mpp == nil {
-	// do.
+		return updateLegacy(ctx, inv)
+	}
+
+	return updateMpp(ctx, inv)
+}
+
+// updateMpp is a callback for DB.UpdateInvoice that contains the invoice
+// settlement logic for mpp payments.
+func updateMpp(ctx *invoiceUpdateCtx, inv *channeldb.Invoice) (
+	*channeldb.InvoiceUpdateDesc, updateResult, error) {
+
+	// Start building the accept descriptor.
+	acceptDesc := &channeldb.HtlcAcceptDesc{
+		Amt:           ctx.amtPaid,
+		Expiry:        ctx.expiry,
+		AcceptHeight:  ctx.currentHeight,
+		MppTotalAmt:   ctx.mpp.TotalMsat(),
+		CustomRecords: ctx.customRecords,
+	}
+
+	// Only accept payments to open invoices. This behaviour differs from
+	// non-mpp payments that are accepted even after the invoice is settled.
+	// Because non-mpp payments don't have a payment address, this is needed
+	// to thwart probing.
+	if inv.State != channeldb.ContractOpen {
+		return nil, resultInvoiceNotOpen, nil
+	}
+
+	// Check the payment address that authorizes the payment.
+	if ctx.mpp.PaymentAddr() != inv.Terms.PaymentAddr {
+		return nil, resultAddressMismatch, nil
+	}
+
+	// Don't accept zero-valued sets.
+	if ctx.mpp.TotalMsat() == 0 {
+		return nil, resultHtlcSetTotalTooLow, nil
+	}
+
+	// Check that the total amt of the htlc set is high enough. In case this
+	// is a zero-valued invoice, it will always be enough.
+	if ctx.mpp.TotalMsat() < inv.Terms.Value {
+		return nil, resultHtlcSetTotalTooLow, nil
+	}
+
+	// Check whether total amt matches other htlcs in the set.
+	var newSetTotal lnwire.MilliSatoshi
+	for _, htlc := range inv.Htlcs {
+		// Only consider accepted mpp htlcs. It is possible that there
+		// are htlcs registered in the invoice database that previously
+		// timed out and are in the canceled state now.
+		if htlc.State != channeldb.HtlcStateAccepted {
+			continue
+		}
+
+		if ctx.mpp.TotalMsat() != htlc.MppTotalAmt {
+			return nil, resultHtlcSetTotalMismatch, nil
+		}
+
+		newSetTotal += htlc.Amt
+	}
+
+	// Add amount of new htlc.
+	newSetTotal += ctx.amtPaid
+
+	// Make sure the communicated set total isn't overpaid.
+	if newSetTotal > ctx.mpp.TotalMsat() {
+		return nil, resultHtlcSetOverpayment, nil
+	}
+
+	// The invoice is still open. Check the expiry.
+	if ctx.expiry < uint32(ctx.currentHeight+ctx.finalCltvRejectDelta) {
+		return nil, resultExpiryTooSoon, nil
+	}
+
+	if ctx.expiry < uint32(ctx.currentHeight+inv.Terms.FinalCltvDelta) {
+		return nil, resultExpiryTooSoon, nil
+	}
+
+	// Record HTLC in the invoice database.
+	newHtlcs := map[channeldb.CircuitKey]*channeldb.HtlcAcceptDesc{
+		ctx.circuitKey: acceptDesc,
+	}
+
+	update := channeldb.InvoiceUpdateDesc{
+		AddHtlcs: newHtlcs,
+	}
+
+	// If the invoice cannot be settled yet, only record the htlc.
+	setComplete := newSetTotal == ctx.mpp.TotalMsat()
+	if !setComplete {
+		return &update, resultPartialAccepted, nil
+	}
+
+	// Check to see if we can settle or this is an hold invoice and
+	// we need to wait for the preimage.
+	holdInvoice := inv.Terms.PaymentPreimage == channeldb.UnknownPreimage
+	if holdInvoice {
+		update.State = &channeldb.InvoiceStateUpdateDesc{
+			NewState: channeldb.ContractAccepted,
+		}
+		return &update, resultAccepted, nil
+	}
+
+	update.State = &channeldb.InvoiceStateUpdateDesc{
+		NewState: channeldb.ContractSettled,
+		Preimage: inv.Terms.PaymentPreimage,
+	}
+
+	return &update, resultSettled, nil
+}
+
+// updateLegacy is a callback for DB.UpdateInvoice that contains the invoice
+// settlement logic for legacy payments.
+func updateLegacy(ctx *invoiceUpdateCtx, inv *channeldb.Invoice) (
+	*channeldb.InvoiceUpdateDesc, updateResult, error) {
+
+	// If the invoice is already canceled, there is no further
+	// checking to do.
 	if inv.State == channeldb.ContractCanceled {
 		return nil, resultInvoiceAlreadyCanceled, nil
 	}
@@ -116,6 +263,20 @@ func updateInvoice(ctx *invoiceUpdateCtx, inv *channeldb.Invoice) (
 		return nil, resultAmountTooLow, nil
 	}
 
