Fix bug in onion payment payload decode

Prior to this change, we could have failed to decode a valid payload of size
>253. This is because we were decoding the length (a BigSize, big-endian) as a
VarInt (little-endian).

Found in #1652.

lightning/src/ln/msgs.rs

@@ -42,7 +42,7 @@ use io_extras::read_to_end;
 
 use util::events::MessageSendEventsProvider;
 use util::logger;
-use util::ser::{LengthReadable, Readable, ReadableArgs, Writeable, Writer, FixedLengthReader, HighZeroBytesDroppedVarInt, Hostname};
+use util::ser::{BigSize, LengthReadable, Readable, ReadableArgs, Writeable, Writer, FixedLengthReader, HighZeroBytesDroppedVarInt, Hostname};
 
 use ln::{PaymentPreimage, PaymentHash, PaymentSecret};
 
@@ -1418,16 +1418,11 @@ impl Writeable for OnionHopData {
 }
 
 impl Readable for OnionHopData {
-	fn read<R: Read>(mut r: &mut R) -> Result<Self, DecodeError> {
-		use bitcoin::consensus::encode::{Decodable, Error, VarInt};
-		let v: VarInt = Decodable::consensus_decode(&mut r)
-			.map_err(|e| match e {
-				Error::Io(ioe) => DecodeError::from(ioe),
-				_ => DecodeError::InvalidValue
-			})?;
+	fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
+		let b: BigSize = Readable::read(r)?;
 		const LEGACY_ONION_HOP_FLAG: u64 = 0;
-		let (format, amt, cltv_value) = if v.0 != LEGACY_ONION_HOP_FLAG {
-			let mut rd = FixedLengthReader::new(r, v.0);
+		let (format, amt, cltv_value) = if b.0 != LEGACY_ONION_HOP_FLAG {
+			let mut rd = FixedLengthReader::new(r, b.0);
 			let mut amt = HighZeroBytesDroppedVarInt(0u64);
 			let mut cltv_value = HighZeroBytesDroppedVarInt(0u32);
 			let mut short_id: Option<u64> = None;
@@ -2824,4 +2819,40 @@ mod tests {
 		assert_eq!(gossip_timestamp_filter.first_timestamp, 1590000000);
 		assert_eq!(gossip_timestamp_filter.timestamp_range, 0xffff_ffff);
 	}
+
+	#[test]
+	fn decode_onion_hop_data_len_as_bigsize() {
+		// Tests that we can decode an onion payload that is >253 bytes.
+		// Previously, receiving a payload of this size could've caused us to fail to decode a valid
+		// payload, because we were decoding the length (a BigSize, big-endian) as a VarInt
+		// (little-endian).
+
+		// Encode a test onion payload with a big custom TLV such that it's >253 bytes, forcing the
+		// payload length to be encoded over multiple bytes rather than a single u8.
+		let big_payload = encode_big_payload().unwrap();
+		let mut rd = Cursor::new(&big_payload[..]);
+		<msgs::OnionHopData as Readable>::read(&mut rd).unwrap();
+	}
+	// see above test, needs to be a separate method for use of the serialization macros.
+	fn encode_big_payload() -> Result<Vec<u8>, std::io::Error> {
+		use util::ser::HighZeroBytesDroppedVarInt;
+		let payload = msgs::OnionHopData {
+			format: OnionHopDataFormat::NonFinalNode {
+				short_channel_id: 0xdeadbeef1bad1dea,
+			},
+			amt_to_forward: 1000,
+			outgoing_cltv_value: 0xffffffff,
+		};
+		let mut encoded_payload = Vec::new();
+		let test_bytes = vec![42 as u8; 1000];
+		if let OnionHopDataFormat::NonFinalNode { short_channel_id } = payload.format {
+			encode_varint_length_prefixed_tlv!(&mut encoded_payload, {
+				(1, test_bytes, vec_type),
+				(2, HighZeroBytesDroppedVarInt(payload.amt_to_forward), required),
+				(4, HighZeroBytesDroppedVarInt(payload.outgoing_cltv_value), required),
+				(6, short_channel_id, required)
+			});
+		}
+		Ok(encoded_payload)
+	}
 }