Broadcast holder commitment for currently confirmed funding #3939

wpaulino · 2025-07-17T17:12:16Z

A splice's FundingScope can only be promoted once a ChannelMonitorUpdateStep::RenegotiatedFundingLocked is applied, or if the monitor is no longer accepting updates, once the splice transaction is no longer under reorg risk. Because of this, our current FundingScope may not reflect the latest confirmed state in the chain. Before making a holder commitment broadcast, we must check which FundingScope is currently confirmed to ensure that it can propagate throughout the network.

ldk-reviews-bot · 2025-07-17T17:12:19Z

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codecov · 2025-07-18T22:42:46Z

Codecov Report

❌ Patch coverage is 47.20812% with 104 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.84%. Comparing base (ecce268) to head (9051703).

Files with missing lines	Patch %	Lines
lightning/src/chain/channelmonitor.rs	43.40%	92 Missing and 11 partials ⚠️
lightning/src/chain/onchaintx.rs	93.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3939      +/-   ##
==========================================
- Coverage   88.93%   88.84%   -0.10%     
==========================================
  Files         174      174              
  Lines      123842   123983     +141     
  Branches   123842   123983     +141     
==========================================
+ Hits       110142   110150       +8     
- Misses      11254    11373     +119     
- Partials     2446     2460      +14

Flag	Coverage Δ
fuzzing	`22.57% <21.31%> (-0.04%)`	⬇️
tests	`88.67% <47.20%> (-0.10%)`	⬇️

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ldk-reviews-bot · 2025-07-21T00:01:01Z

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lightning/src/chain/channelmonitor.rs

ldk-reviews-bot · 2025-07-22T00:15:42Z

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lightning/src/chain/channelmonitor.rs

ldk-reviews-bot · 2025-07-26T00:00:27Z

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lightning/src/chain/channelmonitor.rs

ldk-reviews-bot · 2025-07-30T00:01:40Z

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lightning/src/chain/channelmonitor.rs

TheBlueMatt · 2025-07-30T13:37:11Z

lightning/src/chain/channelmonitor.rs

@@ -4627,7 +4698,8 @@ impl<Signer: EcdsaChannelSigner> ChannelMonitorImpl<Signer> {
 			is_holder_tx = true;
 			log_info!(logger, "Got broadcast of latest holder commitment tx {}, searching for available HTLCs to claim", commitment_txid);
 			let holder_commitment_tx = &self.funding.current_holder_commitment_tx;
-			let res = self.get_broadcasted_holder_claims(holder_commitment_tx, height);
+			let res =
+				self.get_broadcasted_holder_claims(&self.funding, holder_commitment_tx, height);


Don't these (and the above reference to self.funding in the if) need to look at the funding confirmed? If we first confirm an alt funding, then close the channel, we will want to check against a commitment tx spending the new funding.

Those changes are coming in a separate PR.

TheBlueMatt · 2025-07-30T13:51:24Z

lightning/src/chain/channelmonitor.rs

+							// We've just seen the counterparty commitment confirm, which conflicts
+							// with our holder commitment, so make sure we no longer attempt to
+							// broadcast it.
+							should_broadcast_commitment = false;


Shouldn't this move out of the else? If we see a holder commitment tx confirm in the above case we'll call check_spend_holder_transaction which will build the required spends of the commitment tx and we don't need to broadcast manually either (and if the thing that confirmed is our previous (unrevoked) commitment tx we'll be trying to double-spend).

Yeah, I hadn't considered a watchtower being able to broadcast as well.

TheBlueMatt · 2025-07-30T13:55:02Z

lightning/src/chain/channelmonitor.rs

+			if !retain && matches!(entry.event, OnchainEvent::AlternativeFundingConfirmation {})
+				&& self.holder_tx_signed
+			{
+				queue_new_commitment_claims = true;


Don't we need to queue new claims even if we've broadcasted without a AlternativeFundingConfirmation? I think in the case that (a) a new funding confirms, then (b) the channel closes (at which point we won't have an AlternativeFundingConfirmation because it only gets inserted when the funding confirms if we're closed then), then (c) we get a reorg of the new funding we need to broadcast but we wouldn't here.

Same in transaction_unconfirmed, and it might be nice to DRY that code up a bit.