+	// TODO(joostjager): Check invoice mpp required feature
+	// bit when feature becomes mandatory.
+
+	// Don't allow settling the invoice with an old style
+	// htlc if we are already in the process of gathering an
+	// mpp set.
+	for _, htlc := range inv.Htlcs {
+		if htlc.State == channeldb.HtlcStateAccepted &&
+			htlc.MppTotalAmt > 0 {
+
+			return nil, resultMppInProgress, nil
+		}
+	}
+
 	// The invoice is still open. Check the expiry.
 	if ctx.expiry < uint32(ctx.currentHeight+ctx.finalCltvRejectDelta) {
 		return nil, resultExpiryTooSoon, nil

lnrpc/invoicesrpc/utils.go

@@ -75,14 +75,15 @@ func CreateRPCInvoice(invoice *channeldb.Invoice,
 		}
 
 		rpcHtlc := lnrpc.InvoiceHTLC{
-			ChanId:        key.ChanID.ToUint64(),
+			ChanId:          key.ChanID.ToUint64(),
-			HtlcIndex:     key.HtlcID,
+			HtlcIndex:       key.HtlcID,
-			AcceptHeight:  int32(htlc.AcceptHeight),
+			AcceptHeight:    int32(htlc.AcceptHeight),
-			AcceptTime:    htlc.AcceptTime.Unix(),
+			AcceptTime:      htlc.AcceptTime.Unix(),
-			ExpiryHeight:  int32(htlc.Expiry),
+			ExpiryHeight:    int32(htlc.Expiry),
-			AmtMsat:       uint64(htlc.Amt),
+			AmtMsat:         uint64(htlc.Amt),
-			State:         state,
+			State:           state,
-			CustomRecords: htlc.CustomRecords,
+			CustomRecords:   htlc.CustomRecords,
+			MppTotalAmtMsat: uint64(htlc.MppTotalAmt),
 		}
 
 		// Only report resolved times if htlc is resolved.

lnrpc/rpc.proto

@@ -2407,6 +2407,9 @@ message InvoiceHTLC {
 
     /// Custom tlv records.
     map<uint64, bytes> custom_records = 9 [json_name = "custom_records"];
+
+    /// The total amount of the mpp payment in msat.
+    uint64 mpp_total_amt_msat = 10 [json_name = "mpp_total_amt_msat"];
 }
 
 message AddInvoiceResponse {

lnrpc/rpc.swagger.json

@@ -2854,6 +2854,11 @@
             "format": "byte"
           },
           "description": "/ Custom tlv records."
+        },
+        "mpp_total_amt_msat": {
+          "type": "string",
+          "format": "uint64",
+          "description": "/ The total amount of the mpp payment in msat."
         }
       },
       "title": "/ Details of an HTLC that paid to an invoice"

lntest/itest/lnd_test.go

@@ -4712,13 +4712,29 @@ func testSingleHopSendToRouteCase(net *lntest.NetworkHarness, t *harnessTest,
 	}
 
 	// Create invoices for Dave, which expect a payment from Carol.
-	_, rHashes, _, err := createPayReqs(
+	payReqs, rHashes, _, err := createPayReqs(
 		dave, paymentAmtSat, numPayments,
 	)
 	if err != nil {
 		t.Fatalf("unable to create pay reqs: %v", err)
 	}
 
+	// Reconstruct payment addresses.
+	var payAddrs [][]byte
+	for _, payReq := range payReqs {
+		ctx, _ := context.WithTimeout(
+			context.Background(), defaultTimeout,
+		)
+		resp, err := dave.DecodePayReq(
+			ctx,
+			&lnrpc.PayReqString{PayReq: payReq},
+		)
+		if err != nil {
+			t.Fatalf("decode pay req: %v", err)
+		}
+		payAddrs = append(payAddrs, resp.PaymentAddr)
+	}
+
 	// Query for routes to pay from Carol to Dave.
 	// We set FinalCltvDelta to 40 since by default QueryRoutes returns
 	// the last hop with a final cltv delta of 9 where as the default in
@@ -4741,12 +4757,10 @@ func testSingleHopSendToRouteCase(net *lntest.NetworkHarness, t *harnessTest,
 	// Construct a closure that will set MPP fields on the route, which
 	// allows us to test MPP payments.
 	setMPPFields := func(i int) {
-		addr := [32]byte{byte(i)}
-
 		hop := r.Hops[len(r.Hops)-1]
 		hop.TlvPayload = true
 		hop.MppRecord = &lnrpc.MPPRecord{
-			PaymentAddr:  addr[:],
+			PaymentAddr:  payAddrs[i],
 			TotalAmtMsat: paymentAmtSat * 1000,
 		}
 	}
@@ -4930,8 +4944,8 @@ func testSingleHopSendToRouteCase(net *lntest.NetworkHarness, t *harnessTest,
 					hop.MppRecord.TotalAmtMsat)
 			}
 
-			expAddr := [32]byte{byte(i)}
+			expAddr := payAddrs[i]
-			if !bytes.Equal(hop.MppRecord.PaymentAddr, expAddr[:]) {
+			if !bytes.Equal(hop.MppRecord.PaymentAddr, expAddr) {
 				t.Fatalf("incorrect mpp payment addr for payment %d "+
 					"want: %x, got: %x",
 					i, expAddr, hop.MppRecord.PaymentAddr)

server.go

@@ -380,6 +380,9 @@ func newServer(listenAddrs []net.Addr, chanDB *channeldb.DB,
 
 	registryConfig := invoices.RegistryConfig{
 		FinalCltvRejectDelta: defaultFinalCltvRejectDelta,
+		HtlcHoldDuration:     invoices.DefaultHtlcHoldDuration,
+		Now:                  time.Now,
+		TickAfter:            time.After,
 	}
 
 	s := &server{