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

@@ -9,6 +9,7 @@ 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"
@@ -109,6 +110,10 @@ 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.
@@ -123,6 +128,14 @@ type FwdResolution struct {
 	// FwdActionSettle.
 	Preimage lntypes.Preimage
 
+	// OutAmountMsat is the amount that is to be used for forwarding if
+	// Action is FwdActionResumeModified.
+	OutAmountMsat fn.Option[lnwire.MilliSatoshi]
+
+	// OutWireCustomRecords is the custom records that are to be used for
+	// forwarding if Action is FwdActionResumeModified.
+	OutWireCustomRecords fn.Option[lnwire.CustomRecords]
+
 	// FailureMessage is the encrypted failure message that is to be passed
 	// back to the sender if action is FwdActionFail.
 	FailureMessage []byte
@@ -399,6 +412,11 @@ func (s *InterceptableSwitch) resolve(res *FwdResolution) error {
 	case FwdActionResume:
 		return intercepted.Resume()
 
+	case FwdActionResumeModified:
+		return intercepted.ResumeModified(
+			res.OutAmountMsat, res.OutWireCustomRecords,
+		)
+
 	case FwdActionSettle:
 		return intercepted.Settle(res.Preimage)
 
@@ -641,6 +659,64 @@ func (f *interceptedForward) Resume() error {
 	return f.htlcSwitch.ForwardPackets(nil, f.packet)
 }
 
+// ResumeModified resumes the default behavior with field modifications. The
+// input amount (if provided) specifies that the value of the inbound HTLC
+// should be interpreted differently from the on-chain amount during further
+// validation. The presence of an output amount and/or custom records indicates
+// that those values should be modified on the outgoing HTLC.
+func (f *interceptedForward) ResumeModified(
+	outAmountMsat fn.Option[lnwire.MilliSatoshi],
+	outWireCustomRecords fn.Option[lnwire.CustomRecords]) error {
+
+	// Convert the optional custom records to the correct type and validate
+	// them.
+	validatedRecords, err := fn.MapOptionZ(
+		outWireCustomRecords,
+		func(cr lnwire.CustomRecords) fn.Result[lnwire.CustomRecords] {
+			if len(cr) == 0 {
+				return fn.Ok[lnwire.CustomRecords](nil)
+			}
+
+			// Type cast and validate custom records.
+			err := cr.Validate()
+			if err != nil {
+				return fn.Err[lnwire.CustomRecords](
+					fmt.Errorf("failed to validate "+
+						"custom records: %w", err),
+				)
+			}
+
+			return fn.Ok(cr)
+		},
+	).Unpack()
+	if err != nil {
+		return fmt.Errorf("failed to encode custom records: %w",
+			err)
+	}
+
+	// Modify the wire message contained in the packet.
+	switch htlc := f.packet.htlc.(type) {
+	case *lnwire.UpdateAddHTLC:
+		outAmountMsat.WhenSome(func(amount lnwire.MilliSatoshi) {
+			f.packet.amount = amount
+			htlc.Amount = amount
+		})
+
+		if len(validatedRecords) > 0 {
+			htlc.CustomRecords = validatedRecords
+		}
+
+	case *lnwire.UpdateFulfillHTLC:
+		if len(validatedRecords) > 0 {
+			htlc.CustomRecords = validatedRecords
+		}
+	}
+
+	// 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

@@ -383,6 +383,11 @@ 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[lnwire.CustomRecords]) error
+
 	// Settle notifies the intention to settle an existing hold
 	// forward with a given preimage.
 	Settle(lntypes.Preimage) error

intercepted_forward.go

@@ -3,6 +3,7 @@ package lnd
 import (
 	"errors"
 
+	"github.com/lightningnetwork/lnd/fn"
 	"github.com/lightningnetwork/lnd/htlcswitch"
 	"github.com/lightningnetwork/lnd/lntypes"
 	"github.com/lightningnetwork/lnd/lnwire"
@@ -51,6 +52,14 @@ 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[lnwire.CustomRecords]) error {
+
+	return ErrCannotResume
+}
+
 // Fail notifies the intention to fail an existing hold forward with an
 // encrypted failure reason.
 func (f *interceptedForward) Fail(_ []byte) error {

invoices/invoices_test.go

@@ -176,7 +176,7 @@ func TestInvoices(t *testing.T) {
 			test: testQueryInvoices,
 		},
 		{
-			name: "CustomRecords",
+			name: "OutWireCustomRecords",
 			test: testCustomRecords,
 		},
 		{