f also assert closing updates are closing

Author: TheBlueMatt

lightning/src/ln/channelmanager.rs

@@ -5633,8 +5633,18 @@ where
 											} else { false }
 										},
 										// or the channel we'd unblock is already closed,
-										BackgroundEvent::ClosedMonitorUpdateRegeneratedOnStartup((funding_txo, ..))
-											=> *funding_txo == next_channel_outpoint,
+										BackgroundEvent::ClosedMonitorUpdateRegeneratedOnStartup(
+											(funding_txo, monitor_update)
+										) => {
+											if *funding_txo == next_channel_outpoint {
+												assert_eq!(monitor_update.updates.len(), 1);
+												assert!(matches!(
+													monitor_update.updates[0],
+													ChannelMonitorUpdateStep::ChannelForceClosed { .. }
+												));
+												true
+											} else { false }
+										},
 										// or the monitor update has completed and will unblock
 										// immediately once we get going.
 										BackgroundEvent::MonitorUpdatesComplete {