Add ChainMonitor::archive_fully_resolved_monitor_channels

  Archives fully resolved channel monitors by adding them to a backup
  location and removing them from the primary storage & the monitor set.

  This is useful for pruning fully resolved monitors from the monitor
  set and primary storage so they are not reloaded on every new new
  block connection.

  We also add a new function, `archive_persisted_channel` to the
  `Persist` trait that writes the monitor to an archive storage and
  removes it from the primary storage.

fuzz/src/utils/test_persister.rs

@@ -17,4 +17,7 @@ impl chainmonitor::Persist<TestChannelSigner> for TestPersister {
 	fn update_persisted_channel(&self, _funding_txo: OutPoint, _update: Option<&channelmonitor::ChannelMonitorUpdate>, _data: &channelmonitor::ChannelMonitor<TestChannelSigner>, _update_id: MonitorUpdateId) -> chain::ChannelMonitorUpdateStatus {
 		self.update_ret.lock().unwrap().clone()
 	}
+
+	fn archive_persisted_channel(&self, _: OutPoint) {
+	}
 }

lightning/src/chain/chainmonitor.rs

@@ -194,6 +194,11 @@ pub trait Persist<ChannelSigner: WriteableEcdsaChannelSigner> {
 	///
 	/// [`Writeable::write`]: crate::util::ser::Writeable::write
 	fn update_persisted_channel(&self, channel_funding_outpoint: OutPoint, update: Option<&ChannelMonitorUpdate>, data: &ChannelMonitor<ChannelSigner>, update_id: MonitorUpdateId) -> ChannelMonitorUpdateStatus;
+	/// Prevents the channel monitor from being loaded on startup.
+	///
+	/// Archiving the data in a backup location (rather than deleting it fully) is useful for
+	/// hedging against data loss in case of unexpected failure.
+	fn archive_persisted_channel(&self, channel_funding_outpoint: OutPoint);
 }
 
 struct MonitorHolder<ChannelSigner: WriteableEcdsaChannelSigner> {
@@ -656,6 +661,26 @@ where C::Target: chain::Filter,
 			}
 		}
 	}
+
+	/// Archives fully resolved channel monitors by calling [`Persist::archive_persisted_channel`].
+	///
+	/// This is useful for pruning fully resolved monitors from the monitor set and primary
+	/// storage so they are not kept in memory and reloaded on restart.
+	///
+	/// Depending on the implementation of [`Persist::archive_persisted_channel`] the monitor
+	/// data could be moved to an archive location or/and removed entirely.
+	pub fn archive_fully_resolved_channel_monitors(&self) {
+		let mut monitors = self.monitors.write().unwrap();
+		monitors.retain(|funding_txo, monitor_holder| {
+			if monitor_holder.monitor.is_fully_resolved() {
+				self.persister.archive_persisted_channel(*funding_txo);
+				false
+			} else {
+				true
+			}
+		});
+	}
+
 }
 
 impl<ChannelSigner: WriteableEcdsaChannelSigner, C: Deref, T: Deref, F: Deref, L: Deref, P: Deref>

lightning/src/ln/monitor_tests.rs

@@ -158,6 +158,60 @@ fn revoked_output_htlc_resolution_timing() {
 	expect_payment_failed!(nodes[1], payment_hash_1, false);
 }
 
