Track recent monitor updates in TestChainMonitor

And log them in check_added_monitors if it fails.

Author: valentinewallace

lightning/src/ln/functional_test_utils.rs

@@ -1274,7 +1274,20 @@ pub fn check_added_monitors<CM: AChannelManager, H: NodeHolder<CM = CM>>(node: &
 	if let Some(chain_monitor) = node.chain_monitor() {
 		let mut added_monitors = chain_monitor.added_monitors.lock().unwrap();
 		let n = added_monitors.len();
-		assert_eq!(n, count, "expected {} monitors to be added, not {}", count, n);
+		if n != count {
+			let recent = chain_monitor.recent_monitor_updates.lock().unwrap();
+			let mut desc = String::new();
+			for (i, (chan_id, update)) in recent.iter().take(n).enumerate() {
+				desc += &format!(
+					"\n  [{}] chan={} update_id={} steps={:?}",
+					i, chan_id, update.update_id, update.updates
+				);
+			}
+			panic!(
+				"expected {} monitors to be added, not {}. Last {} updates (most recent first):{}",
+				count, n, n, desc
+			);
+		}
 		added_monitors.clear();
 	}
 }

lightning/src/util/test_utils.rs

@@ -521,6 +521,8 @@ pub struct TestChainMonitor<'a> {
 	/// deferred operations. This allows tests to control exactly when queued monitor updates
 	/// are applied to the in-memory monitor.
 	pub pause_flush: AtomicBool,
+	/// Buffer of the last 20 monitor updates, most recent first.
+	pub recent_monitor_updates: Mutex<Vec<(ChannelId, ChannelMonitorUpdate)>>,
 }
 impl<'a> TestChainMonitor<'a> {
 	pub fn new(
@@ -581,6 +583,7 @@ impl<'a> TestChainMonitor<'a> {
 			#[cfg(feature = "std")]
 			write_blocker: Mutex::new(None),
 			pause_flush: AtomicBool::new(false),
+			recent_monitor_updates: Mutex::new(Vec::new()),
 		}
 	}
 
@@ -679,6 +682,12 @@ impl<'a> chain::Watch<TestChannelSigner> for TestChainMonitor<'a> {
 			.or_insert(Vec::new())
 			.push(update.clone());
 
+		{
+			let mut recent = self.recent_monitor_updates.lock().unwrap();
+			recent.insert(0, (channel_id, update.clone()));
+			recent.truncate(20);
+		}
+
 		if let Some(exp) = self.expect_channel_force_closed.lock().unwrap().take() {
 			assert_eq!(channel_id, exp.0);
 			assert_eq!(update.updates.len(), 1);