Use explicit WithoutLength for BOLT 12 features

Most BOLT 12 features are used as the value of a TLV record and thus
don't use an explicit length. One exception is the features inside the
blinded payinfo subtype since the TLV record contains a list of them.
However, these features are also used in the BOLT 4 encrypted_data_tlv
TLV stream as a single record, where the length is implicit.

Implement Readable and Writeable for Features wrapped in WithoutLength
such that either serialization can be used where required.

lightning/src/ln/features.rs

@@ -65,7 +65,7 @@ use core::marker::PhantomData;
 use bitcoin::bech32;
 use bitcoin::bech32::{Base32Len, FromBase32, ToBase32, u5, WriteBase32};
 use crate::ln::msgs::DecodeError;
-use crate::util::ser::{Readable, Writeable, Writer};
+use crate::util::ser::{Readable, WithoutLength, Writeable, Writer};
 
 mod sealed {
 	use crate::prelude::*;
@@ -725,26 +725,40 @@ macro_rules! impl_feature_tlv_write {
 	($features: ident) => {
 		impl Writeable for $features {
 			fn write<W: Writer>(&self, w: &mut W) -> Result<(), io::Error> {
-				self.write_be(w)
+				WithoutLength(self).write(w)
 			}
 		}
 		impl Readable for $features {
 			fn read<R: io::Read>(r: &mut R) -> Result<Self, DecodeError> {
-				let v = io_extras::read_to_end(r)?;
-				Ok(Self::from_be_bytes(v))
+				Ok(WithoutLength::<Self>::read(r)?.0)
 			}
 		}
 	}
 }
 
 impl_feature_tlv_write!(ChannelTypeFeatures);
-impl_feature_tlv_write!(OfferFeatures);
-impl_feature_tlv_write!(InvoiceRequestFeatures);
+
+// Some features may appear both in a TLV record and as part of a TLV subtype sequence. The latter
+// requires a length but the former does not.
+
+impl<T: sealed::Context> Writeable for WithoutLength<&Features<T>> {
+	fn write<W: Writer>(&self, w: &mut W) -> Result<(), io::Error> {
+		self.0.write_be(w)
+	}
+}
+
+impl<T: sealed::Context> Readable for WithoutLength<Features<T>> {
+	fn read<R: io::Read>(r: &mut R) -> Result<Self, DecodeError> {
+		let v = io_extras::read_to_end(r)?;
+		Ok(WithoutLength(Features::<T>::from_be_bytes(v)))
+	}
+}
 
 #[cfg(test)]
 mod tests {
-	use super::{ChannelFeatures, ChannelTypeFeatures, InitFeatures, InvoiceFeatures, NodeFeatures, sealed};
+	use super::{ChannelFeatures, ChannelTypeFeatures, InitFeatures, InvoiceFeatures, NodeFeatures, OfferFeatures, sealed};
 	use bitcoin::bech32::{Base32Len, FromBase32, ToBase32, u5};
+	use crate::util::ser::{Readable, WithoutLength, Writeable};
 
 	#[test]
 	fn sanity_test_unknown_bits() {
@@ -838,6 +852,20 @@ mod tests {
 		assert!(features.supports_payment_secret());
 	}
 
+	#[test]
+	fn encodes_features_without_length() {
+		let features = OfferFeatures::from_le_bytes(vec![1, 2, 3, 4, 5, 42, 100, 101]);
+		assert_eq!(features.flags.len(), 8);
+
+		let mut serialized_features = Vec::new();
+		WithoutLength(&features).write(&mut serialized_features).unwrap();
+		assert_eq!(serialized_features.len(), 8);
+
+		let deserialized_features =
+			WithoutLength::<OfferFeatures>::read(&mut &serialized_features[..]).unwrap().0;
+		assert_eq!(features, deserialized_features);
+	}
+
 	#[test]
 	fn invoice_features_encoding() {
 		let features_as_u5s = vec![

lightning/src/offers/invoice_request.rs

@@ -380,7 +380,7 @@ impl Writeable for InvoiceRequestContents {
 tlv_stream!(InvoiceRequestTlvStream, InvoiceRequestTlvStreamRef, 80..160, {
 	(80, chain: ChainHash),
 	(82, amount: (u64, HighZeroBytesDroppedBigSize)),
-	(84, features: InvoiceRequestFeatures),
+	(84, features: (InvoiceRequestFeatures, WithoutLength)),
 	(86, quantity: (u64, HighZeroBytesDroppedBigSize)),
 	(88, payer_id: PublicKey),
 	(89, payer_note: (String, WithoutLength)),

lightning/src/offers/offer.rs

@@ -567,7 +567,7 @@ tlv_stream!(OfferTlvStream, OfferTlvStreamRef, 1..80, {
 	(6, currency: CurrencyCode),
 	(8, amount: (u64, HighZeroBytesDroppedBigSize)),
 	(10, description: (String, WithoutLength)),
-	(12, features: OfferFeatures),
+	(12, features: (OfferFeatures, WithoutLength)),
 	(14, absolute_expiry: (u64, HighZeroBytesDroppedBigSize)),
 	(16, paths: (Vec<BlindedPath>, WithoutLength)),
 	(18, issuer: (String, WithoutLength)),