Automatically archive resolved `ChannelMonitor`s in the BP

diff --git a/lightning-background-processor/src/lib.rs b/lightning-background-processor/src/lib.rs
index aae738a..c794e46 100644
--- a/lightning-background-processor/src/lib.rs
+++ b/lightning-background-processor/src/lib.rs
@@ -171,6 +171,16 @@ const SWEEPER_TIMER: Duration = Duration::from_secs(30);
 #[cfg(test)]
 const SWEEPER_TIMER: Duration = Duration::from_secs(1);
 
+#[cfg(not(test))]
+const FIRST_ARCHIVE_STALE_MONITORS_TIMER: Duration = Duration::from_secs(15);
+#[cfg(test)]
+const FIRST_ARCHIVE_STALE_MONITORS_TIMER: Duration = Duration::ZERO;
+
+#[cfg(not(test))]
+const ARCHIVE_STALE_MONITORS_TIMER: Duration = Duration::from_secs(60 * 10);
+#[cfg(test)]
+const ARCHIVE_STALE_MONITORS_TIMER: Duration = Duration::from_secs(1);
+
 /// core::cmp::min is not currently const, so we define a trivial (and equivalent) replacement
 const fn min_duration(a: Duration, b: Duration) -> Duration {
 	if a.as_nanos() < b.as_nanos() {
@@ -1018,8 +1028,10 @@ where
 	let mut last_scorer_persist_call = sleeper(SCORER_PERSIST_TIMER);
 	let mut last_rebroadcast_call = sleeper(REBROADCAST_TIMER);
 	let mut last_sweeper_call = sleeper(SWEEPER_TIMER);
+	let mut last_archive_call = sleeper(FIRST_ARCHIVE_STALE_MONITORS_TIMER);
 	let mut have_pruned = false;
 	let mut have_decayed_scorer = false;
+	let mut have_archived = false;
 
 	let mut last_forwards_processing_call = sleeper(batch_delay.get());
 
@@ -1147,11 +1159,31 @@ where
 			log_trace!(logger, "Done persisting ChannelManager.");
 		}
 
-		// Note that we want to run a graph prune once not long after startup before
-		// falling back to our usual hourly prunes. This avoids short-lived clients never
-		// pruning their network graph. We run once 60 seconds after startup before
-		// continuing our normal cadence. For RGS, since 60 seconds is likely too long,
-		// we prune after an initial sync completes.
+		// Note that we want to archive stale ChannelMonitors and run a network graph prune once
+		// not long after startup before falling back to their usual infrequent runs. This avoids
+		// short-lived clients never archiving stale ChannelMonitors or pruning their network
+		// graph. For network graph pruning, in the case of RGS sync, we run a prune immediately
+		// after initial sync completes, otherwise we do so on a timer which should be long enough
+		// to give us a chance to get most of the network graph from our peers.
+		let archive_timer = if have_archived {
+			ARCHIVE_STALE_MONITORS_TIMER
+		} else {
+			FIRST_ARCHIVE_STALE_MONITORS_TIMER
+		};
+		let archive_timer_elapsed = {
+			match check_and_reset_sleeper(&mut last_archive_call, || sleeper(archive_timer)) {
+				Some(false) => true,
+				Some(true) => break,
+				None => false,
+			}
+		};
+		if archive_timer_elapsed {
+			log_trace!(logger, "Archiving stale ChannelMonitors.");
+			chain_monitor.archive_fully_resolved_channel_monitors();
+			have_archived = true;
+			log_trace!(logger, "Archived stale ChannelMonitors.");
+		}
+
 		let prune_timer = if gossip_sync.prunable_network_graph().is_some() {
 			NETWORK_PRUNE_TIMER
 		} else {
@@ -1601,8 +1633,10 @@ impl BackgroundProcessor {
 			let mut last_scorer_persist_call = Instant::now();
 			let mut last_rebroadcast_call = Instant::now();
 			let mut last_sweeper_call = Instant::now();
+			let mut last_archive_call = Instant::now();
 			let mut have_pruned = false;
 			let mut have_decayed_scorer = false;
+			let mut have_archived = false;
 
 			let mut cur_batch_delay = batch_delay.get();
 			let mut last_forwards_processing_call = Instant::now();
@@ -1691,11 +1725,26 @@ impl BackgroundProcessor {
 					});
 				}
 
