Save first confirmed funding txo in ChannelMonitor

Currently, when a ChannelMonitor is persisted, it's funding txo is used
as the key. However, when a channel is spliced, there is a new funding
txo. Store the first confirmed funding txo in ChannelMonitor such that
the persistence does not change. This will be used for v1 channels for
backward compatibility while v2 channels will use the channel ID, which
for them is not derived from the funding txo. This prevents needing to
migrate previously-persisted ChannelMonitors.

Author: jkczyz

lightning/src/chain/channelmonitor.rs

@@ -879,6 +879,7 @@ pub(crate) struct ChannelMonitorImpl<Signer: EcdsaChannelSigner> {
 	holder_revocation_basepoint: RevocationBasepoint,
 	channel_id: ChannelId,
 	funding_info: (OutPoint, ScriptBuf),
+	first_confirmed_funding_txo: OutPoint,
 	current_counterparty_commitment_txid: Option<Txid>,
 	prev_counterparty_commitment_txid: Option<Txid>,
 
@@ -1246,6 +1247,7 @@ impl<Signer: EcdsaChannelSigner> Writeable for ChannelMonitorImpl<Signer> {
 			(21, self.balances_empty_height, option),
 			(23, self.holder_pays_commitment_tx_fee, option),
 			(25, self.payment_preimages, required),
+			(27, self.first_confirmed_funding_txo, required),
 		});
 
 		Ok(())
@@ -1398,6 +1400,8 @@ impl<Signer: EcdsaChannelSigner> ChannelMonitor<Signer> {
 		let mut outputs_to_watch = new_hash_map();
 		outputs_to_watch.insert(funding_info.0.txid, vec![(funding_info.0.index as u32, funding_info.1.clone())]);
 
+		let first_confirmed_funding_txo = funding_info.0;
+
 		Self::from_impl(ChannelMonitorImpl {
 			latest_update_id: 0,
 			commitment_transaction_number_obscure_factor,
@@ -1411,6 +1415,7 @@ impl<Signer: EcdsaChannelSigner> ChannelMonitor<Signer> {
 			holder_revocation_basepoint,
 			channel_id,
 			funding_info,
+			first_confirmed_funding_txo,
 			current_counterparty_commitment_txid: None,
 			prev_counterparty_commitment_txid: None,
 
@@ -5033,6 +5038,7 @@ impl<'a, 'b, ES: EntropySource, SP: SignerProvider> ReadableArgs<(&'a ES, &'b SP
 		let mut channel_id = None;
 		let mut holder_pays_commitment_tx_fee = None;
 		let mut payment_preimages_with_info: Option<HashMap<_, _>> = None;
+		let mut first_confirmed_funding_txo = RequiredWrapper(None);
 		read_tlv_fields!(reader, {
 			(1, funding_spend_confirmed, option),
 			(3, htlcs_resolved_on_chain, optional_vec),
@@ -5047,6 +5053,7 @@ impl<'a, 'b, ES: EntropySource, SP: SignerProvider> ReadableArgs<(&'a ES, &'b SP
 			(21, balances_empty_height, option),
 			(23, holder_pays_commitment_tx_fee, option),
 			(25, payment_preimages_with_info, option),
+			(27, first_confirmed_funding_txo, (default_value, funding_info.0)),
 		});
 		if let Some(payment_preimages_with_info) = payment_preimages_with_info {
 			if payment_preimages_with_info.len() != payment_preimages.len() {
@@ -5099,6 +5106,7 @@ impl<'a, 'b, ES: EntropySource, SP: SignerProvider> ReadableArgs<(&'a ES, &'b SP
 			holder_revocation_basepoint,
 			channel_id: channel_id.unwrap_or(ChannelId::v1_from_funding_outpoint(outpoint)),
 			funding_info,
+			first_confirmed_funding_txo: first_confirmed_funding_txo.0.unwrap(),
 			current_counterparty_commitment_txid,
 			prev_counterparty_commitment_txid,
 

lightning/src/ln/channel.rs

@@ -1965,6 +1965,8 @@ trait InitialRemoteCommitmentReceiver<SP: Deref> where SP::Target: SignerProvide
 		let shutdown_script = context.shutdown_scriptpubkey.clone().map(|script| script.into_inner());
 		let mut monitor_signer = signer_provider.derive_channel_signer(context.channel_value_satoshis, context.channel_keys_id);
 		monitor_signer.provide_channel_parameters(&context.channel_transaction_parameters);
+		// TODO(RBF): When implementing RBF, the funding_txo passed here must only update
+		// ChannelMonitorImp::first_confirmed_funding_txo during channel establishment, not splicing
 		let channel_monitor = ChannelMonitor::new(context.secp_ctx.clone(), monitor_signer,
 		                                          shutdown_script, context.get_holder_selected_contest_delay(),
 		                                          &context.destination_script, (funding_txo, funding_txo_script),