[fuzz] Check that channels don't get stuck in chanmon_consistency

This adds a new command string in the chanmon_consistency fuzzer
which tests that, once all pending HTLCs are settled, at least one
side of a channel can still send funds.

While this should have caught the recent(ish) spec bug where
channels could get stuck, I did not attempt to reproduce said bug
with this patch.

Author: TheBlueMatt

fuzz/src/chanmon_consistency.rs

@@ -871,6 +871,60 @@ pub fn do_test<Out: test_logger::Output>(data: &[u8], out: Out) {
 			0x5c => { send_hop_payment(&nodes[0], &nodes[1], chan_a, &nodes[2], chan_b, 1, &mut payment_id); },
 			0x5d => { send_hop_payment(&nodes[2], &nodes[1], chan_b, &nodes[0], chan_a, 1, &mut payment_id); },
 
+			0xff => {
+				// Test that no channel is in a stuck state where neither party can send funds even
+				// after we resolve all pending events.
+				// First make sure there are no pending monitor updates, resetting the error state
+				// and calling channel_monitor_updated for each monitor.
+				*monitor_a.update_ret.lock().unwrap() = Ok(());
+				*monitor_b.update_ret.lock().unwrap() = Ok(());
+				*monitor_c.update_ret.lock().unwrap() = Ok(());
+
+				if let Some((id, _)) = monitor_a.latest_monitors.lock().unwrap().get(&chan_1_funding) {
+					nodes[0].channel_monitor_updated(&chan_1_funding, *id);
+				}
+				if let Some((id, _)) = monitor_b.latest_monitors.lock().unwrap().get(&chan_1_funding) {
+					nodes[1].channel_monitor_updated(&chan_1_funding, *id);
+				}
+				if let Some((id, _)) = monitor_b.latest_monitors.lock().unwrap().get(&chan_2_funding) {
+					nodes[1].channel_monitor_updated(&chan_2_funding, *id);
+				}
+				if let Some((id, _)) = monitor_c.latest_monitors.lock().unwrap().get(&chan_2_funding) {
+					nodes[2].channel_monitor_updated(&chan_2_funding, *id);
+				}
+
+				// Next, make sure peers are all connected to each other
+				if chan_a_disconnected {
+					nodes[0].peer_connected(&nodes[1].get_our_node_id(), &Init { features: InitFeatures::empty() });
+					nodes[1].peer_connected(&nodes[0].get_our_node_id(), &Init { features: InitFeatures::empty() });
+					chan_a_disconnected = false;
+				}
+				if chan_b_disconnected {
+					nodes[1].peer_connected(&nodes[2].get_our_node_id(), &Init { features: InitFeatures::empty() });
+					nodes[2].peer_connected(&nodes[1].get_our_node_id(), &Init { features: InitFeatures::empty() });
+					chan_b_disconnected = false;
+				}
+
+				for _ in 0..13 {
+					// Then, make sure any current forwards make their way to their destination
+					process_msg_events!(0, false);
+					process_msg_events!(1, false);
+					process_msg_events!(2, false);
+					// ...making sure any pending PendingHTLCsForwardable events are handled and
+					// payments claimed.
+					process_events!(0, false);
+					process_events!(1, false);
+					process_events!(2, false);
+				}
+
+				// Finally, make sure that at least one end of each channel can make a substantial payment.
+				assert!(
+					send_payment(&nodes[0], &nodes[1], chan_a, 10_000_000, &mut payment_id) ||
+					send_payment(&nodes[1], &nodes[0], chan_a, 10_000_000, &mut payment_id));
+				assert!(
+					send_payment(&nodes[1], &nodes[2], chan_b, 10_000_000, &mut payment_id) ||
+					send_payment(&nodes[2], &nodes[1], chan_b, 10_000_000, &mut payment_id));
+			},
 			_ => test_return!(),
 		}