Yeah, this is something I forgot to change after reworking AlternativeFundingConfirmation.

TheBlueMatt · 2025-07-30T13:57:54Z

lightning/src/chain/channelmonitor.rs

-		//- htlc update there as failure-trigger tx (revoked commitment tx, non-revoked commitment tx, HTLC-timeout tx) has been disconnected
-		//- maturing spendable output has transaction paying us has been disconnected
-		self.onchain_events_awaiting_threshold_conf.retain(|ref entry| entry.height < height);
-
 		let bounded_fee_estimator = LowerBoundedFeeEstimator::new(fee_estimator);
 		let conf_target = self.closure_conf_target();
 		self.onchain_tx_handler.block_disconnected(


Shouldn't this happen after we call self.cancel_prev_commitment_claims? That way we aren't RBF bumping spends of something that doesn't exist (though its not entirely clear to me that its an issue, but easier to avoid). Same in transactions_unconfirmed and might be nice to DRY these up.

Fixed, but I don't see a good way of DRYing this up very much.

lightning/src/chain/channelmonitor.rs

TheBlueMatt · 2025-07-30T14:45:20Z

lightning/src/chain/channelmonitor.rs

+						debug_assert!(self.alternative_funding_confirmed.is_none());
+						self
+							.pending_funding
+							.drain(..)


This looks like it needs to handle multiple incomplete 0conf fundings too, though - we can accept one 0conf splice then another before either confirm, or accept another after the first only has 1 conf. We'd need to handle that here by only dropping some fundings...

I kinda feel like we should just drop the last commit and do it later, rather than trying to support it right now.

Ah right, the key aspect here is that the zero conf splices chain upon one another, unlike RBFs.

Whether it's a splice, or a dual-funded RBF, we need to know which funding transaction out of all of the negotiated ones is currently confirmed in case we need to broadcast the holder commitment.

A `FundingScope` can only be promoted once a `ChannelMonitorUpdateStep::RenegotiatedFundingLocked` is applied, or if the monitor is no longer accepting updates, once the renegotiated funding transaction is no longer under reorg risk. Because of this, our current `FundingScope` may not reflect the latest confirmed state in the chain. Before making a holder commitment broadcast, we must check which `FundingScope` is currently confirmed to ensure that it can propogate throughout the network.

wpaulino added this to Weekly Goals Jul 17, 2025

ldk-reviews-bot requested a review from joostjager July 17, 2025 17:23

jkczyz mentioned this pull request Jul 17, 2025

Dual-funded channels and Splicing Project Tracking #1621

Open

7 tasks

wpaulino removed the request for review from joostjager July 17, 2025 21:00

wpaulino force-pushed the splice-funding-commitment-broadcast branch from a783444 to a8ae4b7 Compare July 18, 2025 22:23

wpaulino requested a review from TheBlueMatt July 18, 2025 22:23

wpaulino mentioned this pull request Jul 21, 2025

Introduce RenegotiatedFundingLocked monitor update variant #3894

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wpaulino force-pushed the splice-funding-commitment-broadcast branch from a8ae4b7 to 86b53fa Compare July 22, 2025 16:39

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wpaulino force-pushed the splice-funding-commitment-broadcast branch from 86b53fa to 48629c7 Compare July 22, 2025 23:15

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wpaulino force-pushed the splice-funding-commitment-broadcast branch from 48629c7 to 60d97d0 Compare July 24, 2025 00:10

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wpaulino force-pushed the splice-funding-commitment-broadcast branch 3 times, most recently from 582ad71 to 511fdc8 Compare July 29, 2025 19:39

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wpaulino added 2 commits July 30, 2025 11:27

Detect channel alternative funding transaction confirmation

5b32677

Whether it's a splice, or a dual-funded RBF, we need to know which funding transaction out of all of the negotiated ones is currently confirmed in case we need to broadcast the holder commitment.

wpaulino force-pushed the splice-funding-commitment-broadcast branch from 511fdc8 to 9051703 Compare July 30, 2025 18:27

wpaulino requested a review from TheBlueMatt July 30, 2025 18:30

Broadcast holder commitment for currently confirmed funding #3939

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Broadcast holder commitment for currently confirmed funding #3939

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wpaulino commented Jul 17, 2025 •

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