Add deferred monitor write support to chanmon_consistency fuzz target

Add a deferred flag to TestChainMonitor that controls whether the
underlying ChainMonitor queues operations instead of executing them
immediately. The flag is derived from the first fuzz input byte so each
of the three nodes can independently run in deferred or immediate mode,
while the same byte also selects the channel type.

In deferred mode, watch_channel and update_channel always return
InProgress. A new flush_and_update_latest_monitors method drains the
queued operations and, when the persister reports Completed, promotes
the pending shadow monitor snapshots to persisted state. This method is
called before release_pending_monitor_events, at each point where the
fuzzer completes pending monitor updates, and after watch_channel during
node reload so the node starts from a consistent state.

The first-byte config layout now uses bits 0 to 2 for initial monitor
status, bits 3 to 4 for channel type, and bits 5 to 7 for deferred
monitor writes. This fixes the rebase conflict where deferred mode
still referenced the pre-channel-type bit layout.

AI tools were used in preparing this commit.

Author: joostjager

fuzz/src/chanmon_consistency.rs

@@ -267,12 +267,13 @@ struct TestChainMonitor {
 			Arc<KeyProvider>,
 		>,
 	>,
+	pub deferred: bool,
 	pub latest_monitors: Mutex<HashMap<ChannelId, LatestMonitorState>>,
 }
 impl TestChainMonitor {
 	pub fn new(
 		broadcaster: Arc<TestBroadcaster>, logger: Arc<dyn Logger>, feeest: Arc<FuzzEstimator>,
-		persister: Arc<TestPersister>, keys: Arc<KeyProvider>,
+		persister: Arc<TestPersister>, keys: Arc<KeyProvider>, deferred: bool,
 	) -> Self {
 		Self {
 			chain_monitor: Arc::new(chainmonitor::ChainMonitor::new(
@@ -283,14 +284,44 @@ impl TestChainMonitor {
 				Arc::clone(&persister),
 				Arc::clone(&keys),
 				keys.get_peer_storage_key(),
-				false,
+				deferred,
 			)),
 			logger,
 			keys,
 			persister,
+			deferred,
 			latest_monitors: Mutex::new(new_hash_map()),
 		}
 	}
+
+	/// Flushes all deferred monitor operations and, if the persister reports success, promotes
+	/// pending monitor states to persisted in our shadow records. `TestChainMonitor` maintains
+	/// its own `latest_monitors` map that tracks serialized monitor snapshots independently of
+	/// `ChainMonitor`, so that the fuzzer can simulate node restarts by deserializing from these
+	/// snapshots rather than relying on the persister's storage.
+	///
+	/// This simulates the pattern of snapshotting the pending count, persisting the
+	/// `ChannelManager`, then flushing the queued monitor writes.
+	fn flush_and_update_latest_monitors(&self) {
+		let count = self.chain_monitor.pending_operation_count();
+		if count == 0 {
+			return;
+		}
+		// Execute all queued watch_channel/update_channel operations inside the ChainMonitor.
+		self.chain_monitor.flush(count, &self.logger);
+		let persister_res = *self.persister.update_ret.lock().unwrap();
+		// Only update our local tracking state when the persister signals completion. When
+		// persistence is still in-progress, the monitors stay in the pending set so that a
+		// simulated restart can still reload from the last fully-persisted snapshot.
+		if persister_res == chain::ChannelMonitorUpdateStatus::Completed {
+			for (_channel_id, state) in self.latest_monitors.lock().unwrap().iter_mut() {
+				if let Some((id, data)) = state.pending_monitors.drain(..).last() {
+					state.persisted_monitor_id = id;
+					state.persisted_monitor = data;
+				}
+			}
+		}
+	}
 }
 impl chain::Watch<TestChannelSigner> for TestChainMonitor {
 	fn watch_channel(
@@ -300,6 +331,9 @@ impl chain::Watch<TestChannelSigner> for TestChainMonitor {
 		monitor.write(&mut ser).unwrap();
 		let monitor_id = monitor.get_latest_update_id();
 		let res = self.chain_monitor.watch_channel(channel_id, monitor);
+		if self.deferred {
+			assert_eq!(res, Ok(chain::ChannelMonitorUpdateStatus::InProgress));
+		}
 		let state = match res {
 			Ok(chain::ChannelMonitorUpdateStatus::Completed) => LatestMonitorState {
 				persisted_monitor_id: monitor_id,
@@ -349,6 +383,9 @@ impl chain::Watch<TestChannelSigner> for TestChainMonitor {
 		let mut ser = VecWriter(Vec::new());
 		deserialized_monitor.write(&mut ser).unwrap();
 		let res = self.chain_monitor.update_channel(channel_id, update);
+		if self.deferred {
+			assert_eq!(res, chain::ChannelMonitorUpdateStatus::InProgress);
+		}
 		match res {
 			chain::ChannelMonitorUpdateStatus::Completed => {
 				map_entry.persisted_monitor_id = update.update_id;
@@ -365,6 +402,9 @@ impl chain::Watch<TestChannelSigner> for TestChainMonitor {
 	fn release_pending_monitor_events(
 		&self,
 	) -> Vec<(OutPoint, ChannelId, Vec<MonitorEvent>, PublicKey)> {
+		if self.deferred {
+			self.flush_and_update_latest_monitors();
+		}
 		return self.chain_monitor.release_pending_monitor_events();
 	}
 }
@@ -876,9 +916,10 @@ pub fn do_test<Out: Output + MaybeSend + MaybeSync>(data: &[u8], out: Out) {
 	let broadcast_c = Arc::new(TestBroadcaster { txn_broadcasted: RefCell::new(Vec::new()) });
 	let router = FuzzRouter {};
 
