Allow stale ChannelMonitors if we are sure they don't have funds

In 601bf4b we started refusing to
load `ChannelMonitor`s which were created prior to LDK 0.0.110 and
which were last updated prior to LDK 0.0.116. While this is likely
fine for nearly all of our users, there's some risk that some (like
me) have ancient un-archived `ChannelMonitor`s.

We do need to start auto-archiving `ChannelMonitor`s but for now we
need some way for such users to at least not fail on startup.
Sadly, ancient `ChannelMonitor`s often don't contain some of the
data we need to accurately calculate `Balance`s, but in cases where
they do, and where there are no claimable `Balance`s, we can be
confident we don't need the `ChannelMonitor`s.

Thus, here, we adapt the `ChannelMonitor` `Readable` implementation
used in the persistence wrappers to detect this case and simply
skip loading such `ChannelMonitor`s. Its not clear if this is
sufficient, but its at least better than the current state of
affairs after 601bf4b.

Author: TheBlueMatt

lightning/src/chain/channelmonitor.rs

@@ -1059,11 +1059,18 @@ impl Readable for IrrevocablyResolvedHTLC {
 /// You MUST ensure that no ChannelMonitors for a given channel anywhere contain out-of-date
 /// information and are actively monitoring the chain.
 ///
-/// Note that the deserializer is only implemented for (BlockHash, ChannelMonitor), which
-/// tells you the last block hash which was block_connect()ed. You MUST rescan any blocks along
-/// the "reorg path" (ie disconnecting blocks until you find a common ancestor from both the
-/// returned block hash and the the current chain and then reconnecting blocks to get to the
-/// best chain) upon deserializing the object!
+/// Like the [`ChannelManager`], deserialization is implemented for `(BlockHash, ChannelMonitor)`,
+/// providing you with the last block hash which was connected before shutting down. You must begin
+/// syncing the chain from that point, disconnecting and connecting blocks as required to get to
+/// the best chain on startup. Note that all [`ChannelMonitor`]s passed to a [`ChainMonitor`] must
+/// by synced as of the same block, so syncing must happen prior to [`ChainMonitor`]
+/// initialization.
+///
+/// For those loading potentially-ancient [`ChannelMonitor`]s, deserialization is also implemented
+/// for `Option<(BlockHash, ChannelMonitor)>`. LDK can no longer deserialize a [`ChannelMonitor`]
+/// that was first created in LDK prior to 0.0.110 and last updated prior to LDK 0.0.116. In such
+/// cases, the `Option<(..)>` deserialization option may return `Ok(None)` rather than failing to
+/// deserialize, allowing you to differentiate between the two cases.
 pub struct ChannelMonitor<Signer: EcdsaChannelSigner> {
 	pub(crate) inner: Mutex<ChannelMonitorImpl<Signer>>,
 }
@@ -6251,6 +6258,18 @@ const MAX_ALLOC_SIZE: usize = 64 * 1024;
 
 impl<'a, 'b, ES: EntropySource, SP: SignerProvider> ReadableArgs<(&'a ES, &'b SP)>
 	for (BlockHash, ChannelMonitor<SP::EcdsaSigner>)
+{
+	fn read<R: io::Read>(reader: &mut R, args: (&'a ES, &'b SP)) -> Result<Self, DecodeError> {
+		match <Option<Self>>::read(reader, args) {
+			Ok(Some(res)) => Ok(res),
+			Ok(None) => Err(DecodeError::UnknownRequiredFeature),
+			Err(e) => Err(e),
+		}
+	}
+}
+
+impl<'a, 'b, ES: EntropySource, SP: SignerProvider> ReadableArgs<(&'a ES, &'b SP)>
+	for Option<(BlockHash, ChannelMonitor<SP::EcdsaSigner>)>
 {
 	#[rustfmt::skip]
 	fn read<R: io::Read>(reader: &mut R, args: (&'a ES, &'b SP)) -> Result<Self, DecodeError> {
@@ -6528,11 +6547,6 @@ impl<'a, 'b, ES: EntropySource, SP: SignerProvider> ReadableArgs<(&'a ES, &'b SP
 		}
 
 		let channel_id = channel_id.unwrap_or(ChannelId::v1_from_funding_outpoint(outpoint));
-		if counterparty_node_id.is_none() {
-			panic!("Found monitor for channel {} with no updates since v0.0.118.\
-				These monitors are no longer supported.\
-				To continue, run a v0.1 release, send/route a payment over the channel or close it.", channel_id);
-		}
 
 		let (current_holder_commitment_tx, current_holder_htlc_data) = {
 			let holder_commitment_tx = onchain_tx_handler.current_holder_commitment_tx();
@@ -6586,7 +6600,8 @@ impl<'a, 'b, ES: EntropySource, SP: SignerProvider> ReadableArgs<(&'a ES, &'b SP
 				(None, None)
 			};
 
