Add funding_locked_txid TLVs to channel_reestablish

The splicing spec extends the channel_reestablish message with two more
TLVs indicating which funding txid the sender has sent/received either
explicitly via splice_locked or implicitly via channel_ready. This
allows peers to detect if a splice_locked was lost during disconnection
and must be retransmitted. This commit updates channel_reestablish with
the TLVs. Subsequent commits will implement the spec requirements.

Author: jkczyz

lightning/src/ln/channel.rs

@@ -11013,6 +11013,7 @@ where
 			your_last_per_commitment_secret: remote_last_secret,
 			my_current_per_commitment_point: dummy_pubkey,
 			next_funding_txid: self.maybe_get_next_funding_txid(),
+			my_current_funding_locked: None,
 		}
 	}
 

lightning/src/ln/channelmanager.rs

@@ -11088,6 +11088,7 @@ This indicates a bug inside LDK. Please report this error at https://github.com/
 							your_last_per_commitment_secret: [1u8; 32],
 							my_current_per_commitment_point: PublicKey::from_slice(&[2u8; 33]).unwrap(),
 							next_funding_txid: None,
+							my_current_funding_locked: None,
 						},
 					});
 					return Err(MsgHandleErrInternal::send_err_msg_no_close(

lightning/src/ln/msgs.rs

@@ -926,6 +926,28 @@ pub struct ChannelReestablish {
 	///   * `channel_reestablish`-sending node: https:///github.com/lightning/bolts/blob/247e83d/02-peer-protocol.md?plain=1#L2466-L2470
 	///   * `channel_reestablish`-receiving node: https:///github.com/lightning/bolts/blob/247e83d/02-peer-protocol.md?plain=1#L2520-L2531
 	pub next_funding_txid: Option<Txid>,
+	/// The last funding txid sent by the sending node, which may be:
+	/// - the txid of the last `splice_locked` it sent, otherwise
+	/// - the txid of the funding transaction if it sent `channel_ready`, or else
+	/// - `None` if it has never sent `channel_ready` or `splice_locked`
+	///
+	/// Also contains a bitfield indicating which messages should be retransmitted.
+	pub my_current_funding_locked: Option<FundingLocked>,
+}
+
+/// Information exchanged during channel reestablishment about the last funding locked.
+#[derive(Clone, Debug, Hash, PartialEq, Eq)]
+pub struct FundingLocked {
+	/// The last txid sent by the sending node, which may be either from the last `splice_locked` or
+	/// for the initial funding transaction if it sent `channel_ready`.
+	pub txid: Txid,
+
+	/// A bitfield indicating which messages should be retransmitted by the receiving node.
+	///
+	/// | Bit Position  | Name                      |
+	/// | ------------- | --------------------------|
+	/// | 0             | `announcement_signatures` |
+	pub retransmit_flags: u8,
 }
 
 /// An [`announcement_signatures`] message to be sent to or received from a peer.
@@ -2852,6 +2874,12 @@ impl_writeable_msg!(ChannelReestablish, {
 	my_current_per_commitment_point,
 }, {
 	(0, next_funding_txid, option),
+	(5, my_current_funding_locked, option),
+});
+
+impl_writeable!(FundingLocked, {
+	txid,
+	retransmit_flags
 });
 
 impl_writeable_msg!(ClosingSigned,
@@ -4321,6 +4349,7 @@ mod tests {
 			your_last_per_commitment_secret: [9; 32],
 			my_current_per_commitment_point: public_key,
 			next_funding_txid: None,
+			my_current_funding_locked: None,
 		};
 
 		let encoded_value = cr.encode();
@@ -4372,6 +4401,7 @@ mod tests {
 				])
 				.unwrap(),
 			)),
+			my_current_funding_locked: None,
 		};
 
 		let encoded_value = cr.encode();
@@ -4395,6 +4425,65 @@ mod tests {
 		);
 	}
 
+	#[test]
+	fn encoding_channel_reestablish_with_funding_locked_txid() {
+		let public_key = {
+			let secp_ctx = Secp256k1::new();
+			PublicKey::from_secret_key(
+				&secp_ctx,
+				&SecretKey::from_slice(
+					&<Vec<u8>>::from_hex(
+						"0101010101010101010101010101010101010101010101010101010101010101",
+					)
+					.unwrap()[..],
+				)
+				.unwrap(),
+			)
+		};
+
+		let cr = msgs::ChannelReestablish {
+			channel_id: ChannelId::from_bytes([
+				4, 0, 0, 0, 0, 0, 0, 0, 5, 0, 0, 0, 0, 0, 0, 0, 6, 0, 0, 0, 0, 0, 0, 0, 7, 0, 0, 0,
+				0, 0, 0, 0,
+			]),
+			next_local_commitment_number: 3,
+			next_remote_commitment_number: 4,
+			your_last_per_commitment_secret: [9; 32],
+			my_current_per_commitment_point: public_key,
+			next_funding_txid: None,
+			my_current_funding_locked: Some(msgs::FundingLocked {
+				txid: Txid::from_raw_hash(
+					bitcoin::hashes::Hash::from_slice(&[
+						21, 167, 250, 69, 152, 48, 103, 172, 164, 99, 59, 19, 23, 11, 92, 84, 15,
+						80, 4, 12, 98, 82, 75, 31, 201, 11, 91, 23, 98, 23, 53, 124,
+					])
+					.unwrap(),
+				),
+				retransmit_flags: 1,
+			}),
+		};
+
+		let encoded_value = cr.encode();
+		assert_eq!(
+			encoded_value,
+			vec![
+				4, 0, 0, 0, 0, 0, 0, 0, 5, 0, 0, 0, 0, 0, 0, 0, 6, 0, 0, 0, 0, 0, 0, 0, 7, 0, 0, 0,
+				0, 0, 0, 0, // channel_id
+				0, 0, 0, 0, 0, 0, 0, 3, // next_local_commitment_number
+				0, 0, 0, 0, 0, 0, 0, 4, // next_remote_commitment_number
+				9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9,
+				9, 9, 9, 9, // your_last_per_commitment_secret
+				3, 27, 132, 197, 86, 123, 18, 100, 64, 153, 93, 62, 213, 170, 186, 5, 101, 215, 30,
+				24, 52, 96, 72, 25, 255, 156, 23, 245, 233, 213, 221, 7,
+				143, // my_current_per_commitment_point
+				5,   // Type (my_current_funding_locked)
+				33,  // Length
+				21, 167, 250, 69, 152, 48, 103, 172, 164, 99, 59, 19, 23, 11, 92, 84, 15, 80, 4,
+				12, 98, 82, 75, 31, 201, 11, 91, 23, 98, 23, 53, 124, 1, // Value
+			]
+		);
+	}
+
 	macro_rules! get_keys_from {
 		($slice: expr, $secp_ctx: expr) => {{
 			let privkey = SecretKey::from_slice(&<Vec<u8>>::from_hex($slice).unwrap()[..]).unwrap();