htlcswitch: add resume modified HTLC action to switch

Introduce `ResumeModified` action to resume standard behavior of a p2p
message with optional modifications as specified by the client during
interception.

htlcswitch/interceptable_switch.go

@@ -8,9 +8,11 @@ import (
 	"github.com/go-errors/errors"
 	"github.com/lightningnetwork/lnd/chainntnfs"
 	"github.com/lightningnetwork/lnd/channeldb/models"
+	"github.com/lightningnetwork/lnd/fn"
 	"github.com/lightningnetwork/lnd/htlcswitch/hop"
 	"github.com/lightningnetwork/lnd/lntypes"
 	"github.com/lightningnetwork/lnd/lnwire"
+	"github.com/lightningnetwork/lnd/record"
 )
 
 var (
@@ -105,6 +107,10 @@ const (
 
 	// FwdActionFail fails the intercepted packet back to the sender.
 	FwdActionFail
+
+	// FwdActionResumeModified forwards the intercepted packet to the switch
+	// with modifications.
+	FwdActionResumeModified
 )
 
 // FwdResolution defines the action to be taken on an intercepted packet.
@@ -119,6 +125,14 @@ type FwdResolution struct {
 	// FwdActionSettle.
 	Preimage lntypes.Preimage
 
+	// OutgoingAmountMsat is the amount that is to be used for forwarding if
+	// Action is FwdActionResumeModified.
+	OutgoingAmountMsat fn.Option[lnwire.MilliSatoshi]
+
+	// CustomRecords is the custom records that are to be used for
+	// forwarding if Action is FwdActionResumeModified.
+	CustomRecords fn.Option[record.CustomSet]
+
 	// FailureMessage is the encrypted failure message that is to be passed
 	// back to the sender if action is FwdActionFail.
 	FailureMessage []byte
@@ -375,6 +389,11 @@ func (s *InterceptableSwitch) resolve(res *FwdResolution) error {
 	case FwdActionResume:
 		return intercepted.Resume()
 
+	case FwdActionResumeModified:
+		return intercepted.ResumeModified(
+			res.OutgoingAmountMsat, res.CustomRecords,
+		)
+
 	case FwdActionSettle:
 		return intercepted.Settle(res.Preimage)
 
@@ -617,6 +636,67 @@ func (f *interceptedForward) Resume() error {
 	return f.htlcSwitch.ForwardPackets(nil, f.packet)
 }
 
+// ResumeModified resumes the default behavior with field modifications.
+func (f *interceptedForward) ResumeModified(
+	outgoingAmountMsat fn.Option[lnwire.MilliSatoshi],
+	customRecords fn.Option[record.CustomSet]) error {
+
+	// Modify the wire message contained in the packet.
+	switch htlc := f.packet.htlc.(type) {
+	case *lnwire.UpdateAddHTLC:
+		outgoingAmountMsat.WhenSome(func(amount lnwire.MilliSatoshi) {
+			htlc.Amount = amount
+		})
+
+		//nolint:lll
+		err := fn.MapOptionZ(customRecords, func(records record.CustomSet) error {
+			if len(records) == 0 {
+				return nil
+			}
+
+			// Type cast and validate custom records.
+			htlc.CustomRecords = lnwire.CustomRecords(records)
+			err := htlc.CustomRecords.Validate()
+			if err != nil {
+				return fmt.Errorf("failed to validate custom "+
+					"records: %w", err)
+			}
+
+			return nil
+		})
+		if err != nil {
+			return fmt.Errorf("failed to encode custom records: %w",
+				err)
+		}
+
+	case *lnwire.UpdateFulfillHTLC:
+		//nolint:lll
+		err := fn.MapOptionZ(customRecords, func(records record.CustomSet) error {
+			if len(records) == 0 {
+				return nil
+			}
+
+			// Type cast and validate custom records.
+			htlc.CustomRecords = lnwire.CustomRecords(records)
+			err := htlc.CustomRecords.Validate()
+			if err != nil {
+				return fmt.Errorf("failed to validate custom "+
+					"records: %w", err)
+			}
+
+			return nil
+		})
+		if err != nil {
+			return fmt.Errorf("failed to encode custom records: %w",
+				err)
+		}
+	}
+
+	// Forward to the switch. A link quit channel isn't needed, because we
+	// are on a different thread now.
+	return f.htlcSwitch.ForwardPackets(nil, f.packet)
+}
+
 // Fail notifies the intention to Fail an existing hold forward with an
 // encrypted failure reason.
 func (f *interceptedForward) Fail(reason []byte) error {

htlcswitch/interfaces.go

@@ -6,6 +6,7 @@ import (
 	"github.com/btcsuite/btcd/wire"
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/channeldb/models"
+	"github.com/lightningnetwork/lnd/fn"
 	"github.com/lightningnetwork/lnd/invoices"
 	"github.com/lightningnetwork/lnd/lntypes"
 	"github.com/lightningnetwork/lnd/lnwallet/chainfee"
@@ -375,6 +376,11 @@ type InterceptedForward interface {
 	// this htlc which usually means forward it.
 	Resume() error
 
+	// ResumeModified notifies the intention to resume an existing hold
+	// forward with modified fields.
+	ResumeModified(outgoingAmountMsat fn.Option[lnwire.MilliSatoshi],
+		customRecords fn.Option[record.CustomSet]) error
+
 	// Settle notifies the intention to settle an existing hold
 	// forward with a given preimage.
 	Settle(lntypes.Preimage) error

intercepted_forward.go

@@ -3,9 +3,11 @@ package lnd
 import (
 	"errors"
 
+	"github.com/lightningnetwork/lnd/fn"
 	"github.com/lightningnetwork/lnd/htlcswitch"
 	"github.com/lightningnetwork/lnd/lntypes"
 	"github.com/lightningnetwork/lnd/lnwire"
+	"github.com/lightningnetwork/lnd/record"
 )
 
 var (
@@ -51,6 +53,14 @@ func (f *interceptedForward) Resume() error {
 	return ErrCannotResume
 }
 
+// ResumeModified notifies the intention to resume an existing hold forward with
+// a modified htlc.
+func (f *interceptedForward) ResumeModified(_ fn.Option[lnwire.MilliSatoshi],
+	_ fn.Option[record.CustomSet]) error {
+
+	return ErrCannotResume
+}
+
 // Fail notifies the intention to fail an existing hold forward with an
 // encrypted failure reason.
 func (f *interceptedForward) Fail(_ []byte) error {