keysend: Check that the destination supports keysend upon init

We were blindly initiating the keysend payment, which could lead to
confusing outcomes. This adds a very specific error message to the
error returned.

Changelog-Fixed: keysend: Keysend now checks whether the destination supports keysend before attempting a payment. If not a more informative error is returned.

plugins/keysend.c

@@ -1,6 +1,7 @@
 #include <bitcoin/preimage.h>
 #include <ccan/array_size/array_size.h>
 #include <ccan/tal/str/str.h>
+#include <common/gossmap.h>
 #include <plugins/libplugin-pay.h>
 #include <plugins/libplugin.h>
 #include <wire/onion_wire.h>
@@ -54,6 +55,24 @@ static struct keysend_data *keysend_init(struct payment *p)
 static void keysend_cb(struct keysend_data *d, struct payment *p) {
 	struct createonion_hop *last_payload;
 	size_t hopcount;
+	struct gossmap_node *node;
+	bool enabled;
+	const struct gossmap *gossmap = get_gossmap(p->plugin);
+
+	/* On the root payment we perform the featurebit check. */
+	if (p->parent == NULL && p->step == PAYMENT_STEP_INITIALIZED) {
+		node = gossmap_find_node(gossmap, p->destination);
+
+		enabled = gossmap_node_get_feature(gossmap, node,
+						   KEYSEND_FEATUREBIT) != -1;
+		if (!enabled)
+			return payment_fail(
+			    p,
+			    "Recipient %s does not support keysend payments "
+			    "(feature bit %d missing in node announcement)",
+			    node_id_to_hexstr(tmpctx, p->destination),
+			    KEYSEND_FEATUREBIT);
+	}
 
 	if (p->step != PAYMENT_STEP_ONION_PAYLOAD)
 		return payment_continue(p);

tests/test_pay.py

@@ -3373,7 +3373,7 @@ def test_listpay_result_with_paymod(node_factory, bitcoind):
 
     amount_sat = 10 ** 6
 
-    l1, l2, l3 = node_factory.line_graph(3)
+    l1, l2, l3 = node_factory.line_graph(3, wait_for_announce=True)
 
     invl2 = l2.rpc.invoice(amount_sat * 2, "inv_l2", "inv_l2")
     l1.rpc.pay(invl2['bolt11'])