configure: make partid payments only available with EXPERIMENTAL_FEATURES and payment_secret

Explicit #if EXPERIMENTAL_FEATURES check in case we enable them at different
times, but it requires a payment_secret since we put them in the same field.

This incidently stops it working on legacy nodes.

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

lightningd/pay.c

@@ -978,6 +978,11 @@ send_payment(struct lightningd *ld,
 	if (!final_tlv && payment_secret)
 		final_tlv = true;
 
+	/* Parallel payments are invalid for legacy. */
+	if (partid && !final_tlv)
+		return command_fail(cmd, PAY_DESTINATION_PERM_FAIL,
+				    "Cannot do parallel payments to legacy node");
+
 	onion = onion_final_hop(cmd,
 				final_tlv,
 				route[i].amount,
@@ -1306,6 +1311,10 @@ static struct command_result *json_sendpay(struct command *cmd,
 	}
 #endif
 
+	if (*partid && !payment_secret)
+		return command_fail(cmd, JSONRPC2_INVALID_PARAMS,
+				    "partid requires payment_secret");
+
 	return send_payment(cmd->ld, cmd, rhash, *partid,
 			    route,
 			    route[routetok->size-1].amount,

tests/test_pay.py

@@ -2570,6 +2570,7 @@ def test_sendonion_rpc(node_factory):
 
 
 @unittest.skipIf(not DEVELOPER, "needs DEVELOPER=1")
+@unittest.skipIf(not EXPERIMENTAL_FEATURES, "needs partid support")
 def test_partial_payment(node_factory, bitcoind, executor):
     # We want to test two payments at the same time, before we send commit
     l1, l2, l3, l4 = node_factory.get_nodes(4, [{}] + [{'disconnect': ['=WIRE_UPDATE_ADD_HTLC-nocommit']}] * 2 + [{}])