Work around LND bug 6039

LND hasn't properly handled shutdown messages ever, and
force-closes any time we send one while HTLCs are still present.
The issue is tracked at
lightningnetwork/lnd#6039 and has had
multiple patches to fix it but none so far have managed to land
upstream. The issue appears to be very low priority for the LND
team despite being marked "P1".

We're not going to bother handling this in a sensible way, instead
simply repeated the Shutdown message on repeat until morale
improves.

Author: TheBlueMatt

lightning/src/ln/channel.rs

@@ -3811,6 +3811,17 @@ impl<Signer: WriteableEcdsaChannelSigner> Channel<Signer> {
 		}
 	}
 
+	/// Gets the `Shutdown` message we should send our peer on reconnect, if any.
+	pub fn get_outbound_shutdown(&self) -> Option<msgs::Shutdown> {
+		if self.context.channel_state & (ChannelState::LocalShutdownSent as u32) != 0 {
+			assert!(self.context.shutdown_scriptpubkey.is_some());
+			Some(msgs::Shutdown {
+				channel_id: self.context.channel_id,
+				scriptpubkey: self.get_closing_scriptpubkey(),
+			})
+		} else { None }
+	}
+
 	/// May panic if some calls other than message-handling calls (which will all Err immediately)
 	/// have been called between remove_uncommitted_htlcs_and_mark_paused and this call.
 	///
@@ -3877,13 +3888,7 @@ impl<Signer: WriteableEcdsaChannelSigner> Channel<Signer> {
 		self.context.channel_state &= !(ChannelState::PeerDisconnected as u32);
 		self.context.sent_message_awaiting_response = None;
 
-		let shutdown_msg = if self.context.channel_state & (ChannelState::LocalShutdownSent as u32) != 0 {
-			assert!(self.context.shutdown_scriptpubkey.is_some());
-			Some(msgs::Shutdown {
-				channel_id: self.context.channel_id,
-				scriptpubkey: self.get_closing_scriptpubkey(),
-			})
-		} else { None };
+		let shutdown_msg = self.get_outbound_shutdown();
 
 		let announcement_sigs = self.get_announcement_sigs(node_signer, genesis_block_hash, user_config, best_block.height(), logger);
 

lightning/src/ln/channelmanager.rs

@@ -7489,6 +7489,36 @@ where
 	fn handle_error(&self, counterparty_node_id: &PublicKey, msg: &msgs::ErrorMessage) {
 		let _persistence_guard = PersistenceNotifierGuard::notify_on_drop(self);
 
+		match &msg.data as &str {
+			"cannot co-op close channel w/ active htlcs"|
+			"link failed to shutdown" =>
+			{
+				// LND hasn't properly handled shutdown messages ever, and force-closes any time we
+				// send one while HTLCs are still present. The issue is tracked at
+				// https://github.com/lightningnetwork/lnd/issues/6039 and has had multiple patches
+				// to fix it but none so far have managed to land upstream. The issue appears to be
+				// very low priority for the LND team despite being marked "P1".
+				// We're not going to bother handling this in a sensible way, instead simply
+				// repeating the Shutdown message on repeat until morale improves.
+				if msg.channel_id != [0; 32] {
+					let per_peer_state = self.per_peer_state.read().unwrap();
+					let peer_state_mutex_opt = per_peer_state.get(counterparty_node_id);
+					if peer_state_mutex_opt.is_none() { return; }
+					let mut peer_state = peer_state_mutex_opt.unwrap().lock().unwrap();
+					if let Some(chan) = peer_state.channel_by_id.get(&msg.channel_id) {
+						if let Some(msg) = chan.get_outbound_shutdown() {
+							peer_state.pending_msg_events.push(events::MessageSendEvent::SendShutdown {
+								node_id: *counterparty_node_id,
+								msg,
+							});
+						}
+					}
+				}
+				return;
+			}
+			_ => {}
+		}
+
 		if msg.channel_id == [0; 32] {
 			let channel_ids: Vec<[u8; 32]> = {
 				let per_peer_state = self.per_peer_state.read().unwrap();