offers: don't send blinded path to neighbor for *invoices*.

In 6e4ff6a ("offers: add a blinded path
if we have no advertized address") we were overzealous, and set blinded
paths not just for offers and invoicerequests, but for invoices themselves.

This has revealed various interop issues (which is great, but not good
for our users!) so we should disable that.  It also reduces the reliability
of payments in general.

Changelog-None: fixes previously overzealous addition
Signed-off-by: Rusty Russell <[email protected]>

Author: rustyrussell

plugins/offers.c

@@ -67,7 +67,7 @@ struct gossmap *get_gossmap(struct plugin *plugin)
  *     - MUST include `offer_paths` containing one or more paths to the node
  *       from publicly reachable nodes.
  */
-bool we_want_blinded_path(struct plugin *plugin)
+bool we_want_blinded_path(struct plugin *plugin, bool for_payment)
 {
 	struct node_id local_nodeid;
 	const struct gossmap_node *node;
@@ -87,7 +87,11 @@ bool we_want_blinded_path(struct plugin *plugin)
 		return true;
 
 	/* Matt Corallo also suggests we do this (for now) if we don't
-	 * advertize an address to connect to. */
+	 * advertize an address to connect to, so they can fetch the
+	 * invoice for the offer, or send the invoice for the invoicerequest.
+	 * For actual payments they can use any route. */
+	if (for_payment)
+		return false;
 
 	/* We expect to know our own node announcements, but just in case. */
 	nannounce = gossmap_node_get_announce(tmpctx, gossmap, node);

plugins/offers.h

@@ -93,5 +93,5 @@ struct command_result *find_best_peer_(struct command *cmd,
 			(arg))
 
 /* Do we want a blinded path from a peer? */
-bool we_want_blinded_path(struct plugin *plugin);
+bool we_want_blinded_path(struct plugin *plugin, bool for_payment);
 #endif /* LIGHTNING_PLUGINS_OFFERS_H */

plugins/offers_invreq_hook.c

@@ -352,7 +352,7 @@ static struct command_result *found_best_peer(struct command *cmd,
 static struct command_result *add_blindedpaths(struct command *cmd,
 					       struct invreq *ir)
 {
-	if (!we_want_blinded_path(cmd->plugin))
+	if (!we_want_blinded_path(cmd->plugin, true))
 		return create_invoicereq(cmd, ir);
 
 	return find_best_peer(cmd, OPT_ROUTE_BLINDING,

plugins/offers_offer.c

@@ -329,7 +329,7 @@ static struct command_result *maybe_add_path(struct command *cmd,
 	 *       publicly reachable nodes.
 	 */
 	if (!offinfo->offer->offer_paths) {
-		if (we_want_blinded_path(cmd->plugin))
+		if (we_want_blinded_path(cmd->plugin, false))
 			return find_best_peer(cmd, OPT_ONION_MESSAGES,
 					      found_best_peer, offinfo);
 	}
@@ -726,9 +726,9 @@ struct command_result *json_invoicerequest(struct command *cmd,
 	 *   - MUST set `invreq_features`.`features` to the bitmap of features.
 	 */
 
-	/* FIXME: We only set blinded path if private, we should allow
+	/* FIXME: We only set blinded path if private/noaddr, we should allow
 	 * setting otherwise! */
-	if (we_want_blinded_path(cmd->plugin)) {
+	if (we_want_blinded_path(cmd->plugin, false)) {
 		struct invrequest_data *idata = tal(cmd, struct invrequest_data);
 		idata->invreq = invreq;
 		idata->single_use = *single_use;

tests/test_xpay.py

@@ -789,11 +789,17 @@ def test_xpay_twohop_bug(node_factory):
    "status": "complete"
 }
     """
-    l1, l2, l3, l4 = node_factory.line_graph(4, wait_for_announce=True,
-                                             opts=[{'cltv-delta': 50},
-                                                   {'cltv-delta': 100},
-                                                   {'cltv-delta': 200},
-                                                   {'cltv-final': 400}])
+    l1, l2, l3 = node_factory.line_graph(3, wait_for_announce=True,
+                                         opts=[{'cltv-delta': 50},
+                                               {'cltv-delta': 100},
+                                               {'cltv-delta': 200}])
+
+    # Connect l3->l4 via private channel, to force blinded path
+    l4 = node_factory.get_node(options={'cltv-final': 400})
+    node_factory.join_nodes([l3, l4], announce_channels=False)
+
+    # Make sure l4 sees all the gossip
+    wait_for(lambda: len(l4.rpc.listchannels()['channels']) == 2 * 2)
 
     offer = l4.rpc.offer('any')
     inv = l1.rpc.fetchinvoice(offer['bolt12'], '15000msat')['invoice']
@@ -806,10 +812,10 @@ def test_xpay_twohop_bug(node_factory):
 
     # This works.
     l1.rpc.pay(inv)
-    # CLTV is blockheight (108) + 1 + 100 + 200 + 400
-    l1.daemon.wait_for_log(f'Adding HTLC 0 amount=15002msat cltv={108 + 1 + 100 + 200 + 400}')
+    # CLTV is blockheight (110) + 1 + 100 + 200 + 400
+    l1.daemon.wait_for_log(f'Adding HTLC 0 amount=15002msat cltv={110 + 1 + 100 + 200 + 400}')
 
     inv = l1.rpc.fetchinvoice(offer['bolt12'], '15000msat')['invoice']
     # This doesn't!
     l1.rpc.xpay(inv)
-    l1.daemon.wait_for_log(f'Adding HTLC 1 amount=15002msat cltv={108 + 1 + 100 + 200 + 400}')
+    l1.daemon.wait_for_log(f'Adding HTLC 1 amount=15002msat cltv={110 + 1 + 100 + 200 + 400}')