+#[test]
+fn archive_fully_resolved_monitors() {
+	// Test we can archive fully resolved channel monitor.
+	let chanmon_cfgs = create_chanmon_cfgs(2);
+	let node_cfgs = create_node_cfgs(2, &chanmon_cfgs);
+	let mut user_config = test_default_channel_config();
+	let node_chanmgrs = create_node_chanmgrs(2, &node_cfgs, &[Some(user_config), Some(user_config)]);
+	let mut nodes = create_network(2, &node_cfgs, &node_chanmgrs);
+
+	let (_, _, chan_id, funding_tx) =
+		create_announced_chan_between_nodes_with_value(&nodes, 0, 1, 1_000_000, 1_000_000);
+
+	nodes[0].node.close_channel(&chan_id, &nodes[1].node.get_our_node_id()).unwrap();
+	let node_0_shutdown = get_event_msg!(nodes[0], MessageSendEvent::SendShutdown, nodes[1].node.get_our_node_id());
+	nodes[1].node.handle_shutdown(&nodes[0].node.get_our_node_id(), &node_0_shutdown);
+	let node_1_shutdown = get_event_msg!(nodes[1], MessageSendEvent::SendShutdown, nodes[0].node.get_our_node_id());
+	nodes[0].node.handle_shutdown(&nodes[1].node.get_our_node_id(), &node_1_shutdown);
+
+	let node_0_closing_signed = get_event_msg!(nodes[0], MessageSendEvent::SendClosingSigned, nodes[1].node.get_our_node_id());
+	nodes[1].node.handle_closing_signed(&nodes[0].node.get_our_node_id(), &node_0_closing_signed);
+	let node_1_closing_signed = get_event_msg!(nodes[1], MessageSendEvent::SendClosingSigned, nodes[0].node.get_our_node_id());
+	nodes[0].node.handle_closing_signed(&nodes[1].node.get_our_node_id(), &node_1_closing_signed);
+	let (_, node_0_2nd_closing_signed) = get_closing_signed_broadcast!(nodes[0].node, nodes[1].node.get_our_node_id());
+	nodes[1].node.handle_closing_signed(&nodes[0].node.get_our_node_id(), &node_0_2nd_closing_signed.unwrap());
+	let (_, _) = get_closing_signed_broadcast!(nodes[1].node, nodes[0].node.get_our_node_id());
+
+	let shutdown_tx = nodes[0].tx_broadcaster.txn_broadcasted.lock().unwrap().split_off(0);
+
+	mine_transaction(&nodes[0], &shutdown_tx[0]);
+	mine_transaction(&nodes[1], &shutdown_tx[0]);
+
+	connect_blocks(&nodes[0], 6);
+	connect_blocks(&nodes[1], 6);
+
+	check_closed_event!(nodes[0], 1, ClosureReason::LocallyInitiatedCooperativeClosure, [nodes[1].node.get_our_node_id()], 1000000);
+	check_closed_event!(nodes[1], 1, ClosureReason::CounterpartyInitiatedCooperativeClosure, [nodes[0].node.get_our_node_id()], 1000000);
+
+	assert_eq!(nodes[0].chain_monitor.chain_monitor.list_monitors().len(), 1);
+	// First archive should set balances_empty_height to current block height
+	nodes[0].chain_monitor.chain_monitor.archive_fully_resolved_channel_monitors(); 
+	assert_eq!(nodes[0].chain_monitor.chain_monitor.list_monitors().len(), 1);
+	connect_blocks(&nodes[0], 2016);
+	// Second call after 2016 blocks, should archive the monitor
+	nodes[0].chain_monitor.chain_monitor.archive_fully_resolved_channel_monitors();
+	// Should have no monitors left
+	assert_eq!(nodes[0].chain_monitor.chain_monitor.list_monitors().len(), 0);
+	// Remove the corresponding outputs and transactions the chain source is
+	// watching. This is to make sure the `Drop` function assertions pass.
+	nodes.get_mut(0).unwrap().chain_source.remove_watched_txn_and_outputs(
+		OutPoint { txid: funding_tx.txid(), index: 0 },
+		funding_tx.output[0].script_pubkey.clone()
+	);
+}
+
 fn do_chanmon_claim_value_coop_close(anchors: bool) {
 	// Tests `get_claimable_balances` returns the correct values across a simple cooperative claim.
 	// Specifically, this tests that the channel non-HTLC balances show up in

lightning/src/util/persist.rs

@@ -56,6 +56,11 @@ pub const CHANNEL_MONITOR_PERSISTENCE_SECONDARY_NAMESPACE: &str = "";
 /// The primary namespace under which [`ChannelMonitorUpdate`]s will be persisted.
 pub const CHANNEL_MONITOR_UPDATE_PERSISTENCE_PRIMARY_NAMESPACE: &str = "monitor_updates";
 
