offers: honor payment-fronting-nodes when creating invoices.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Author: rustyrussell

plugins/offers_invreq_hook.c

@@ -274,6 +274,12 @@ static struct command_result *found_best_peer(struct command *cmd,
 	 *   for each `blinded_path` in `paths`, in order.
 	 */
 	if (!best) {
+		/* Don't allow bare invoices if they explicitly told us to front */
+		if (od->fronting_nodes) {
+			return fail_invreq(cmd, ir,
+					   "Could not find path from payment-fronting-node");
+		}
+
 		/* Note: since we don't make one, createinvoice adds a dummy. */
 		plugin_log(cmd->plugin, LOG_UNUSUAL,
 			   "No incoming channel for %s, so no blinded path",
@@ -384,6 +390,8 @@ static struct command_result *found_best_peer(struct command *cmd,
 static struct command_result *add_blindedpaths(struct command *cmd,
 					       struct invreq *ir)
 {
+	const struct offers_data *od = get_offers_data(cmd->plugin);
+
 	if (!we_want_blinded_path(cmd->plugin, true))
 		return create_invoicereq(cmd, ir);
 
@@ -393,7 +401,7 @@ static struct command_result *add_blindedpaths(struct command *cmd,
 	 * us onion messaging. */
 	return find_best_peer(cmd,
 			      (1ULL << OPT_ROUTE_BLINDING) | (1ULL << OPT_ONION_MESSAGES),
-			      NULL, found_best_peer, ir);
+			      od->fronting_nodes, found_best_peer, ir);
 }
 
 static struct command_result *cancel_invoice(struct command *cmd,

tests/test_invoices.py

@@ -976,8 +976,15 @@ def test_payment_fronting(node_factory):
     l4offer = l4.rpc.offer(1000, 'l4offer', 'l4offer')['bolt12']
     assert [r['first_node_id'] for r in l4.rpc.decode(l4offer)['offer_paths']] == [l1.info['id'], l2.info['id']]
 
-    l1.rpc.fetchinvoice(l3offer)['invoice']
-    l1.rpc.fetchinvoice(l4offer)['invoice']
+    l3invb12 = l1.rpc.fetchinvoice(l3offer)['invoice']
+    l4invb12 = l1.rpc.fetchinvoice(l4offer)['invoice']
+
+    assert only_one(l3.rpc.decode(l3invb12)['invoice_paths'])['first_node_id'] == l1.info['id']
+    # Given multiple, it will pick one.
+    assert only_one(l3.rpc.decode(l4invb12)['invoice_paths'])['first_node_id'] in (l1.info['id'], l2.info['id'])
+
+    l1.rpc.xpay(l3invb12)
+    l1.rpc.xpay(l4invb12)
 
 
 def test_invoice_maxdesc(node_factory, chainparams):