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Introduce RenegotiatedFunding monitor update variant #3822

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@wpaulino wpaulino commented Jun 2, 2025

This is a new ChannelMonitorUpdateStep variant intended to be used whenever a new funding transaction for the channel has been negotiated via the InteractiveTxConstructor. This commit primarily focuses on its use for splices, but future work will expand where needed to support RBFs (both for the initial dual funding transaction, and splice transactions).

To draw a parallel to channel open, we generally want to have the commitment transactions negotiated for the funding transaction and committed to the respective ChannelMonitor before attempting to sign the funding transaction itself. This monitor update fulfills this need for a newly negotiated splice; it includes both the new holder and counterparty commitment transactions, and the new set of applicable ChannelTransactionParameters. Once the monitor update has been applied to the monitor and persisted, we allow the release of our tx_signatures for the splice transaction to wait for its confirmation.

Depends on #3774.

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ldk-reviews-bot commented Jun 2, 2025

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

@wpaulino wpaulino requested a review from jkczyz June 2, 2025 23:53
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codecov bot commented Jun 3, 2025

Codecov Report

Attention: Patch coverage is 33.89831% with 117 lines in your changes missing coverage. Please review.

Project coverage is 89.76%. Comparing base (0848e7a) to head (689d0c1).
Report is 32 commits behind head on main.

Files with missing lines Patch % Lines
lightning/src/chain/channelmonitor.rs 10.31% 113 Missing ⚠️
lightning/src/chain/chainmonitor.rs 83.33% 3 Missing and 1 partial ⚠️
Additional details and impacted files
@@            Coverage Diff             @@
##             main    #3822      +/-   ##
==========================================
- Coverage   89.88%   89.76%   -0.12%     
==========================================
  Files         160      162       +2     
  Lines      129654   130586     +932     
  Branches   129654   130586     +932     
==========================================
+ Hits       116534   117215     +681     
- Misses      10425    10671     +246     
- Partials     2695     2700       +5     

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@wpaulino wpaulino force-pushed the renegotiated-funding-monitor-update branch from 6c78591 to ac8c8c5 Compare June 9, 2025 00:35
@wpaulino wpaulino requested a review from jkczyz June 9, 2025 00:35
@tankyleo tankyleo self-requested a review June 9, 2025 19:55
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First pass in channel monitor

@wpaulino wpaulino force-pushed the renegotiated-funding-monitor-update branch from ac8c8c5 to 5f0709f Compare June 10, 2025 18:58
@wpaulino wpaulino requested a review from TheBlueMatt June 10, 2025 18:58
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🔔 1st Reminder

Hey @TheBlueMatt @jkczyz! 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.

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stared at the code again today, picked up nits on the way.

@@ -1677,7 +1677,33 @@ impl<SP: Deref> Channel<SP> where
res
},
ChannelPhase::Funded(mut funded_channel) => {
let res = funded_channel.commitment_signed(msg, logger).map(|monitor_update_opt| (None, monitor_update_opt));
#[cfg(splicing)]
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Sheesh, is it too late to make this a new damn message? We now have three totally separate handling logic for commitment_signed, and this one doesn't even make any sense - we're receiving a single commitment_signed but we actually need two if we were in a normal state...

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Sheesh, is it too late to make this a new damn message?

Probably not? Would be a trivial thing to change, but it would basically be a copy of the commitment_signed message.

this one doesn't even make any sense - we're receiving a single commitment_signed but we actually need two if we were in a normal state...

I'm not following, what do you mean by "normal state"? This is handling the case where we haven't finished negotiating a splice transaction. We expect a single commitment_signed to be exchanged for the alternative commitments at the current commitment number. After the negotiation finishes, if we're expecting a batch and they didn't send it, we'll fail the channel. If we're not expecting a batch and they did send it, we'll also fail the channel.

pending_splice_funding, &self.holder_commitment_point, msg, logger,
)?;

let counterparty_commitment_tx = self.context.build_commitment_transaction(
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Bleh, we already build the commitment tx in validate_commitment_signed, lets not do it again.

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We need to build both, validate_commitment_signed builds the holder one, this builds the counterparty one

// There may be a pending monitor update for a `revoke_and_ack` or preimage we have to
// handle first.
} else {
self.monitor_updating_paused(false, false, false, Vec::new(), Vec::new(), Vec::new());
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Note that you can call monitor_updating_paused even if its already paused.

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I figured, just seemed redundant. Do you want it to be called anyway?

This is a new `ChannelMonitorUpdateStep` variant intended to be used
whenever a new funding transaction for the channel has been negotiated
via the `InteractiveTxConstructor`. This commit primarily focuses on its
use for splices, but future work will expand where needed to support
RBFs (both for the initial dual funding transaction, and splice
transactions).

To draw a parallel to channel open, we generally want to have the
commitment transactions negotiated for the funding transaction and
committed to the respective `ChannelMonitor` before attempting to sign
the funding transaction itself. This monitor update fulfills this need
for a newly negotiated splice; it includes both the new holder
and counterparty commitment transactions, and the new set of applicable
`ChannelTransactionParameters`. Once the monitor update has been applied
to the monitor and persisted, we allow the release of our
`tx_signatures` for the splice transaction to wait for its confirmation.
@wpaulino wpaulino force-pushed the renegotiated-funding-monitor-update branch from 5f0709f to 689d0c1 Compare June 11, 2025 17:04
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5 participants