-				// Note that we want to run a graph prune once not long after startup before
-				// falling back to our usual hourly prunes. This avoids short-lived clients never
-				// pruning their network graph. We run once 60 seconds after startup before
-				// continuing our normal cadence. For RGS, since 60 seconds is likely too long,
-				// we prune after an initial sync completes.
+				// Note that we want to archive stale ChannelMonitors and run a network graph prune once
+				// not long after startup before falling back to their usual infrequent runs. This avoids
+				// short-lived clients never archiving stale ChannelMonitors or pruning their network
+				// graph. For network graph pruning, in the case of RGS sync, we run a prune immediately
+				// after initial sync completes, otherwise we do so on a timer which should be long enough
+				// to give us a chance to get most of the network graph from our peers.
+				let archive_timer = if have_archived {
+					ARCHIVE_STALE_MONITORS_TIMER
+				} else {
+					FIRST_ARCHIVE_STALE_MONITORS_TIMER
+				};
+				let archive_timer_elapsed = last_archive_call.elapsed() > archive_timer;
+				if archive_timer_elapsed {
+					log_trace!(logger, "Archiving stale ChannelMonitors.");
+					chain_monitor.archive_fully_resolved_channel_monitors();
+					have_archived = true;
+					last_archive_call = Instant::now();
+					log_trace!(logger, "Archived stale ChannelMonitors.");
+				}
+
 				let prune_timer =
 					if have_pruned { NETWORK_PRUNE_TIMER } else { FIRST_NETWORK_PRUNE_TIMER };
 				let prune_timer_elapsed = last_prune_call.elapsed() > prune_timer;
@@ -3698,4 +3747,127 @@ mod tests {
 		exit_sender.send(()).unwrap();
 		t1.await.unwrap().unwrap();
 	}
+
+	#[test]
+	fn test_monitor_archive() {
+		let (persist_dir, nodes) = create_nodes(2, "test_monitor_archive");
+		// Open a channel, but don't confirm it so that it prunes immediately on FC.
+		open_channel!(nodes[0], nodes[1], 100000);
+
+		let data_dir = nodes[1].kv_store.get_data_dir();
+		let persister = Arc::new(Persister::new(data_dir));
+		let event_handler = |_: _| Ok(());
+		let bp = BackgroundProcessor::start(
+			persister,
+			event_handler,
+			Arc::clone(&nodes[1].chain_monitor),
+			Arc::clone(&nodes[1].node),
+			Some(Arc::clone(&nodes[1].messenger)),
+			nodes[1].p2p_gossip_sync(),
+			Arc::clone(&nodes[1].peer_manager),
+			Some(Arc::clone(&nodes[1].liquidity_manager)),
+			Some(Arc::clone(&nodes[1].sweeper)),
+			Arc::clone(&nodes[1].logger),
+			Some(Arc::clone(&nodes[1].scorer)),
+		);
+
+		let dir = format!("{}_persister_1/monitors", &persist_dir);
+		let mut mons = std::fs::read_dir(&dir).unwrap();
+		let mut mon = mons.next().unwrap().unwrap();
+		if mon.path().to_str().unwrap().ends_with(".tmp") {
+			mon = mons.next().unwrap().unwrap();
+			assert_eq!(mon.path().extension(), None);
+		}
+		assert!(mons.next().is_none());
+
+		// Because the channel wasn't funded, we'll archive the ChannelMonitor immedaitely after
+		// its force-closed (at least on node B, which didn't put their money into it).
