offer: allow offers in other currencies if we can convert.

This avoids a footgun where they create an offer then we can't create
the invoice because they don't have a converter plugin.

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

plugins/offers_offer.c

@@ -249,22 +249,57 @@ static struct command_result *param_invoice_payment_hash(struct command *cmd,
 	return NULL;
 }
 
+struct offer_info {
+	const struct tlv_offer *offer;
+	const char *label;
+	bool *single_use;
+};
+
+static struct command_result *create_offer(struct command *cmd,
+					   struct offer_info *offinfo)
+{
+	struct out_req *req;
+
+	/* We simply pass this through. */
+	req = jsonrpc_request_start(cmd->plugin, cmd, "createoffer",
+				    forward_result, forward_error,
+				    offinfo);
+	json_add_string(req->js, "bolt12",
+			offer_encode(tmpctx, offinfo->offer));
+	if (offinfo->label)
+		json_add_string(req->js, "label", offinfo->label);
+	json_add_bool(req->js, "single_use", *offinfo->single_use);
+
+	return send_outreq(cmd->plugin, req);
+}
+
+static struct command_result *currency_done(struct command *cmd,
+					    const char *buf,
+					    const jsmntok_t *result,
+					    struct offer_info *offinfo)
+{
+	/* Fail in this case, by forwarding warnings. */
+	if (!json_get_member(buf, result, "msat"))
+		return forward_error(cmd, buf, result, offinfo);
+
+	return create_offer(cmd, offinfo);
+}
+
 struct command_result *json_offer(struct command *cmd,
 				  const char *buffer,
 				  const jsmntok_t *params)
 {
-	const char *desc, *vendor, *label;
+	const char *desc, *vendor;
 	struct tlv_offer *offer;
-	struct out_req *req;
-	bool *single_use;
+	struct offer_info *offinfo = tal(cmd, struct offer_info);
 
-	offer = tlv_offer_new(cmd);
+	offinfo->offer = offer = tlv_offer_new(offinfo);
 
 	if (!param(cmd, buffer, params,
 		   p_req("amount", param_amount, offer),
 		   p_req("description", param_escaped_string, &desc),
 		   p_opt("vendor", param_escaped_string, &vendor),
-		   p_opt("label", param_escaped_string, &label),
+		   p_opt("label", param_escaped_string, &offinfo->label),
 		   p_opt("quantity_min", param_u64, &offer->quantity_min),
 		   p_opt("quantity_max", param_u64, &offer->quantity_max),
 		   p_opt("absolute_expiry", param_u64, &offer->absolute_expiry),
@@ -278,7 +313,8 @@ struct command_result *json_offer(struct command *cmd,
 		   p_opt("recurrence_limit",
 			 param_number,
 			 &offer->recurrence_limit),
-		   p_opt_def("single_use", param_bool, &single_use, false),
+		   p_opt_def("single_use", param_bool,
+			     &offinfo->single_use, false),
 		   /* FIXME: hints support! */
 		   NULL))
 		return command_param_failed();
@@ -318,16 +354,22 @@ struct command_result *json_offer(struct command *cmd,
 
 	offer->node_id = tal_dup(offer, struct pubkey32, &id);
 
-	/* We simply pass this through. */
-	req = jsonrpc_request_start(cmd->plugin, cmd, "createoffer",
-				    forward_result, forward_error,
-				    offer);
-	json_add_string(req->js, "bolt12", offer_encode(tmpctx, offer));
-	if (label)
-		json_add_string(req->js, "label", label);
-	json_add_bool(req->js, "single_use", *single_use);
+	/* If they specify a different currency, warn if we can't
+	 * convert it! */
+	if (offer->currency) {
+		struct out_req *req;
+
+		req = jsonrpc_request_start(cmd->plugin, cmd, "currencyconvert",
+					    currency_done, forward_error,
+					    offinfo);
+		json_add_u32(req->js, "amount", 1);
+		json_add_stringn(req->js, "currency",
+				 (const char *)offer->currency,
+				 tal_bytelen(offer->currency));
+		return send_outreq(cmd->plugin, req);
+	}
 
-	return send_outreq(cmd->plugin, req);
+	return create_offer(cmd, offinfo);
 }
 
 struct command_result *json_offerout(struct command *cmd,

tests/plugins/currencyUSDAUD5000.py

@@ -0,0 +1,18 @@
+#!/usr/bin/env python3
+"""
+This plugin is used to test the currency command
+"""
+from pyln.client import Plugin, Millisatoshi
+
+plugin = Plugin()
+
+
+@plugin.method("currencyconvert")
+def currencyconvert(plugin, amount, currency):
+    """Converts currency using given APIs."""
+    if currency in ('USD', 'AUD'):
+        return {"msat": Millisatoshi(round(amount * 5000))}
+    raise Exception("No values available for currency {}".format(currency.upper()))
+
+
+plugin.run()

tests/test_pay.py

@@ -3691,7 +3691,8 @@ def test_mpp_overload_payee(node_factory, bitcoind):
 
 @unittest.skipIf(not EXPERIMENTAL_FEATURES, "offers are experimental")
 def test_offer(node_factory, bitcoind):
-    l1 = node_factory.get_node()
+    plugin = os.path.join(os.path.dirname(__file__), 'plugins/currencyUSDAUD5000.py')
+    l1 = node_factory.get_node(options={'plugin': plugin})
 
     bolt12tool = os.path.join(os.path.dirname(__file__), "..", "devtools", "bolt12-cli")
     # Try different amount strings
@@ -3719,6 +3720,11 @@ def test_offer(node_factory, bitcoind):
         l1.rpc.call('offer', {'amount': '1.1AUD',
                               'description': 'test for invalid amount'})
 
+    # Make sure it fails on unknown currencies.
+    with pytest.raises(RpcError, match='No values available for currency EUR'):
+        l1.rpc.call('offer', {'amount': '1.00EUR',
+                              'description': 'test for unknown currency'})
+
     # Test label and description
     weird_label = 'label \\ " \t \n'
     weird_desc = 'description \\ " \t \n ナンセンス 1杯'