xpay: don't place global reservations on generated channels.

We generate fake scids for routehints and blinded paths.  But then we were
placing reservations on them as if they were global.  If there are two xpays
going at once these reservations will clash, even though the same scid refers
to different channels.

Reported-by: @Lagrang3
Changelog-Fixed: xpay: fixed theoretical clash with simultanous payments via routehints and blinded paths.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Author: rustyrussell

plugins/xpay/xpay.c

@@ -145,6 +145,8 @@ struct hop {
 	u32 cltv_value_in;
 	/* This is the delay, out from node. */
 	u32 cltv_value_out;
+	/* This is a fake channel. */
+	bool fake_channel;
 };
 
 /* Each actual payment attempt */
@@ -478,18 +480,6 @@ static void payment_give_up(struct command *aux_cmd,
 		cleanup(aux_cmd, payment);
 }
 
-/* We usually add things we learned to the global layer, but not
- * if it's a fake channel */
-static const char *layer_of(const struct payment *payment,
-			    const struct short_channel_id_dir *scidd)
-{
-	struct gossmap *gossmap = get_gossmap(xpay_of(payment->plugin));
-
-	if (gossmap_find_chan(gossmap, &scidd->scid))
-		return "xpay";
-	return payment->private_layer;
-}
-
 static void add_result_summary(struct attempt *attempt,
 			       enum log_level level,
 			       const char *fmt, ...)
@@ -697,8 +687,11 @@ static void update_knowledge_from_error(struct command *aux_cmd,
 	/* We learned something about prior nodes */
 	for (size_t i = 0; i < index; i++) {
 		req = payment_ignored_req(aux_cmd, attempt, "askrene-inform-channel");
+		/* Put what we learned in xpay, unless it's a fake channel */
 		json_add_string(req->js, "layer",
-				layer_of(attempt->payment, &attempt->hops[i].scidd));
+				attempt->hops[i].fake_channel
+				? attempt->payment->private_layer
+				: "xpay");
 		json_add_short_channel_id_dir(req->js,
 					      "short_channel_id_dir",
 					      attempt->hops[i].scidd);
@@ -855,8 +848,11 @@ static void update_knowledge_from_error(struct command *aux_cmd,
 
 channel_capacity:
 	req = payment_ignored_req(aux_cmd, attempt, "askrene-inform-channel");
+	/* Put what we learned in xpay, unless it's a fake channel */
 	json_add_string(req->js, "layer",
-			layer_of(attempt->payment, &attempt->hops[index].scidd));
+			attempt->hops[index].fake_channel
+			? attempt->payment->private_layer
+			: "xpay");
 	json_add_short_channel_id_dir(req->js,
 				      "short_channel_id_dir",
 				      attempt->hops[index].scidd);
@@ -892,6 +888,8 @@ static struct command_result *unreserve_path(struct command *aux_cmd,
 		json_object_start(req->js, NULL);
 		json_add_short_channel_id_dir(req->js, "short_channel_id_dir", hop->scidd);
 		json_add_amount_msat(req->js, "amount_msat", hop->amount_out);
+		if (hop->fake_channel)
+			json_add_string(req->js, "layer", attempt->payment->private_layer);
 		json_object_end(req->js);
 	}
 	json_array_end(req->js);
@@ -1177,6 +1175,7 @@ static struct command_result *getroutes_done(struct command *aux_cmd,
 	const jsmntok_t *t, *routes;
 	size_t i;
 	struct amount_msat needs_routing, was_routing;
+	struct gossmap *gossmap = get_gossmap(xpay_of(payment->plugin));
 
 	payment_log(payment, LOG_DBG, "getroutes_done: %s",
 		    payment->cmd ? "continuing" : "ignoring");
@@ -1244,6 +1243,7 @@ static struct command_result *getroutes_done(struct command *aux_cmd,
 			if (err)
 				plugin_err(aux_cmd->plugin, "Malformed routes: %s",
 					   err);
+			hop->fake_channel = !gossmap_find_chan(gossmap, &hop->scidd.scid);
 			if (j > 0) {
 				hops[j-1].amount_out = hop->amount_in;
 				hops[j-1].cltv_value_out = hop->cltv_value_in;
@@ -1268,6 +1268,8 @@ static struct command_result *getroutes_done(struct command *aux_cmd,
 			json_add_short_channel_id_dir(req->js, "short_channel_id_dir",
 						      hop->scidd);
 			json_add_amount_msat(req->js, "amount_msat", hop->amount_out);
+			if (hop->fake_channel)
+				json_add_string(req->js, "layer", payment->private_layer);
 			json_object_end(req->js);
 		}
 		json_array_end(req->js);