htlcswitch+lnrpc: report node-ID next hop to the off-chain HTLC interceptor

When the switch forwards a blinded hop identified by node ID, it has not
yet resolved a concrete outgoing channel at interception time. Expose the
next hop to the interceptor: InterceptedForward.Packet() reports the
packet's outgoing channel as-is (hop.Exit, since none is selected yet) and
carries the requested pubkey in OutgoingNodeID.

At the RPC boundary, forwardInterceptor.onIntercept maps a node-ID hop to
the reserved NodeIDForwardSCID sentinel in outgoing_requested_chan_id and
the pubkey in outgoing_requested_node_id, so a client switching on a zero
channel ID to detect the exit hop does not misread the forward as a final
receive. The sentinel is a wire-only concern, applied where the request is
built rather than in the switch's internal InterceptedPacket, which stays
truthful (OutgoingNodeID.IsSome() is the node-ID discriminator).

(cherry picked from commit 32373b76c7)

htlcswitch/interceptable_switch.go

@@ -705,6 +705,7 @@ func (f *interceptedForward) Packet() InterceptedPacket {
 			HtlcID: f.packet.incomingHTLCID,
 		},
 		OutgoingChanID:       f.packet.outgoingChanID,
+		OutgoingNodeID:       f.packet.outgoingHop.RightToSome(),
 		Hash:                 f.htlc.PaymentHash,
 		OutgoingExpiry:       f.htlc.Expiry,
 		OutgoingAmount:       f.htlc.Amount,

htlcswitch/interfaces.go

@@ -381,6 +381,14 @@ type InterceptableHtlcForwarder interface {
 // and resolve it later or let the switch execute its default behavior.
 type ForwardInterceptor func(InterceptedPacket) error
 
+// NodeIDForwardSCID is the sentinel outgoing SCID reported to HTLC interceptor
+// clients (at the RPC boundary) for a next hop identified by node ID (BOLT 4
+// next_node_id) rather than by channel. All bits are set, an out-of-range value
+// that can never match a real or alias channel, so a client switching on a zero
+// SCID to detect the exit hop does not read the forward as a final receive. The
+// pubkey is in InterceptedPacket.OutgoingNodeID.
+const NodeIDForwardSCID uint64 = ^uint64(0)
+
 // InterceptedPacket contains the relevant information for the interceptor about
 // an HTLC.
 type InterceptedPacket struct {
@@ -388,9 +396,17 @@ type InterceptedPacket struct {
 	// packet.
 	IncomingCircuit models.CircuitKey
 
-	// OutgoingChanID is the destination channel for this packet.
+	// OutgoingChanID is the destination channel for this packet. For a
+	// node-ID next hop with no concrete channel known yet it is hop.Exit
+	// and OutgoingNodeID holds the pubkey; the RPC layer maps that to the
+	// NodeIDForwardSCID sentinel before reporting it to a client.
 	OutgoingChanID lnwire.ShortChannelID
 
+	// OutgoingNodeID is the next hop's compressed pubkey for a blinded
+	// route that identifies it by node ID (next_node_id). None in the
+	// common channel-ID case.
+	OutgoingNodeID fn.Option[[33]byte]
+
 	// Hash is the payment hash of the htlc.
 	Hash lntypes.Hash
 

lnrpc/routerrpc/forward_interceptor.go

@@ -100,6 +100,17 @@ func (r *forwardInterceptor) onIntercept(
 		InWireCustomRecords:     htlc.InWireCustomRecords,
 	}
 
+	// A node-ID forward has no requested outgoing channel. Expose the
+	// requested pubkey and report the reserved NodeIDForwardSCID sentinel
+	// rather than a zero SCID. Older un-upgraded protobuf clients do not
+	// know about outgoing_requested_node_id and would otherwise interpret
+	// a zero SCID as an exit hop.
+	htlc.OutgoingNodeID.WhenSome(func(nodeID [33]byte) {
+		interceptionRequest.OutgoingRequestedNodeId = nodeID[:]
+		interceptionRequest.OutgoingRequestedChanId =
+			htlcswitch.NodeIDForwardSCID
+	})
+
 	return r.stream.Send(interceptionRequest)
 }