fuzz: add force-close support to chanmon_consistency

Add two new fuzzer actions (0xc0, 0xc1) to force-close one channel
on each peer pair (A-B and B-C). This tests channel monitor
consistency when channels are intentionally closed.

Changes:
- Add fc_ab/fc_bc bools to track force-closed channels
- Handle HandleError, BroadcastChannelUpdate, and ChannelClosed
  events generated by force-close
- Update test_return! macro to account for closed channels
- Skip force-closed channels in 0xff validation

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: joostjager

fuzz/src/chanmon_consistency.rs

@@ -1270,16 +1270,29 @@ pub fn do_test<Out: Output>(data: &[u8], underlying_out: Out, anchors: bool) {
 
 	let pending_payments = RefCell::new([Vec::new(), Vec::new(), Vec::new()]);
 	let resolved_payments = RefCell::new([Vec::new(), Vec::new(), Vec::new()]);
+	let mut fc_ab = false; // Force-closed one A-B channel
+	let mut fc_bc = false; // Force-closed one B-C channel
 
 	macro_rules! test_return {
 		() => {{
-			assert_eq!(nodes[0].list_channels().len(), 3);
-			assert_eq!(nodes[1].list_channels().len(), 6);
-			assert_eq!(nodes[2].list_channels().len(), 3);
-
-			// At no point should we have broadcasted any transactions after the initial channel
-			// opens.
-			assert!(broadcast.txn_broadcasted.borrow().is_empty());
+			// Node A has 3 A-B channels, minus 1 if we force-closed.
+			let expected_a = if fc_ab { 2 } else { 3 };
+			assert_eq!(nodes[0].list_channels().len(), expected_a);
+			// Node B has 3 A-B + 3 B-C channels. Counterparty may not have processed
+			// the force-close yet, so we bound the count.
+			let node_b_chans = nodes[1].list_channels().len();
+			let min_b = 6 - (if fc_ab { 1 } else { 0 }) - (if fc_bc { 1 } else { 0 });
+			let max_b = 6 - (if fc_bc { 1 } else { 0 });
+			assert!(node_b_chans >= min_b && node_b_chans <= max_b);
+			// Node C has 3 B-C channels. Counterparty may not have processed force-close.
+			let node_c_chans = nodes[2].list_channels().len();
+			let min_c = if fc_bc { 2 } else { 3 };
+			assert!(node_c_chans >= min_c && node_c_chans <= 3);
+
+			// Only check for no broadcasts if no force-closes happened.
+			if !fc_ab && !fc_bc {
+				assert!(broadcast.txn_broadcasted.borrow().is_empty());
+			}
 
 			return;
 		}};
@@ -1352,6 +1365,8 @@ pub fn do_test<Out: Output>(data: &[u8], underlying_out: Out, anchors: bool) {
 						},
 						MessageSendEvent::SendChannelReady { .. } => continue,
 						MessageSendEvent::SendAnnouncementSignatures { .. } => continue,
+						MessageSendEvent::HandleError { .. } => continue,
+						MessageSendEvent::BroadcastChannelUpdate { .. } => continue,
 						MessageSendEvent::SendChannelUpdate { ref node_id, ref msg } => {
 							assert_eq!(msg.contents.channel_flags & 2, 0); // The disable bit must never be set!
 							if Some(*node_id) == expect_drop_id { panic!("peer_disconnected should drop msgs bound for the disconnected peer"); }
@@ -1573,6 +1588,12 @@ pub fn do_test<Out: Output>(data: &[u8], underlying_out: Out, anchors: bool) {
 							// force-close which we should detect as an error).
 							assert_eq!(msg.contents.channel_flags & 2, 0);
 						},
+						MessageSendEvent::HandleError { .. } => {
+							// Can be generated by force-close
+						},
+						MessageSendEvent::BroadcastChannelUpdate { .. } => {
+							// Can be generated by force-close (with disable bit set)
+						},
 						_ => if out.may_fail.load(atomic::Ordering::Acquire) {
 							return;
 						} else {
@@ -1616,6 +1637,8 @@ pub fn do_test<Out: Output>(data: &[u8], underlying_out: Out, anchors: bool) {
 							MessageSendEvent::SendChannelUpdate { ref msg, .. } => {
 								assert_eq!(msg.contents.channel_flags & 2, 0); // The disable bit must never be set!
 							},
+							MessageSendEvent::HandleError { .. } => {},
+							MessageSendEvent::BroadcastChannelUpdate { .. } => {},
 							_ => {
 								if out.may_fail.load(atomic::Ordering::Acquire) {
 									return;
@@ -1643,6 +1666,8 @@ pub fn do_test<Out: Output>(data: &[u8], underlying_out: Out, anchors: bool) {
 							MessageSendEvent::SendChannelUpdate { ref msg, .. } => {
 								assert_eq!(msg.contents.channel_flags & 2, 0); // The disable bit must never be set!
 							},
+							MessageSendEvent::HandleError { .. } => {},
+							MessageSendEvent::BroadcastChannelUpdate { .. } => {},
 							_ => {
 								if out.may_fail.load(atomic::Ordering::Acquire) {
 									return;
@@ -1737,6 +1762,7 @@ pub fn do_test<Out: Output>(data: &[u8], underlying_out: Out, anchors: bool) {
 						},
 						events::Event::SplicePending { .. } => {},
 						events::Event::SpliceFailed { .. } => {},
+						events::Event::ChannelClosed { .. } => {},
 
 						_ => {
 							if out.may_fail.load(atomic::Ordering::Acquire) {
@@ -2326,6 +2352,25 @@ pub fn do_test<Out: Output>(data: &[u8], underlying_out: Out, anchors: bool) {
 				}
 			},
 
+			// Force close A-B channel 0
+			0xc0 => {
+				let _ = nodes[0].force_close_broadcasting_latest_txn(
+					&chan_a_id,
+					&nodes[1].get_our_node_id(),
+					"]]]]".to_string(),
+				);
+				fc_ab = true;
+			},
+			// Force close B-C channel 0
+			0xc1 => {
+				let _ = nodes[1].force_close_broadcasting_latest_txn(
+					&chan_b_id,
+					&nodes[2].get_our_node_id(),
+					"]]]]".to_string(),
+				);
+				fc_bc = true;
+			},
+
 			0xb0 | 0xb1 | 0xb2 => {
 				// Restart node A, picking among the in-flight `ChannelMonitor`s to use based on
 				// the value of `v` we're matching.
@@ -2545,13 +2590,20 @@ pub fn do_test<Out: Output>(data: &[u8], underlying_out: Out, anchors: bool) {
 				process_all_events!();
 
 				// Finally, make sure that at least one end of each channel can make a substantial payment
-				for &scid in &chan_ab_scids {
+				// (skip channel 0 if it was force-closed)
+				for (i, &scid) in chan_ab_scids.iter().enumerate() {
+					if fc_ab && i == 0 {
+						continue;
+					}
 					assert!(
 						send(0, 1, scid, 10_000_000, &mut p_ctr)
 							|| send(1, 0, scid, 10_000_000, &mut p_ctr)
 					);
 				}
-				for &scid in &chan_bc_scids {
+				for (i, &scid) in chan_bc_scids.iter().enumerate() {
+					if fc_bc && i == 0 {
+						continue;
+					}
 					assert!(
 						send(1, 2, scid, 10_000_000, &mut p_ctr)
 							|| send(2, 1, scid, 10_000_000, &mut p_ctr)