Exchange `splice_locked` messages #3741

jkczyz · 2025-04-16T17:11:41Z

After a splice has been negotiated, each party must send a splice_locked message to the other party once the splice transaction has had an acceptable number of confirmations. Update the logic for processing newly confirmed transactions and updated best block to send splice_locked when appropriate.

TODO: Likewise, handle splice_locked and promote the channel's FundingScope once both splice_locked messages have been exchanged.

ldk-reviews-bot · 2025-04-16T17:11:44Z

👋 Thanks for assigning @wpaulino as a reviewer!
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Once they submit their review, I'll check if a second reviewer would be helpful.

jkczyz · 2025-04-16T17:13:51Z

The logic around determining if the splice_locked messages have been exchanged is still a work in progress. It may need to be re-considered to work with chained 0-conf splices. See lightning/bolts#1160 (comment).

When processing confirmed transactions and updates to the best block, ChannelManager may be instructed to send a channel_ready message when a channel's funding transaction has confirmed and has met the required number of confirmations. A similar action is needed for sending splice_locked once splice transaction has confirmed with required number of confirmations. Generalize do_chain_event signature to allow for either scenario.

When processing confirmed transactions, if the funding transaction is found then information about it in the ChannelContext is updated. In preparation for splicing, move this data to FundingScope.

When checking if channel_ready should be sent, the funding transaction must reach minimum_depth confirmations. The same logic is needed for splicing a channel, so refactor it into a helper method.

jkczyz · 2025-04-18T23:17:53Z

@wpaulino Ok, this is in better shape for a high-level look. I don't believe it correctly handles unconfirmed splice transactions yet. Also, doesn't yet re-send splice_locked on channel reestablishment, thought that may come in a follow-up.

codecov · 2025-04-18T23:24:11Z

Codecov Report

Attention: Patch coverage is 88.63636% with 40 lines in your changes missing coverage. Please review.

Project coverage is 90.15%. Comparing base (7b45811) to head (e4c0566).
Report is 49 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/ln/channel.rs	86.47%	30 Missing and 8 partials ⚠️
lightning/src/ln/channelmanager.rs	96.72%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3741      +/-   ##
==========================================
+ Coverage   89.10%   90.15%   +1.04%     
==========================================
  Files         156      156              
  Lines      123431   132183    +8752     
  Branches   123431   132183    +8752     
==========================================
+ Hits       109985   119164    +9179     
+ Misses      10760    10435     -325     
+ Partials     2686     2584     -102

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ldk-reviews-bot · 2025-04-19T00:00:14Z

🔔 1st Reminder

Hey @wpaulino! This PR has been waiting for your review.
Please take a look when you have a chance. If you're unable to review, please let us know so we can find another reviewer.

optout21

LGTM. The preparational changes are very clear. The WIP part also makes sense so far.

optout21 · 2025-04-19T09:37:16Z

lightning/src/ln/channelmanager.rs

@@ -11725,6 +11727,13 @@ where
 											log_trace!(logger, "Sending channel_ready WITHOUT channel_update for {}", funded_channel.context.channel_id());
 										}
 									},
+									#[cfg(splicing)]
+									Some(FundingConfirmedMessage::Splice(splice_locked)) => {
+										pending_msg_events.push(MessageSendEvent::SendSpliceLocked {


TODO note: An event will have to be sent here as well, similar to ChannelReady -- ChannelReady with a flag indicating it is for splicing, or a new, splicing-specific event.

optout21 · 2025-04-19T09:44:42Z

lightning/src/ln/channel.rs

@@ -1864,6 +1864,8 @@ impl FundingScope {
 #[cfg(splicing)]
 struct PendingSplice {
 	pub our_funding_contribution: i64,
+	sent_funding_txid: Option<Txid>,


Where is this field set? Is it when signatures have been exchanged for the the negotiated funding transaction / it has been broadcast?