+		nodes[1].node.force_close_all_channels_broadcasting_latest_txn("".to_owned());
+		loop {
+			let mut mons = std::fs::read_dir(&dir).unwrap();
+			if let Some(new_mon) = mons.next() {
+				let mut new_mon = new_mon.unwrap();
+				if new_mon.path().to_str().unwrap().ends_with(".tmp") {
+					new_mon = mons.next().unwrap().unwrap();
+					assert_eq!(new_mon.path().extension(), None);
+				}
+				assert_eq!(new_mon.path(), mon.path());
+				assert!(mons.next().is_none());
+			} else {
+				break;
+			}
+		}
+
+		bp.stop().unwrap();
+	}
+
+	#[tokio::test]
+	#[cfg(not(c_bindings))]
+	async fn test_monitor_archive_async() {
+		let (persist_dir, nodes) = create_nodes(2, "test_monitor_archive_async");
+		// Open a channel, but don't confirm it so that it prunes immediately on FC.
+		open_channel!(nodes[0], nodes[1], 100000);
+
+		let kv_store = KVStoreSyncWrapper(Arc::clone(&nodes[0].kv_store));
+		let sweeper_async: &'static OutputSweeper<_, _, _, _, _, _, _> = unsafe {
+			&*(nodes[0].sweeper.sweeper_async() as *const OutputSweeper<_, _, _, _, _, _, _>)
+				as &'static OutputSweeper<_, _, _, _, _, _, _>
+		};
+		let (exit_sender, exit_receiver) = tokio::sync::watch::channel(());
+		let bp_future = tokio::spawn(super::process_events_async(
+			kv_store,
+			move |_: Event| async move { Ok(()) },
+			Arc::clone(&nodes[1].chain_monitor),
+			Arc::clone(&nodes[1].node),
+			crate::NO_ONION_MESSENGER,
+			nodes[1].no_gossip_sync(),
+			Arc::clone(&nodes[1].peer_manager),
+			crate::NO_LIQUIDITY_MANAGER,
+			Some(sweeper_async),
+			Arc::clone(&nodes[1].logger),
+			Some(Arc::clone(&nodes[1].scorer)),
+			move |dur: Duration| {
+				let mut exit_receiver = exit_receiver.clone();
+				Box::pin(async move {
+					tokio::select! {
+						_ = tokio::time::sleep(dur) => false,
+						_ = exit_receiver.changed() => true,
+					}
+				})
+			},
+			false,
+			|| Some(Duration::ZERO),
+		));
+
+		let dir = format!("{}_persister_1/monitors", &persist_dir);
+		let mut mons = std::fs::read_dir(&dir).unwrap();
+		let mut mon = mons.next().unwrap().unwrap();
+		if mon.path().to_str().unwrap().ends_with(".tmp") {
+			mon = mons.next().unwrap().unwrap();
+			assert_eq!(mon.path().extension(), None);
+		}
+		assert!(mons.next().is_none());
+
+		// Because the channel wasn't funded, we'll archive the ChannelMonitor immedaitely after
+		// its force-closed (at least on node B, which didn't put their money into it).
+		nodes[1].node.force_close_all_channels_broadcasting_latest_txn("".to_owned());
+		loop {
+			let mut mons = std::fs::read_dir(&dir).unwrap();
+			if let Some(new_mon) = mons.next() {
+				let mut new_mon = new_mon.unwrap();
+				if new_mon.path().to_str().unwrap().ends_with(".tmp") {
+					new_mon = mons.next().unwrap().unwrap();
+					assert_eq!(new_mon.path().extension(), None);
+				}
+				assert_eq!(new_mon.path(), mon.path());
+				assert!(mons.next().is_none());
+			} else {
+				break;
+			}
+			tokio::task::yield_now().await;
+		}
+
+		exit_sender.send(()).unwrap();
+		bp_future.await.unwrap().unwrap();
+	}
 }