routerrpc: extend HTLC forward interceptor resp with modification fields

This commit extends the forward HTLC intercept response with fields that
can be used in conjunction with a `ResumeModified` action to modify the
intercepted HTLC p2p message.

htlcswitch/interceptable_switch.go

@@ -128,6 +128,10 @@ type FwdResolution struct {
 	// FwdActionSettle.
 	Preimage lntypes.Preimage
 
+	// InAmountMsat is the amount that is to be used for validating if
+	// Action is FwdActionResumeModified.
+	InAmountMsat fn.Option[lnwire.MilliSatoshi]
+
 	// OutAmountMsat is the amount that is to be used for forwarding if
 	// Action is FwdActionResumeModified.
 	OutAmountMsat fn.Option[lnwire.MilliSatoshi]
@@ -414,7 +418,8 @@ func (s *InterceptableSwitch) resolve(res *FwdResolution) error {
 
 	case FwdActionResumeModified:
 		return intercepted.ResumeModified(
-			res.OutAmountMsat, res.OutWireCustomRecords,
+			res.InAmountMsat, res.OutAmountMsat,
+			res.OutWireCustomRecords,
 		)
 
 	case FwdActionSettle:
@@ -665,6 +670,7 @@ func (f *interceptedForward) Resume() error {
 // 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(
+	inAmountMsat fn.Option[lnwire.MilliSatoshi],
 	outAmountMsat fn.Option[lnwire.MilliSatoshi],
 	outWireCustomRecords fn.Option[lnwire.CustomRecords]) error {
 
@@ -694,6 +700,11 @@ func (f *interceptedForward) ResumeModified(
 			err)
 	}
 
+	// Set the incoming amount, if it is provided, on the packet.
+	inAmountMsat.WhenSome(func(amount lnwire.MilliSatoshi) {
+		f.packet.incomingAmount = amount
+	})
+
 	// Modify the wire message contained in the packet.
 	switch htlc := f.packet.htlc.(type) {
 	case *lnwire.UpdateAddHTLC:

htlcswitch/interfaces.go

@@ -385,8 +385,9 @@ type InterceptedForward interface {
 
 	// 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
+	ResumeModified(inAmountMsat,
+		outAmountMsat fn.Option[lnwire.MilliSatoshi],
+		outWireCustomRecords fn.Option[lnwire.CustomRecords]) error
 
 	// Settle notifies the intention to settle an existing hold
 	// forward with a given preimage.