-		Ok((best_block.block_hash, ChannelMonitor::from_impl(ChannelMonitorImpl {
+		let dummy_node_id = PublicKey::from_slice(&[2; 33]).unwrap();
+		let monitor = ChannelMonitor::from_impl(ChannelMonitorImpl {
 			funding: FundingScope {
 				channel_parameters,
 
@@ -6646,7 +6661,7 @@ impl<'a, 'b, ES: EntropySource, SP: SignerProvider> ReadableArgs<(&'a ES, &'b SP
 			spendable_txids_confirmed: spendable_txids_confirmed.unwrap(),
 
 			best_block,
-			counterparty_node_id: counterparty_node_id.unwrap(),
+			counterparty_node_id: counterparty_node_id.unwrap_or(dummy_node_id),
 			initial_counterparty_commitment_info,
 			initial_counterparty_commitment_tx,
 			balances_empty_height,
@@ -6658,7 +6673,20 @@ impl<'a, 'b, ES: EntropySource, SP: SignerProvider> ReadableArgs<(&'a ES, &'b SP
 			alternative_funding_confirmed,
 
 			written_by_0_1_or_later,
-		})))
+		});
+
+		if counterparty_node_id.is_none() {
+			if (holder_tx_signed || lockdown_from_offchain) && monitor.get_claimable_balances().is_empty() {
+				// If the monitor is no longer readable, but it is a candidate for archiving,
+				// return Ok(None) to allow it to be skipped and not loaded.
+				return Ok(None);
+			} else {
+				panic!("Found monitor for channel {channel_id} with no updates since v0.0.118.\
+					These monitors are no longer supported.\
+					To continue, run a v0.1 release, send/route a payment over the channel or close it.");
+			}
+		}
+		Ok(Some((best_block.block_hash, monitor)))
 	}
 }
 

lightning/src/util/persist.rs

@@ -372,15 +372,15 @@ where
 		CHANNEL_MONITOR_PERSISTENCE_PRIMARY_NAMESPACE,
 		CHANNEL_MONITOR_PERSISTENCE_SECONDARY_NAMESPACE,
 	)? {
-		match <(BlockHash, ChannelMonitor<<SP::Target as SignerProvider>::EcdsaSigner>)>::read(
+		match <Option<(BlockHash, ChannelMonitor<<SP::Target as SignerProvider>::EcdsaSigner>)>>::read(
 			&mut io::Cursor::new(kv_store.read(
 				CHANNEL_MONITOR_PERSISTENCE_PRIMARY_NAMESPACE,
 				CHANNEL_MONITOR_PERSISTENCE_SECONDARY_NAMESPACE,
 				&stored_key,
 			)?),
 			(&*entropy_source, &*signer_provider),
 		) {
-			Ok((block_hash, channel_monitor)) => {
+			Ok(Some((block_hash, channel_monitor))) => {
 				let monitor_name = MonitorName::from_str(&stored_key)?;
 				if channel_monitor.persistence_key() != monitor_name {
 					return Err(io::Error::new(
@@ -391,6 +391,7 @@ where
 
