Workaround lnd sending funding_locked before channel_reestablish

lnd has a long-standing bug where, upon reconnection, if the
channel is not yet confirmed they will not send a
channel_reestablish until the channel locks in. Then, they will
send a funding_locked *before* sending the channel_reestablish
(which is clearly a violation of the BOLT specs). We copy
c-lightning's workaround here and simply store the funding_locked
message until we receive a channel_reestablish.

See-also lightningnetwork/lnd#4006

Fixes lightningdevkit#963

Author: TheBlueMatt

lightning/src/ln/channel.rs

@@ -434,6 +434,15 @@ pub(super) struct Channel<Signer: Sign> {
 	next_local_commitment_tx_fee_info_cached: Mutex<Option<CommitmentTxInfoCached>>,
 	#[cfg(any(test, feature = "fuzztarget"))]
 	next_remote_commitment_tx_fee_info_cached: Mutex<Option<CommitmentTxInfoCached>>,
+
+	/// lnd has a long-standing bug where, upon reconnection, if the channel is not yet confirmed
+	/// they will not send a channel_reestablish until the channel locks in. Then, they will send a
+	/// funding_locked *before* sending the channel_reestablish (which is clearly a violation of
+	/// the BOLT specs). We copy c-lightning's workaround here and simply store the funding_locked
+	/// message until we receive a channel_reestablish.
+	///
+	/// See-also <https://github.com/lightningnetwork/lnd/issues/4006>
+	pub workaround_lnd_bug_4006: Option<msgs::FundingLocked>,
 }
 
 #[cfg(any(test, feature = "fuzztarget"))]
@@ -633,6 +642,8 @@ impl<Signer: Sign> Channel<Signer> {
 			next_local_commitment_tx_fee_info_cached: Mutex::new(None),
 			#[cfg(any(test, feature = "fuzztarget"))]
 			next_remote_commitment_tx_fee_info_cached: Mutex::new(None),
+
+			workaround_lnd_bug_4006: None,
 		})
 	}
 
@@ -876,6 +887,8 @@ impl<Signer: Sign> Channel<Signer> {
 			next_local_commitment_tx_fee_info_cached: Mutex::new(None),
 			#[cfg(any(test, feature = "fuzztarget"))]
 			next_remote_commitment_tx_fee_info_cached: Mutex::new(None),
+
+			workaround_lnd_bug_4006: None,
 		};
 
 		Ok(chan)
@@ -1703,7 +1716,8 @@ impl<Signer: Sign> Channel<Signer> {
 
 	pub fn funding_locked<L: Deref>(&mut self, msg: &msgs::FundingLocked, logger: &L) -> Result<(), ChannelError> where L::Target: Logger {
 		if self.channel_state & (ChannelState::PeerDisconnected as u32) == ChannelState::PeerDisconnected as u32 {
-			return Err(ChannelError::Close("Peer sent funding_locked when we needed a channel_reestablish".to_owned()));
+			self.workaround_lnd_bug_4006 = Some(msg.clone());
+			return Err(ChannelError::Ignore("Peer sent funding_locked when we needed a channel_reestablish. The peer is likely lnd, see their bug #4006.".to_owned()));
 		}
 
 		let non_shutdown_state = self.channel_state & (!MULTI_STATE_FLAGS);
@@ -4903,6 +4917,8 @@ impl<'a, Signer: Sign, K: Deref> ReadableArgs<&'a K> for Channel<Signer>
 			next_local_commitment_tx_fee_info_cached: Mutex::new(None),
 			#[cfg(any(test, feature = "fuzztarget"))]
 			next_remote_commitment_tx_fee_info_cached: Mutex::new(None),
+
+			workaround_lnd_bug_4006: None,
 		})
 	}
 }

lightning/src/ln/channelmanager.rs

@@ -3375,7 +3375,7 @@ impl<Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelMana
 	}
 
 	fn internal_channel_reestablish(&self, counterparty_node_id: &PublicKey, msg: &msgs::ChannelReestablish) -> Result<(), MsgHandleErrInternal> {
-		let (htlcs_failed_forward, chan_restoration_res) = {
+		let (htlcs_failed_forward, need_lnd_workaround, chan_restoration_res) = {
 			let mut channel_state_lock = self.channel_state.lock().unwrap();
 			let channel_state = &mut *channel_state_lock;
 
@@ -3396,13 +3396,19 @@ impl<Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelMana
 							msg,
 						});
 					}
-					(htlcs_failed_forward, handle_chan_restoration_locked!(self, channel_state_lock, channel_state, chan, revoke_and_ack, commitment_update, order, monitor_update_opt, Vec::new(), None, funding_locked))
+					let need_lnd_workaround = chan.get_mut().workaround_lnd_bug_4006.take();
+					(htlcs_failed_forward, need_lnd_workaround,
+						handle_chan_restoration_locked!(self, channel_state_lock, channel_state, chan, revoke_and_ack, commitment_update, order, monitor_update_opt, Vec::new(), None, funding_locked))
 				},
 				hash_map::Entry::Vacant(_) => return Err(MsgHandleErrInternal::send_err_msg_no_close("Failed to find corresponding channel".to_owned(), msg.channel_id))
 			}
 		};
 		post_handle_chan_restoration!(self, chan_restoration_res);
 		self.fail_holding_cell_htlcs(htlcs_failed_forward, msg.channel_id);
+
+		if let Some(funding_locked_msg) = need_lnd_workaround {
+			self.internal_funding_locked(counterparty_node_id, &funding_locked_msg)?;
+		}
 		Ok(())
 	}