Batch `commitment_signed` messages for splicing #3651

jkczyz · 2025-03-06T22:57:28Z

Once a channel is funded, it may be spliced to add or remove funds. The new funding transaction is pending until confirmed on chain and thus needs to be tracked. Additionally, it may be replaced by another transaction using RBF with a higher fee. Hence, there may be more than one pending FundingScope to track for a splice.

This PR adds support for tracking pending funding scopes and accounting for any pending scopes where applicable (e.g., when handling and sending commitment_signed messages).

ldk-reviews-bot · 2025-03-06T22:57:31Z

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jkczyz · 2025-03-06T22:59:04Z

@wpaulino Just looking for a quick concept ACK. Still needed:

serialization of pending_funding, presumably?
tests for commitment_signed
update get_available_balances

wpaulino

Yeah this makes sense. We'll need to support sending a commitment_signed for each scope as well.

lightning/src/ln/channel.rs

ldk-reviews-bot · 2025-03-07T19:03:15Z

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jkczyz · 2025-03-07T23:16:52Z

Yeah this makes sense. We'll need to support sending a commitment_signed for each scope as well.

By this do you mean we'll need msgs::CommitmentUpdate to contain a Vec of msgs::CommitmentSigned messages instead of a single one?

jkczyz · 2025-03-07T23:23:17Z

Pushed another commit for get_available_balances.

wpaulino · 2025-03-10T17:13:50Z

By this do you mean we'll need msgs::CommitmentUpdate to contain a Vec of msgs::CommitmentSigned messages instead of a single one?

Yeah we'll need to go through each case where we owe the counterparty a commitment_signed (except for the initial one sent in dual funding/splicing) and make sure we always consider all scopes.

lightning/src/ln/channel.rs

jkczyz · 2025-03-12T18:24:46Z

Yeah we'll need to go through each case where we owe the counterparty a commitment_signed (except for the initial one sent in dual funding/splicing) and make sure we always consider all scopes.

Pushed a couple commits that I think accomplish this. Though I'm not sure about the following line from build_commitment_no_status_check:

rust-lightning/lightning/src/ln/channel.rs

Lines 8621 to 8622 in c66e554

    
           let (mut htlcs_ref, counterparty_commitment_tx) = 
        
           	self.build_commitment_no_state_update(&self.funding, logger);

It is called from methods like send_htlc_and_commit but for producing a ChannelMonitorUpdate. IIUC, I'll need to do this for all funding scopes?

lightning/src/ln/channel.rs

jkczyz · 2025-03-12T22:47:25Z

Rebased on main.

lightning/src/ln/channel.rs

ldk-reviews-bot · 2025-03-19T15:45:27Z

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jkczyz · 2025-03-19T21:48:16Z

Responded and addressed a couple lingering comments.

wpaulino · 2025-04-02T22:13:55Z

This is pretty much there, will give it a final pass once squashed and rebased

jkczyz · 2025-04-02T23:40:37Z

Rebased and fixed fuzz tests (compilation error and log assertion update).

jkczyz · 2025-04-03T00:02:24Z

Rebased again to resolve a couple merge conflicts and squashed.

jkczyz · 2025-04-03T00:16:41Z

Looks like CI was failing because the benchmarks failed to compile. Fixed in the latest push.

lightning/src/ln/channel.rs

TheBlueMatt · 2025-04-03T15:14:20Z

Oops already needs rebase.

Once a channel is funded, it may be spliced to add or remove funds. The new funding transaction is pending until confirmed on chain and thus needs to be tracked. Additionally, it may be replaced by another transaction using RBF with a higher fee. Hence, there may be more than one pending FundingScope to track for a splice. This commit adds support for tracking pending funding scopes. The following commits will account for any pending scopes where applicable (e.g., when handling commitment_signed).

A FundedChannel may have more than one pending FundingScope during splicing, one for the splice attempt and one or more for any RBF attempts. When this is the case, send a commitment_signed message for each FundingScope and include the necessary batch information (i.e., batch_size and funding_txid) to the counterparty.