+/// The primary namespace under which archived [`ChannelMonitor`]s will be persisted.
+pub const ARCHIVED_CHANNEL_MONITOR_PERSISTENCE_PRIMARY_NAMESPACE: &str = "archived_monitors";
+/// The secondary namespace under which archived [`ChannelMonitor`]s will be persisted.
+pub const ARCHIVED_CHANNEL_MONITOR_PERSISTENCE_SECONDARY_NAMESPACE: &str = "";
+
 /// The primary namespace under which the [`NetworkGraph`] will be persisted.
 pub const NETWORK_GRAPH_PERSISTENCE_PRIMARY_NAMESPACE: &str = "";
 /// The secondary namespace under which the [`NetworkGraph`] will be persisted.
@@ -212,6 +217,34 @@ impl<ChannelSigner: WriteableEcdsaChannelSigner, K: KVStore + ?Sized> Persist<Ch
 			Err(_) => chain::ChannelMonitorUpdateStatus::UnrecoverableError
 		}
 	}
+
+	fn archive_persisted_channel(&self, funding_txo: OutPoint) {
+		let monitor_name = MonitorName::from(funding_txo);
+		let monitor = match self.read(
+			CHANNEL_MONITOR_PERSISTENCE_PRIMARY_NAMESPACE,
+			CHANNEL_MONITOR_PERSISTENCE_SECONDARY_NAMESPACE,
+			monitor_name.as_str(),
+		) {
+			Ok(monitor) => monitor,
+			Err(_) => return
+		};
+		match self.write(
+			ARCHIVED_CHANNEL_MONITOR_PERSISTENCE_PRIMARY_NAMESPACE,
+			ARCHIVED_CHANNEL_MONITOR_PERSISTENCE_SECONDARY_NAMESPACE,
+			monitor_name.as_str(),
+			&monitor 
+		) {
+			Ok(()) => {}
+			Err(_e) => return
+		};
+		let key = format!("{}_{}", funding_txo.txid.to_string(), funding_txo.index);
+		let _ = self.remove(
+			CHANNEL_MONITOR_PERSISTENCE_PRIMARY_NAMESPACE,
+			CHANNEL_MONITOR_PERSISTENCE_SECONDARY_NAMESPACE,
+			&key,
+			false
+		);
+	}
 }
 
 /// Read previously persisted [`ChannelMonitor`]s from the store.
@@ -718,6 +751,31 @@ where
 			self.persist_new_channel(funding_txo, monitor, monitor_update_call_id)
 		}
 	}
+
+	fn archive_persisted_channel(&self, funding_txo: OutPoint) {
+		let monitor_name = MonitorName::from(funding_txo);
+		let monitor = match self.read_monitor(&monitor_name) {
+			Ok((_block_hash, monitor)) => monitor,
+			Err(_) => return
+		};
+		match self.kv_store.write(
+			ARCHIVED_CHANNEL_MONITOR_PERSISTENCE_PRIMARY_NAMESPACE,
+			ARCHIVED_CHANNEL_MONITOR_PERSISTENCE_SECONDARY_NAMESPACE,
+			monitor_name.as_str(),
+			&monitor.encode()
+		) {
+			Ok(()) => {},
+			Err(_e) => {
+				return 
+			} 
+		};
+		let _ = self.kv_store.remove(
+			CHANNEL_MONITOR_PERSISTENCE_PRIMARY_NAMESPACE,
+			CHANNEL_MONITOR_PERSISTENCE_SECONDARY_NAMESPACE,
+			monitor_name.as_str(),
+			false,
+		);
+	}
 }
 
 impl<K: Deref, L: Deref, ES: Deref, SP: Deref> MonitorUpdatingPersister<K, L, ES, SP>

lightning/src/util/test_utils.rs

@@ -504,6 +504,10 @@ impl<Signer: sign::ecdsa::WriteableEcdsaChannelSigner> chainmonitor::Persist<Sig
 		}
 		res
 	}
+
+	fn archive_persisted_channel(&self, funding_txo: OutPoint) {
+		<TestPersister as chainmonitor::Persist<TestChannelSigner>>::archive_persisted_channel(&self.persister, funding_txo);
+	}
 }
 
 pub struct TestPersister {
@@ -552,6 +556,18 @@ impl<Signer: sign::ecdsa::WriteableEcdsaChannelSigner> chainmonitor::Persist<Sig
 		}
 		ret
 	}
+
+	fn archive_persisted_channel(&self, funding_txo: OutPoint) { 
+		// remove the channel from the offchain_monitor_updates map
+		match self.offchain_monitor_updates.lock().unwrap().remove(&funding_txo) {
+			Some(_) => {},
+			None => {
+				// If the channel was not in the offchain_monitor_updates map, it should be in the
+				// chain_sync_monitor_persistences map.
+				assert!(self.chain_sync_monitor_persistences.lock().unwrap().remove(&funding_txo).is_some());
+			}
+		};
+	}
 }
 
 pub struct TestStore {
@@ -1363,6 +1379,10 @@ impl TestChainSource {
 			watched_outputs: Mutex::new(new_hash_set()),
 		}
 	}
+	pub fn remove_watched_txn_and_outputs(&self, outpoint: OutPoint, script_pubkey: ScriptBuf) {
+		self.watched_outputs.lock().unwrap().remove(&(outpoint, script_pubkey.clone())); 
+		self.watched_txn.lock().unwrap().remove(&(outpoint.txid, script_pubkey));
+	}
 }
 
 impl UtxoLookup for TestChainSource {