Encode channel_update type in failure messages.

Author: tnull

lightning/src/ln/channelmanager.rs

@@ -3540,12 +3540,14 @@ impl<Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelMana
 	fn get_htlc_temp_fail_err_and_data(&self, desired_err_code: u16, scid: u64, chan: &Channel<Signer>) -> (u16, Vec<u8>) {
 		debug_assert_eq!(desired_err_code & 0x1000, 0x1000);
 		if let Ok(upd) = self.get_channel_update_for_onion(scid, chan) {
-			let mut enc = VecWriter(Vec::with_capacity(upd.serialized_length() + 4));
+			let mut enc = VecWriter(Vec::with_capacity(upd.serialized_length() + 6));
 			if desired_err_code == 0x1000 | 20 {
 				// TODO: underspecified, follow https://github.com/lightning/bolts/issues/791
 				0u16.write(&mut enc).expect("Writes cannot fail");
 			}
-			(upd.serialized_length() as u16).write(&mut enc).expect("Writes cannot fail");
+			(upd.serialized_length() as u16 + 2).write(&mut enc).expect("Writes cannot fail");
+			let chan_update_type = u16::from_be_bytes([0x01, 0x02]);
+			chan_update_type.write(&mut enc).expect("Writes cannot fail");
 			upd.write(&mut enc).expect("Writes cannot fail");
 			(desired_err_code, enc.0)
 		} else {