offers: add self-fetchinvoices for offers, and self-pay for the resul…

…ting invoice.

Changelog-Added: offers: we can now self-fetch and self-pay BOLT12 offers and invoices.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

plugins/establish_onion_path.c

@@ -215,6 +215,12 @@ struct command_result *establish_onion_path_(struct command *cmd,
 	struct connect_info *ci = tal(cmd, struct connect_info);
 	struct out_req *req;
 
+	/* Talking to ourselves is trivial. */
+	if (pubkey_eq(local_id, dst)) {
+		struct pubkey *path = tal_dup_arr(tmpctx, struct pubkey, local_id, 1, 0);
+		return success(cmd, path, arg);
+	}
+
 	ci->local_id = *local_id;
 	ci->dst = *dst;
 	ci->cb = success;

plugins/pay.c

@@ -1070,8 +1070,18 @@ decrypt_done(struct command *cmd,
 				    tal_hex(tmpctx, encdata));
 	}
 
-	if (tal_count(p->blindedpath->path) == 1)
-		return command_fail(cmd, LIGHTNINGD, "FIXME: self-pay!");
+	/* Was this a self-pay?  Simply remove blinded path. */
+	if (tal_count(p->blindedpath->path) == 1) {
+		p->blindedpath = tal_free(p->blindedpath);
+		tal_free(p->pay_destination);
+		p->pay_destination = tal_dup(p, struct node_id, p->route_destination);
+		/* self-pay will want secret from inside TLV */
+		if (tal_bytelen(enctlv->path_id) == sizeof(*p->payment_secret)) {
+			p->payment_secret = tal(p, struct secret);
+			memcpy(p->payment_secret, enctlv->path_id, sizeof(struct secret));
+		}
+		return start_payment(cmd, p);
+	}
 
 	if (!enctlv->short_channel_id && !enctlv->next_node_id) {
 		return command_fail(cmd, PAY_UNPARSEABLE_ONION,

tests/test_pay.py

@@ -5808,6 +5808,15 @@ def test_offer_path_self(node_factory):
     l2.rpc.pay(inv)
 
 
+def test_offer_selfpay(node_factory):
+    """We can fetch an pay our own offer"""
+    l1 = node_factory.get_node(options={'experimental-offers': None})
+
+    offer = l1.rpc.offer(amount='2msat', description='test_offer_path_self')['bolt12']
+    inv = l1.rpc.fetchinvoice(offer)['invoice']
+    l1.rpc.pay(inv)
+
+
 def test_decryptencrypteddata(node_factory):
     l1, l2, l3 = node_factory.line_graph(3, fundchannel=False,
                                          opts={'experimental-offers': None})