Add start_batch message

Instead of batching commitment_signed messages using a batch TLV, the
splicing spec has been updated to introduce a start_batch messages. It
used to indicate that the next batch_size messages for the channel_id
should be treated as one logical message. This commit simply adds the
message while the following commits will implement the handling logic.

Author: jkczyz

lightning/src/ln/msgs.rs

@@ -686,6 +686,18 @@ pub struct ClosingSigned {
 	pub fee_range: Option<ClosingSignedFeeRange>,
 }
 
+/// A [`start_batch`] message to be sent to group together multiple channel messages as a single
+/// logical message.
+///
+/// [`start_batch`]: https://github.com/lightning/bolts/blob/master/02-peer-protocol.md#batching-channel-messages
+#[derive(Clone, Debug, Hash, PartialEq, Eq)]
+pub struct StartBatch {
+	/// The channel ID of all messages in the batch.
+	pub channel_id: ChannelId,
+	/// The number of messages to follow.
+	pub batch_size: u16,
+}
+
 /// An [`update_add_htlc`] message to be sent to or received from a peer.
 ///
 /// [`update_add_htlc`]: https://github.com/lightning/bolts/blob/master/02-peer-protocol.md#adding-an-htlc-update_add_htlc
@@ -3097,6 +3109,11 @@ impl_writeable_msg!(PeerStorage, { data }, {});
 
 impl_writeable_msg!(PeerStorageRetrieval, { data }, {});
 
+impl_writeable_msg!(StartBatch, {
+	channel_id,
+	batch_size
+}, {});
+
 // Note that this is written as a part of ChannelManager objects, and thus cannot change its
 // serialization format in a way which assumes we know the total serialized length/message end
 // position.

lightning/src/ln/peer_handler.rs

@@ -1901,6 +1901,9 @@ impl<Descriptor: SocketDescriptor, CM: Deref, RM: Deref, OM: Deref, L: Deref, CM
 			},
 
 			// Channel messages:
+			wire::Message::StartBatch(_msg) => {
+				debug_assert!(false);
+			},
 			wire::Message::OpenChannel(msg) => {
 				self.message_handler.chan_handler.handle_open_channel(their_node_id, &msg);
 			},

lightning/src/ln/wire.rs

@@ -82,6 +82,7 @@ pub(crate) enum Message<T: core::fmt::Debug + Type + TestEq> {
 	Shutdown(msgs::Shutdown),
 	ClosingSigned(msgs::ClosingSigned),
 	OnionMessage(msgs::OnionMessage),
+	StartBatch(msgs::StartBatch),
 	UpdateAddHTLC(msgs::UpdateAddHTLC),
 	UpdateFulfillHTLC(msgs::UpdateFulfillHTLC),
 	UpdateFailHTLC(msgs::UpdateFailHTLC),
@@ -142,6 +143,7 @@ impl<T: core::fmt::Debug + Type + TestEq> Writeable for Message<T> {
 			&Message::Shutdown(ref msg) => msg.write(writer),
 			&Message::ClosingSigned(ref msg) => msg.write(writer),
 			&Message::OnionMessage(ref msg) => msg.write(writer),
+			&Message::StartBatch(ref msg) => msg.write(writer),
 			&Message::UpdateAddHTLC(ref msg) => msg.write(writer),
 			&Message::UpdateFulfillHTLC(ref msg) => msg.write(writer),
 			&Message::UpdateFailHTLC(ref msg) => msg.write(writer),
@@ -202,6 +204,7 @@ impl<T: core::fmt::Debug + Type + TestEq> Type for Message<T> {
 			&Message::Shutdown(ref msg) => msg.type_id(),
 			&Message::ClosingSigned(ref msg) => msg.type_id(),
 			&Message::OnionMessage(ref msg) => msg.type_id(),
+			&Message::StartBatch(ref msg) => msg.type_id(),
 			&Message::UpdateAddHTLC(ref msg) => msg.type_id(),
 			&Message::UpdateFulfillHTLC(ref msg) => msg.type_id(),
 			&Message::UpdateFailHTLC(ref msg) => msg.type_id(),
@@ -350,6 +353,9 @@ where
 		msgs::OnionMessage::TYPE => {
 			Ok(Message::OnionMessage(LengthReadable::read_from_fixed_length_buffer(buffer)?))
 		},
+		msgs::StartBatch::TYPE => {
+			Ok(Message::StartBatch(LengthReadable::read_from_fixed_length_buffer(buffer)?))
+		},
 		msgs::UpdateAddHTLC::TYPE => {
 			Ok(Message::UpdateAddHTLC(LengthReadable::read_from_fixed_length_buffer(buffer)?))
 		},
@@ -590,6 +596,10 @@ impl Encode for msgs::OnionMessage {
 	const TYPE: u16 = 513;
 }
 
+impl Encode for msgs::StartBatch {
+	const TYPE: u16 = 127;
+}
+
 impl Encode for msgs::UpdateAddHTLC {
 	const TYPE: u16 = 128;
 }