-	// Read initial monitor styles and channel type from fuzz input byte 0:
+	// Read initial monitor styles, channel type, and deferred write mode from fuzz input byte 0:
 	// bits 0-2: monitor styles (1 bit per node)
 	// bits 3-4: channel type (0=Legacy, 1=KeyedAnchors, 2=ZeroFeeCommitments)
+	// bits 5-7: deferred monitor write mode (1 bit per node)
 	let config_byte = if !data.is_empty() { data[0] } else { 0 };
 	let chan_type = match (config_byte >> 3) & 0b11 {
 		0 => ChanType::Legacy,
@@ -902,6 +943,11 @@ pub fn do_test<Out: Output + MaybeSend + MaybeSync>(data: &[u8], out: Out) {
 			ChannelMonitorUpdateStatus::Completed
 		}),
 	];
+	let deferred = [
+		config_byte & 0b0010_0000 != 0,
+		config_byte & 0b0100_0000 != 0,
+		config_byte & 0b1000_0000 != 0,
+	];
 
 	let mut chain_state = ChainState::new();
 	let mut node_height_a: u32 = 0;
@@ -930,6 +976,7 @@ pub fn do_test<Out: Output + MaybeSend + MaybeSync>(data: &[u8], out: Out) {
 					update_ret: Mutex::new(mon_style[$node_id as usize].borrow().clone()),
 				}),
 				Arc::clone(&keys_manager),
+				deferred[$node_id as usize],
 			));
 
 			let mut config = UserConfig::default();
@@ -993,6 +1040,7 @@ pub fn do_test<Out: Output + MaybeSend + MaybeSync>(data: &[u8], out: Out) {
 				update_ret: Mutex::new(ChannelMonitorUpdateStatus::Completed),
 			}),
 			Arc::clone(keys),
+			deferred[node_id as usize],
 		));
 
 		let mut config = UserConfig::default();
@@ -1070,18 +1118,28 @@ pub fn do_test<Out: Output + MaybeSend + MaybeSync>(data: &[u8], out: Out) {
 		let manager =
 			<(BestBlock, ChanMan)>::read(&mut &ser[..], read_args).expect("Failed to read manager");
 		let res = (manager.1, chain_monitor.clone());
+		let expected_status = if deferred[node_id as usize] {
+			ChannelMonitorUpdateStatus::InProgress
+		} else {
+			ChannelMonitorUpdateStatus::Completed
+		};
 		for (channel_id, mon) in monitors.drain() {
 			assert_eq!(
 				chain_monitor.chain_monitor.watch_channel(channel_id, mon),
-				Ok(ChannelMonitorUpdateStatus::Completed)
+				Ok(expected_status)
 			);
 		}
+		if deferred[node_id as usize] {
+			let count = chain_monitor.chain_monitor.pending_operation_count();
+			chain_monitor.chain_monitor.flush(count, &chain_monitor.logger);
+		}
 		*chain_monitor.persister.update_ret.lock().unwrap() = *mon_style[node_id as usize].borrow();
 		res
 	};
 
 	macro_rules! complete_all_pending_monitor_updates {
 		($monitor: expr) => {{
+			$monitor.flush_and_update_latest_monitors();
 			for (channel_id, state) in $monitor.latest_monitors.lock().unwrap().iter_mut() {
 				for (id, data) in state.pending_monitors.drain(..) {
 					$monitor.chain_monitor.channel_monitor_updated(*channel_id, id).unwrap();
@@ -2060,6 +2118,7 @@ pub fn do_test<Out: Output + MaybeSend + MaybeSync>(data: &[u8], out: Out) {
 			|monitor: &Arc<TestChainMonitor>,
 			 chan_funding,
 			 compl_selector: &dyn Fn(&mut Vec<(u64, Vec<u8>)>) -> Option<(u64, Vec<u8>)>| {
+				monitor.flush_and_update_latest_monitors();
 				if let Some(state) = monitor.latest_monitors.lock().unwrap().get_mut(chan_funding) {
 					assert!(
 						state.pending_monitors.windows(2).all(|pair| pair[0].0 < pair[1].0),
@@ -2076,6 +2135,7 @@ pub fn do_test<Out: Output + MaybeSend + MaybeSync>(data: &[u8], out: Out) {
 			};
 
 		let complete_all_monitor_updates = |monitor: &Arc<TestChainMonitor>, chan_id| {
+			monitor.flush_and_update_latest_monitors();
 			if let Some(state) = monitor.latest_monitors.lock().unwrap().get_mut(chan_id) {
 				assert!(
 					state.pending_monitors.windows(2).all(|pair| pair[0].0 < pair[1].0),