moq-transport: Add lengths to control messages

draft-06 change

See: [PR 520](moq-wg/moq-transport#520), [§6](https://www.ietf.org/archive/id/draft-ietf-moq-transport-06.html#section-6), [§1.3](https://www.ietf.org/archive/id/draft-ietf-moq-transport-06.html#section-1.3-5.2.1)

We're not doing anything particularly useful with this. Partly, this is
because there seems to be agreement that messages shouldn't be skipped,
and it's not clear what else we'd use these lengths for.

moq-transport/src/message/mod.rs

@@ -86,6 +86,11 @@ macro_rules! message_types {
 		impl Decode for Message {
 			fn decode<R: bytes::Buf>(r: &mut R) -> Result<Self, DecodeError> {
 				let t = u64::decode(r)?;
+				let len = u64::decode(r)?;
+
+				if r.remaining() < len as usize {
+					return Err(DecodeError::InvalidLength(len as usize, r.remaining()));
+				}
 
 				match t {
 					$($val => {
@@ -102,7 +107,18 @@ macro_rules! message_types {
 				match self {
 					$(Self::$name(ref m) => {
 						self.id().encode(w)?;
-						m.encode(w)
+
+						// Find out the length of the message
+						// by encoding it into a buffer and then encoding the length.
+						// This is a bit wasteful, but it's the only way to know the length.
+						let mut buf = Vec::new();
+						m.encode(&mut buf).unwrap();
+						(buf.len() as u64).encode(w)?;
+
+						// At least don't encode the message twice.
+						// Instead, write the buffer directly to the writer.
+						w.put_slice(&buf);
+						Ok(())
 					},)*
 				}
 			}