I believe these are specific to splice_locked, so when that is sent/received.

optout21 · 2025-04-19T10:02:09Z

lightning/src/ln/channel.rs

+			return None;
+		}
+
+		let confirmed_funding_txid = match funding.get_funding_txo().map(|txo| txo.txid) {


Nit: FundingScope could offer to return TXID, to avoid the need for such maps.

ldk-reviews-bot · 2025-04-21T00:00:36Z

🔔 2nd Reminder

Hey @wpaulino! This PR has been waiting for your review.
Please take a look when you have a chance. If you're unable to review, please let us know so we can find another reviewer.

wpaulino · 2025-04-21T18:45:04Z

lightning/src/ln/channel.rs

-							self.context.minimum_depth = Some(COINBASE_MATURITY);
+							self.funding.minimum_depth.unwrap_or(0) > 0 &&
+							self.funding.minimum_depth.unwrap_or(0) < COINBASE_MATURITY {
+							self.funding.minimum_depth = Some(COINBASE_MATURITY);


Can we somehow enforce the coinbase maturity without updating minimum_depth? We always keep the funding transaction around now so we should be able to check if it's a coinbase transaction or not. That would prevent us from tracking minimum_depth per scope.

wpaulino · 2025-04-21T18:53:35Z

lightning/src/ln/channel.rs

@@ -1864,6 +1864,8 @@ impl FundingScope {
 #[cfg(splicing)]
 struct PendingSplice {
 	pub our_funding_contribution: i64,
+	sent_funding_txid: Option<Txid>,


I believe these are specific to splice_locked, so when that is sent/received.

wpaulino · 2025-04-21T18:58:29Z

lightning/src/ln/channel.rs

+
+		let funding_tx_confirmations = height as i64 - funding.funding_tx_confirmation_height as i64 + 1;
+		if funding_tx_confirmations <= 0 {
+			funding.funding_tx_confirmation_height = 0;


Feels weird to have this done here, as opposed to doing so in funding_transaction_unconfirmed or when we see the block at funding_tx_confirmation_height get disconnected.

wpaulino · 2025-04-21T19:03:21Z

lightning/src/ln/channel.rs

+				None
+			},
+			_ => {
+				Some(msgs::SpliceLocked {


Shouldn't sent_funding_txid get set here?

wpaulino · 2025-04-21T19:04:07Z

lightning/src/ln/channel.rs

+
+		match pending_splice.sent_funding_txid {
+			Some(sent_funding_txid) if confirmed_funding_txid == sent_funding_txid => {
+				debug_assert!(false);


We may need to replay splice_locked if a disconnect happened before they were able to process it. Unclear if we'd want to use this same code path for it or not.

wpaulino · 2025-04-21T19:11:47Z

lightning/src/ln/channel.rs

+		};
+
+		let mut confirmed_funding_tx = None;
+		for &(index_in_block, tx) in txdata.iter() {


I think we may want to iterate over the funding scopes for each transaction instead, since we may be processing full block data with thousands of transactions and having to compute the txid of each one.

jkczyz requested a review from wpaulino April 16, 2025 17:11

jkczyz added 8 commits April 18, 2025 17:56

Move ChannelContext::funding_tx_confirmed_in to FundingScope

2e85dde

When processing confirmed transactions, if the funding transaction is found then information about it in the ChannelContext is updated. In preparation for splicing, move this data to FundingScope.

Move ChannelContext::funding_tx_confirmation_height to FundingScope

8d605b0

When processing confirmed transactions, if the funding transaction is found then information about it in the ChannelContext is updated. In preparation for splicing, move this data to FundingScope.

Move ChannelContext::short_channel_id to FundingScope

d830342

When processing confirmed transactions, if the funding transaction is found then information about it in the ChannelContext is updated. In preparation for splicing, move this data to FundingScope.

Move ChannelContext::minimum_depth to FundingScope

4b4ce81

When processing confirmed transactions, if the funding transaction is found then information about it in the ChannelContext is updated. In preparation for splicing, move this data to FundingScope.

Refactor funding tx confirmation check into helper

6bbbce8

When checking if channel_ready should be sent, the funding transaction must reach minimum_depth confirmations. The same logic is needed for splicing a channel, so refactor it into a helper method.

WIP: Check all funding transactions

71e27a4

Handle splice_locked message

e4c0566

jkczyz force-pushed the 2025-04-splice-locked branch from dfbc04e to e4c0566 Compare April 18, 2025 23:15

optout21 reviewed Apr 19, 2025

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wpaulino reviewed Apr 21, 2025

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Exchange `splice_locked` messages #3741

Exchange `splice_locked` messages #3741

jkczyz commented Apr 16, 2025

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Exchange splice_locked messages #3741

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Exchange splice_locked messages #3741

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Exchange `splice_locked` messages #3741

Exchange `splice_locked` messages #3741

ldk-reviews-bot commented Apr 16, 2025 •

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codecov bot commented Apr 18, 2025 •

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