 				res.push((block_hash, channel_monitor));
 			},
+			Ok(None) => {},
 			Err(_) => {
 				return Err(io::Error::new(
 					io::ErrorKind::InvalidData,
@@ -783,9 +784,11 @@ where
 		let secondary = CHANNEL_MONITOR_PERSISTENCE_SECONDARY_NAMESPACE;
 		let monitor_list = self.0.kv_store.list(primary, secondary).await?;
 		let mut res = Vec::with_capacity(monitor_list.len());
-		// TODO: Parallelize this loop
 		for monitor_key in monitor_list {
-			res.push(self.read_channel_monitor_with_updates(monitor_key.as_str()).await?)
+			let result = self.maybe_read_channel_monitor_with_updates(monitor_key.as_str()).await?;
+			if let Some(read_res) = result {
+				res.push(read_res);
+			}
 		}
 		Ok(res)
 	}
@@ -922,9 +925,30 @@ where
 	pub async fn read_channel_monitor_with_updates(
 		&self, monitor_key: &str,
 	) -> Result<(BlockHash, ChannelMonitor<<SP::Target as SignerProvider>::EcdsaSigner>), io::Error>
+	{
+		match self.maybe_read_channel_monitor_with_updates(monitor_key).await? {
+			Some(res) => Ok(res),
+			None => {
+				Err(io::Error::new(
+					io::ErrorKind::InvalidData,
+					"ChannelMonitor was stale, with no updates since LDK 0.0.118.\
+					It cannot be read by modern versions of LDK, though also does not contain any funds left to sweep.\
+					You should manually delete it instead",
+				))
+			},
+		}
+	}
+
+	async fn maybe_read_channel_monitor_with_updates(
+		&self, monitor_key: &str,
+	) -> Result<Option<(BlockHash, ChannelMonitor<<SP::Target as SignerProvider>::EcdsaSigner>)>, io::Error>
 	{
 		let monitor_name = MonitorName::from_str(monitor_key)?;
-		let (block_hash, monitor) = self.read_monitor(&monitor_name, monitor_key).await?;
+		let read_res = self.maybe_read_monitor(&monitor_name, monitor_key).await?;
+		let (block_hash, monitor) = match read_res {
+			Some(res) => res,
+			None => return Ok(None),
+		};
 		let mut current_update_id = monitor.get_latest_update_id();
 		// TODO: Parallelize this loop by speculatively reading a batch of updates
 		loop {
@@ -955,13 +979,13 @@ where
 				io::Error::new(io::ErrorKind::Other, "Monitor update failed")
 			})?;
 		}
-		Ok((block_hash, monitor))
+		Ok(Some((block_hash, monitor)))
 	}
 
 	/// Read a channel monitor.
-	async fn read_monitor(
+	async fn maybe_read_monitor(
 		&self, monitor_name: &MonitorName, monitor_key: &str,
-	) -> Result<(BlockHash, ChannelMonitor<<SP::Target as SignerProvider>::EcdsaSigner>), io::Error>
+	) -> Result<Option<(BlockHash, ChannelMonitor<<SP::Target as SignerProvider>::EcdsaSigner>)>, io::Error>
 	{
 		let primary = CHANNEL_MONITOR_PERSISTENCE_PRIMARY_NAMESPACE;
 		let secondary = CHANNEL_MONITOR_PERSISTENCE_SECONDARY_NAMESPACE;
@@ -971,11 +995,12 @@ where
 		if monitor_cursor.get_ref().starts_with(MONITOR_UPDATING_PERSISTER_PREPEND_SENTINEL) {
 			monitor_cursor.set_position(MONITOR_UPDATING_PERSISTER_PREPEND_SENTINEL.len() as u64);
 		}
-		match <(BlockHash, ChannelMonitor<<SP::Target as SignerProvider>::EcdsaSigner>)>::read(
+		match <Option<(BlockHash, ChannelMonitor<<SP::Target as SignerProvider>::EcdsaSigner>)>>::read(
 			&mut monitor_cursor,
 			(&*self.entropy_source, &*self.signer_provider),
 		) {
-			Ok((blockhash, channel_monitor)) => {
+			Ok(None) => Ok(None),
+			Ok(Some((blockhash, channel_monitor))) => {
 				if channel_monitor.persistence_key() != *monitor_name {
 					log_error!(
 						self.logger,
@@ -987,7 +1012,7 @@ where
 						"ChannelMonitor was stored under the wrong key",
 					))
 				} else {
-					Ok((blockhash, channel_monitor))
+					Ok(Some((blockhash, channel_monitor)))
 				}
 			},
 			Err(e) => {
@@ -1027,9 +1052,14 @@ where
 		let monitor_keys = self.kv_store.list(primary, secondary).await?;
 		for monitor_key in monitor_keys {
 			let monitor_name = MonitorName::from_str(&monitor_key)?;
-			let (_, current_monitor) = self.read_monitor(&monitor_name, &monitor_key).await?;
-			let latest_update_id = current_monitor.get_latest_update_id();
-			self.cleanup_stale_updates_for_monitor_to(&monitor_key, latest_update_id).await?;
+			let maybe_monitor = self.maybe_read_monitor(&monitor_name, &monitor_key).await?;
+			if let Some((_, current_monitor)) = maybe_monitor {
+				let latest_update_id = current_monitor.get_latest_update_id();
+				self.cleanup_stale_updates_for_monitor_to(&monitor_key, latest_update_id).await?;
+			} else {
+				// TODO: Also clean up super stale monitors (created pre-0.0.110 and last updated
+				// pre-0.0.116).
+			}
 		}
 		Ok(())
 	}