-Fixing offers when offer_paths are present and offer_issuer_id is no

longer needed.
-There is a missing piece however in plugins/offers_invreq_hook.c, which
causes an error in plugins/fetchinvoice.c when trying to fetch an
invoice for an offer without offer_issuer _id. Comments were added to
describe the remaining issue.

Author: 21M4TW

plugins/fetchinvoice.c

@@ -231,6 +231,11 @@ 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

@@ -993,6 +993,11 @@ static struct command_result *listoffers_done(struct command *cmd,
 	 *     - MUST set `invoice_node_id` to the `offer_issuer_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;
 
 	/* BOLT #12:

plugins/offers_offer.c

@@ -269,6 +269,12 @@ static struct command_result *found_best_peer(struct command *cmd,
 		struct secret blinding_path_secret;
 		struct sha256 offer_id;
 
+		/* offer_issuer_id is not needed when offer_paths are used.
+		 * The following line seems to produce a valid offer with
+		 * offer_issuer_id removed.
+		 */
+		offinfo->offer->offer_issuer_id = NULL;
+
 		/* Note: "id" of offer minus paths */
 		offer_offer_id(offinfo->offer, &offer_id);