Add a relatively simple test of the legacy closing fee negotiation

This doesn't exhaustively test closing fee negotiation at all, but
ensures that it is at least basically able to come to consensus and
sign cooperative closing transactions.

Author: TheBlueMatt

lightning/src/ln/shutdown_tests.rs

@@ -834,3 +834,48 @@ fn test_closing_signed_reinit_timeout() {
 	do_test_closing_signed_reinit_timeout(TimeoutStep::AfterClosingSigned);
 	do_test_closing_signed_reinit_timeout(TimeoutStep::NoTimeout);
 }
+
+fn do_simple_legacy_shutdown_test(high_initiator_fee: bool) {
+	// A simpe test of the legacy shutdown fee negotiation logic.
+	let chanmon_cfgs = create_chanmon_cfgs(2);
+	let node_cfgs = create_node_cfgs(2, &chanmon_cfgs);
+	let node_chanmgrs = create_node_chanmgrs(2, &node_cfgs, &[None, None]);
+	let nodes = create_network(2, &node_cfgs, &node_chanmgrs);
+
+	let chan = create_announced_chan_between_nodes(&nodes, 0, 1, InitFeatures::known(), InitFeatures::known());
+
+	if high_initiator_fee {
+		// If high_initiator_fee is set, set nodes[0]'s feerate significantly higher. This
+		// shouldn't impact the flow at all given nodes[1] will happily accept the higher fee.
+		let mut feerate_lock = chanmon_cfgs[0].fee_estimator.sat_per_kw.lock().unwrap();
+		*feerate_lock *= 10;
+	}
+
+	nodes[0].node.close_channel(&OutPoint { txid: chan.3.txid(), index: 0 }.to_channel_id()).unwrap();
+	let node_0_shutdown = get_event_msg!(nodes[0], MessageSendEvent::SendShutdown, nodes[1].node.get_our_node_id());
+	nodes[1].node.handle_shutdown(&nodes[0].node.get_our_node_id(), &InitFeatures::known(), &node_0_shutdown);
+	let node_1_shutdown = get_event_msg!(nodes[1], MessageSendEvent::SendShutdown, nodes[0].node.get_our_node_id());
+	nodes[0].node.handle_shutdown(&nodes[1].node.get_our_node_id(), &InitFeatures::known(), &node_1_shutdown);
+
+	let mut node_0_closing_signed = get_event_msg!(nodes[0], MessageSendEvent::SendClosingSigned, nodes[1].node.get_our_node_id());
+	node_0_closing_signed.fee_range = None;
+	if high_initiator_fee {
+		assert!(node_0_closing_signed.fee_satoshis > 500);
+	} else {
+		assert!(node_0_closing_signed.fee_satoshis < 500);
+	}
+
+	nodes[1].node.handle_closing_signed(&nodes[0].node.get_our_node_id(), &node_0_closing_signed);
+	let (_, mut node_1_closing_signed) = get_closing_signed_broadcast!(nodes[1].node, nodes[0].node.get_our_node_id());
+	node_1_closing_signed.as_mut().unwrap().fee_range = None;
+
+	nodes[0].node.handle_closing_signed(&nodes[1].node.get_our_node_id(), &node_1_closing_signed.unwrap());
+	let (_, node_0_none) = get_closing_signed_broadcast!(nodes[0].node, nodes[1].node.get_our_node_id());
+	assert!(node_0_none.is_none());
+}
+
+#[test]
+fn simple_legacy_shutdown_test() {
+	do_simple_legacy_shutdown_test(false);
+	do_simple_legacy_shutdown_test(true);
+}