Add a macro which implements Readable/Writeable using TLVs only

This also includes a `VecWriteWrapper` and `VecReadWrapper` which
implements serialization for any `Readable`/`Writeable` type that is
in a Vec. We do this instead of implementing `Readable`/`Writeable`
directly as there isn't always a univerally-defined way to serialize
a Vec and this makes things more explicit.

Finally, this tweaks existing macros (and in the new macros) to
support a trailing `,` after a list, eg
`write_tlv_fields!(stream, {(0, a),}, {});` whereas previously the
trailing `,` after the `(0, a)` would be a compile-error.

Author: TheBlueMatt

lightning/src/util/ser.rs

@@ -222,6 +222,33 @@ pub trait MaybeReadable
 	fn read<R: Read>(reader: &mut R) -> Result<Option<Self>, DecodeError>;
 }
 
+const MAX_ALLOC_SIZE: u64 = 64*1024;
+
+pub(crate) struct VecWriteWrapper<'a, T: Writeable>(pub &'a Vec<T>);
+impl<'a, T: Writeable> Writeable for VecWriteWrapper<'a, T> {
+	fn write<W: Writer>(&self, writer: &mut W) -> Result<(), ::std::io::Error> {
+		(self.0.len() as u64).write(writer)?;
+		for ref v in self.0.iter() {
+			v.write(writer)?;
+		}
+		Ok(())
+	}
+}
+pub(crate) struct VecReadWrapper<T: Readable>(pub Vec<T>);
+impl<T: Readable> Readable for VecReadWrapper<T> {
+	fn read<R: Read>(reader: &mut R) -> Result<Self, DecodeError> {
+		let count: u64 = Readable::read(reader)?;
+		let mut values = Vec::with_capacity(cmp::min(count, MAX_ALLOC_SIZE / (::std::mem::size_of::<T>() as u64)) as usize);
+		for _ in 0..count {
+			match Readable::read(reader) {
+				Ok(v) => { values.push(v); },
+				Err(e) => return Err(e),
+			}
+		}
+		Ok(Self(values))
+	}
+}
+
 pub(crate) struct U48(pub u64);
 impl Writeable for U48 {
 	#[inline]

lightning/src/util/ser_macros.rs

@@ -254,7 +254,7 @@ macro_rules! write_ver_prefix {
 /// This is the preferred method of adding new fields that old nodes can ignore and still function
 /// correctly.
 macro_rules! write_tlv_fields {
-	($stream: expr, {$(($type: expr, $field: expr)),*}, {$(($optional_type: expr, $optional_field: expr)),*}) => {
+	($stream: expr, {$(($type: expr, $field: expr)),* $(,)?}, {$(($optional_type: expr, $optional_field: expr)),* $(,)?}) => {
 		encode_varint_length_prefixed_tlv!($stream, {$(($type, $field)),*} , {$(($optional_type, $optional_field)),*});
 	}
 }
@@ -275,14 +275,86 @@ macro_rules! read_ver_prefix {
 
 /// Reads a suffix added by write_tlv_fields.
 macro_rules! read_tlv_fields {
-	($stream: expr, {$(($reqtype: expr, $reqfield: ident)),*}, {$(($type: expr, $field: ident)),*}) => { {
+	($stream: expr, {$(($reqtype: expr, $reqfield: ident)),* $(,)?}, {$(($type: expr, $field: ident)),* $(,)?}) => { {
 		let tlv_len = ::util::ser::BigSize::read($stream)?;
 		let mut rd = ::util::ser::FixedLengthReader::new($stream, tlv_len.0);
 		decode_tlv!(&mut rd, {$(($reqtype, $reqfield)),*}, {$(($type, $field)),*});
 		rd.eat_remaining().map_err(|_| DecodeError::ShortRead)?;
 	} }
 }
 
+// If we naively create a struct in impl_writeable_tlv_based blow, we may end up returning
+// `Self { ,,vecfield: vecfield }` which is obviously incorrect. Instead, we have to match here to
+// detect at least one empty field set and skip the potentially-extra comma.
+macro_rules! _init_tlv_based_struct {
+	({}, {$($field: ident),*}, {$($vecfield: ident),*}) => {
+		Ok(Self {
+			$($field),*,
+			$($vecfield: $vecfield.unwrap().0),*
+		})
+	};
+	({$($reqfield: ident),*}, {}, {$($vecfield: ident),*}) => {
+		Ok(Self {
+			$($reqfield),*,
+			$($vecfield: $vecfield.unwrap().0),*
+		})
+	};
+	({$($reqfield: ident),*}, {$($field: ident),*}, {}) => {
+		Ok(Self {
+			$($reqfield),*,
+			$($field),*
+		})
+	};
+	({$($reqfield: ident),*}, {$($field: ident),*}, {$($vecfield: ident),*}) => {
+		Ok(Self {
+			$($reqfield),*,
+			$($field),*,
+			$($vecfield: $vecfield.unwrap().0),*
+		})
+	}
+}
+
+/// Implements Readable/Writeable for a struct storing it as a set of TLVs
+/// First block includes all the required fields including a dummy value which is used during
+/// deserialization but which will never be exposed to other code.
+/// The second block includes optional fields.
+macro_rules! impl_writeable_tlv_based {
+	($st: ident, {$(($reqtype: expr, $reqfield: ident, $reqdefault: expr)),* $(,)?}, {$(($type: expr, $field: ident)),* $(,)?}, {$(($vectype: expr, $vecfield: ident)),* $(,)?}) => {
+		impl ::util::ser::Writeable for $st {
+			fn write<W: ::util::ser::Writer>(&self, writer: &mut W) -> Result<(), ::std::io::Error> {
+				write_tlv_fields!(writer, {
+					$(($reqtype, self.$reqfield)),*
+				}, {
+					$(($type, self.$field)),*,
+					$(($vectype, Some(::util::ser::VecWriteWrapper(&self.$vecfield)))),*
+				});
+				Ok(())
+			}
+		}
+
+		impl ::util::ser::Readable for $st {
+			fn read<R: ::std::io::Read>(reader: &mut R) -> Result<Self, ::ln::msgs::DecodeError> {
+				$(
+					let mut $reqfield = $reqdefault;
+				)*
+				$(
+					let mut $field = None;
+				)*
+				$(
+					let mut $vecfield = Some(::util::ser::VecReadWrapper(Vec::new()));
+				)*
+				read_tlv_fields!(reader, {
+					$(($reqtype, $reqfield)),*
+				}, {
+					$(($type, $field)),*,
+					$(($vectype, $vecfield)),*
+				});
+				_init_tlv_based_struct!({$($reqfield),*}, {$($field),*}, {$($vecfield),*})
+			}
+		}
+	}
+}
+
 #[cfg(test)]
 mod tests {
 	use std::io::{Cursor, Read};