Introduce `FundingTransactionReadyForSignatures` event #3889

dunxen · 2025-06-24T12:12:50Z

Cherry-picked from #3735 as it is relevant to splicing and will unblock testing after #3736 lands.

The FundingTransactionReadyForSignatures event requests witnesses from the client for their contributed inputs to an interactively constructed transaction.

The client calls ChannelManager::funding_transaction_signed to provide the witnesses to LDK.

ldk-reviews-bot · 2025-06-24T12:12:52Z

👋 Thanks for assigning @wpaulino as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

lightning/src/events/mod.rs

lightning/src/ln/channelmanager.rs

lightning/src/ln/interactivetxs.rs

codecov · 2025-06-25T11:08:53Z

Codecov Report

❌ Patch coverage is 71.78571% with 158 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.77%. Comparing base (0a23664) to head (abba0a7).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/ln/channelmanager.rs	17.89%	76 Missing and 2 partials ⚠️
lightning/src/ln/interactivetxs.rs	84.65%	54 Missing and 10 partials ⚠️
lightning/src/ln/channel.rs	69.56%	8 Missing and 6 partials ⚠️
lightning/src/events/mod.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3889      +/-   ##
==========================================
- Coverage   89.01%   88.77%   -0.25%     
==========================================
  Files         174      173       -1     
  Lines      124395   124755     +360     
  Branches   124395   124755     +360     
==========================================
+ Hits       110730   110749      +19     
- Misses      11187    11587     +400     
+ Partials     2478     2419      -59

Flag	Coverage Δ
fuzzing	`?`
tests	`88.77% <71.78%> (-0.07%)`	⬇️

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ldk-reviews-bot · 2025-06-26T12:13:27Z

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ldk-reviews-bot · 2025-06-26T12:13:29Z

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optout21

Looks good!

lightning/src/ln/channel.rs

ldk-reviews-bot · 2025-06-28T12:14:21Z

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ldk-reviews-bot · 2025-06-30T12:15:10Z

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jkczyz

Not sure if there was a problem rebasing, but some comments that had been marked resolved weren't fixed.

lightning/src/events/mod.rs

lightning/src/ln/channelmanager.rs

lightning/src/events/mod.rs

dunxen · 2025-07-01T08:08:04Z

Not sure if there was a problem rebasing, but some comments that had been marked resolved weren't fixed.

Yeah, they got lost on a rebase and somehow lost the commit. Rebased to get the one CI fix in. Fixing.

ldk-reviews-bot · 2025-07-02T12:15:51Z

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ldk-reviews-bot · 2025-07-05T00:01:03Z

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ldk-reviews-bot · 2025-07-07T00:01:47Z

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lightning/src/events/mod.rs

lightning/src/ln/channel.rs

lightning/src/ln/interactivetxs.rs

jkczyz

Need to review the last commit still.

lightning/src/events/mod.rs

lightning/src/ln/channel.rs

lightning/src/ln/channelmanager.rs

lightning/src/ln/channel.rs

lightning/src/ln/channelmanager.rs

lightning/src/ln/channel.rs

dunxen

Appreciate the review!

Reworking the PR now.

lightning/src/ln/interactivetxs.rs

lightning/src/ln/channel.rs

lightning/src/ln/channelmanager.rs

lightning/src/ln/interactivetxs.rs

dunxen

Still working through those not marked resolved. I'll re-request review when they're ready.

lightning/src/ln/channelmanager.rs

dunxen

Ok, it should be ready now.

lightning/src/ln/channel.rs

lightning/src/ln/channelmanager.rs

lightning/src/ln/interactivetxs.rs

The `FundingTransactionReadyForSignatures` event requests witnesses from the client for their contributed inputs to an interactively constructed transaction. The client calls `ChannelManager::funding_transaction_signed` to provide the witnesses to LDK. The `handle_channel_resumption` method handles resumption from both a channel re-establish and a monitor update. When the corresponding monitor update for the commitment_signed message completes, we will push the event here. We can thus only ever provide holder signatures after a monitor update has completed. We can also get rid of the reestablish code involved with `monitor_pending_tx_signatures` and remove that field too.

…hecks In a following commit, We'll use the contained scriptPubKeys to validate P2WPKH and P2TR key path spends and to assist in checking that signatures in provided holder witnesses use SIGHASH_ALL to prevent funds being frozen or held ransom.

LDK checks the following: * Each input spends an output that is one of P2WPKH, P2WSH, or P2TR. These were already checked by LDK when the inputs to be contributed were provided. * All signatures use the `SIGHASH_ALL` sighash type. * P2WPKH and P2TR key path spends are valid (verifies signatures) NOTE: * When checking P2WSH spends, LDK tries to decode 70-72 byte witness elements as ECDSA signatures with a sighash flag. If the internal DER-decoding fails, then LDK just assumes it wasn't a signature and carries with checks. If the element can be decoded as an ECDSA signature, the the sighash flag must be `SIGHASH_ALL`. * When checking P2TR script-path spends, LDK assumes all elements of exactly 65 bytes with the last byte matching any valid sighash flag byte are schnorr signatures and checks that the sighash type is `SIGHASH_ALL`. If the last byte is not any valid sighash flag, the element is assumed not to be a signature and is ignored. Elements of 64 bytes are not checked because if they were schnorr signatures then they would implicitly be `SIGHASH_DEFAULT` which is an alias of `SIGHASH_ALL`.

wpaulino · 2025-08-07T00:10:53Z

lightning/src/ln/channelmanager.rs

+	}
+
+	fn maybe_broadcast_interactive_funding(&self, channel: &mut FundedChannel<SP>) {
+		if let Some(ref funding_tx) = channel.funding.get_funding_transaction() {


Because of splicing, it won't always be the channel.funding transaction that needs to be broadcast. It's best to just return the transaction directly to the caller whenever we need to broadcast.

wpaulino · 2025-08-07T00:12:17Z

lightning/src/ln/channel.rs

-			if holder_tx_signatures_opt.is_some() {
-				self.context.channel_state.set_our_tx_signatures_ready();
+			if holder_tx_signatures_opt.is_none() {
+				return Ok((funding_tx_opt, None));


Looks like we can clean up the return flow here with below

wpaulino · 2025-08-07T00:13:47Z

lightning/src/ln/channel.rs

-			}
-			None
-		} else {
+		if signing_session.local_inputs_count() > 0 {


I don't think we need this either, we'd likely reuse the same flow as initiators for acceptors that contributed and need an event

dunxen added Dual-funding Splicing labels Jun 24, 2025

dunxen requested review from wpaulino, optout21 and jkczyz June 24, 2025 12:13

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dunxen mentioned this pull request Jun 25, 2025

Contribute funding inputs on accepting dual-funded channel #3735

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Introduce FundingTransactionReadyForSignatures event #3889

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Introduce FundingTransactionReadyForSignatures event #3889

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Introduce `FundingTransactionReadyForSignatures` event #3889

Introduce `FundingTransactionReadyForSignatures` event #3889

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