Check for dangling monitor events in tests

We want to make sure we don't leak any monitor events by leaving them unacked
forever.

Author: valentinewallace

lightning/src/chain/channelmonitor.rs

@@ -194,7 +194,7 @@ fn push_monitor_event(
 }
 
 /// An event to be processed by the ChannelManager.
-#[derive(Clone, PartialEq, Eq)]
+#[derive(Clone, PartialEq, Eq, Debug)]
 pub enum MonitorEvent {
 	/// A monitor event containing an HTLCUpdate.
 	HTLCEvent(HTLCUpdate),
@@ -256,7 +256,7 @@ impl_writeable_tlv_based_enum_upgradable_legacy!(MonitorEvent,
 /// Simple structure sent back by `chain::Watch` when an HTLC from a forward channel is detected on
 /// chain. Used to update the corresponding HTLC in the backward channel. Failing to pass the
 /// preimage claim backward will lead to loss of funds.
-#[derive(Clone, PartialEq, Eq)]
+#[derive(Clone, PartialEq, Eq, Debug)]
 pub struct HTLCUpdate {
 	pub(crate) payment_hash: PaymentHash,
 	pub(crate) payment_preimage: Option<PaymentPreimage>,
@@ -2272,6 +2272,16 @@ impl<Signer: EcdsaChannelSigner> ChannelMonitor<Signer> {
 		self_inner.next_monitor_event_id = next_id;
 	}
 
+	/// Used by test infra to check for monitor event leaks at the end of a test.
+	#[cfg(any(test, feature = "_test_utils"))]
+	pub fn drain_unacked_monitor_events(&self) -> Vec<(u64, MonitorEvent)> {
+		let mut inner = self.inner.lock().unwrap();
+		let mut events = Vec::new();
+		events.append(&mut inner.pending_monitor_events);
+		events.append(&mut inner.provided_monitor_events);
+		events
+	}
+
 	/// Processes [`SpendableOutputs`] events produced from each [`ChannelMonitor`] upon maturity.
 	///
 	/// For channels featuring anchor outputs, this method will also process [`BumpTransaction`]

lightning/src/ln/functional_test_utils.rs

@@ -801,6 +801,18 @@ impl<'a, 'b, 'c> Drop for Node<'a, 'b, 'c> {
 				panic!("Had excess RAA blockers on node {}: {:?}", self.logger.id, raa_blockers);
 			}
 
+			for channel_id in self.chain_monitor.chain_monitor.list_monitors() {
+				let monitor = self.chain_monitor.chain_monitor.get_monitor(channel_id).unwrap();
+				let unacked_monitor_events = monitor.drain_unacked_monitor_events();
+				if !unacked_monitor_events.is_empty() {
+					panic!(
+						"Node {} channel {channel_id:?} had {} unacked monitor events at drop: {unacked_monitor_events:#?}",
+						unacked_monitor_events.len(),
+						self.logger.id
+					);
+				}
+			}
+
 			// Check that if we serialize the network graph, we can deserialize it again.
 			let network_graph = {
 				let mut w = test_utils::TestVecWriter(Vec::new());