Splicing introduces a concept of batched commitment_signed messages for each pending splice transaction. These can be treated as one logical message, even though the protocol currently defines them as separate commitment_signed messages with a TLV for batch information. Add a LogicalMessage wrapper around wire::Message such that it can be used internally by PeerManager. A CommitmentSignedBatch variant will be added in the next commit.

During splicing, commitment_signed messages need to be collected into a single batch before they are handled. Rather than including this as part of the channel state machine logic, batch when reading messages from the wire since they can be considered one logical message.

A FundedChannel may have more than one pending FundingScope during splicing, one for the splice attempt and one or more for any RBF attempts. The counterparty will send a commitment_signed message for each pending splice transaction and the current funding transaction. Defer handling these commitment_signed messages until the entire batch has arrived. Then validate them individually, also checking if all the pending splice transactions and the current funding transaction have a corresponding commitment_signed in the batch.

A FundedChannel may have more than one pending FundingScope during splicing, one for the splice attempt and one or more for any RBF attempts. When calling get_available_balances, consider all funding scopes and take the minimum by next_outbound_htlc_limit_msat. This is used both informationally and to determine which channel to use to forward an HTLC. The choice of next_outbound_htlc_limit_msat is somewhat arbitrary but matches the field used when determining which channel used to forward an HTLC. Any field should do since each field should be adjusted by the same amount relative to another FundingScope given the nature of the fields (i.e., inbound/outbound capacity, min/max HTLC limit). Using the minimum was chosen since an order for an HTLC to be sent over the channel, it must be possible for each funding scope -- both the confirmed one and any pending scopes, one of which may eventually confirm.

jkczyz

Rebased to resolve merge conflict.

lightning/src/ln/channel.rs

wpaulino · 2025-04-04T18:34:49Z

Merging this as the linting failure has already been fixed separately.

jkczyz requested a review from wpaulino March 6, 2025 22:59

wpaulino reviewed Mar 7, 2025

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jkczyz force-pushed the 2025-03-multiple-funding-scopes branch from eac7be9 to 8b4e46a Compare March 11, 2025 22:44

jkczyz changed the title ~~Add pending funding scopes to FundedChannel~~ Batch commitment_signed messages for splicing Mar 11, 2025

wpaulino reviewed Mar 12, 2025

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jkczyz force-pushed the 2025-03-multiple-funding-scopes branch from c66e554 to 2db5c60 Compare March 12, 2025 22:34

jkczyz marked this pull request as ready for review March 12, 2025 22:35

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jkczyz force-pushed the 2025-03-multiple-funding-scopes branch from e371143 to 0362159 Compare March 18, 2025 23:01

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jkczyz force-pushed the 2025-03-multiple-funding-scopes branch from 0362159 to 7021e01 Compare March 19, 2025 21:45

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Remove data dependency on OnchainTxHandler from onchain claims #3690

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jkczyz force-pushed the 2025-03-multiple-funding-scopes branch from 1695c74 to 1456a7d Compare April 2, 2025 23:40

jkczyz force-pushed the 2025-03-multiple-funding-scopes branch from 1456a7d to 19e8b5c Compare April 3, 2025 00:01

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jkczyz force-pushed the 2025-03-multiple-funding-scopes branch from 19e8b5c to cbd7bf8 Compare April 3, 2025 00:15

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jkczyz added 7 commits April 3, 2025 12:48

Remove unnecessary drain

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jkczyz dismissed TheBlueMatt’s stale review via 80e3235 April 3, 2025 17:56

jkczyz force-pushed the 2025-03-multiple-funding-scopes branch from cbd7bf8 to 80e3235 Compare April 3, 2025 17:56

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wpaulino merged commit c4d23bc into lightningdevkit:main Apr 4, 2025
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jkczyz mentioned this pull request Jun 21, 2025

Dual-funded channels and Splicing Project Tracking #1621

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Batch commitment_signed messages for splicing #3651

Batch commitment_signed messages for splicing #3651

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Batch `commitment_signed` messages for splicing #3651

Batch `commitment_signed` messages for splicing #3651

jkczyz commented Mar 6, 2025 •

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