listoffers_done: Fix invoice_node_id so it is set to the final

blinded_node_id when offer_issuer_id is not present and offers_path is
present.

Author: 21M4TW

plugins/fetchinvoice.c

@@ -240,11 +240,6 @@ static struct command_result *handle_invreq_response(struct command *cmd,
 		badfield = "invoice_payment_hash";
 		goto badinv;
 	}
-	/* Currently the following check fails for offers that do not contain
-	 * offer_issuer_id and for which an invoice is fetched from a CLN node,
-	 * because listoffers_done in offers_invrek_hook.c sets invoice_node_id
-	 * to offer_issuer_id regardless.
-	 */
 	if (!inv->invoice_node_id) {
 		badfield = "invoice_node_id";
 		goto badinv;

plugins/offers_invreq_hook.c

@@ -931,16 +931,15 @@ static struct command_result *listoffers_done(struct command *cmd,
 	assert(ir->inv->invreq_payer_id);
 
 	/* BOLT #12:
-	 *   - if `offer_issuer_id` is present:
+	 *   - if `offer_issuer_id` is present or if `offer_paths` is not present:
 	 *     - MUST set `invoice_node_id` to the `offer_issuer_id`
+	 *   - else:
+	 *     - MUST set `invoice_node_id` to the final `blinded_node_id`
 	 */
-	/* FIXME: We always provide an offer_issuer_id! */
-	/* The following line is wrong when offer_issuer_id is not used.
-	 * According to fetchinvoice.c, it should be "equal to the final
-	 * `blinded_node_id` it [the payer] sent the invoice request to", when
-	 * offer_issuer_id is not set and offer_paths is set
-	 */
-	ir->inv->invoice_node_id = ir->inv->offer_issuer_id;
+	if(!ir->inv->offer_issuer_id && ir->invreq->offer_paths) {
+		ir->inv->invoice_node_id = &(*ir->invreq->offer_paths)->path[tal_count((*ir->invreq->offer_paths)->path)-1]->blinded_node_id;
+
+	} else ir->inv->invoice_node_id = ir->inv->offer_issuer_id;
 
 	/* BOLT #12:
 	 * - MUST set `invoice_created_at` to the number of seconds since