Splicing / Dual Funding

[jkczyz]

Splicing launched as beta in LDK 0.2 and LDK Node 0.7. Full splicing support involves completing the tasks below. This follows on the work detailed in #1621.

Core Splice Features

• Rework ChannelManager::funding_transaction_signed Rework ChannelManager::funding_transaction_signed #4257 Rework ChannelManager::funding_transaction_signed #4336
• Split splice initiation into two phases Split splice initiation into two phases #4290
• Mixed mode splicing (splice-in + splice-out) Mixed mode splicing #4261
• Opportunistic contributions (contribute to splice as acceptor) Contribute to splice as acceptor #4416
• Split DiscardFunding from SpliceFailed event Split DiscardFunding from SpliceFailed event #4388

RBF Support

• RBF splice funding transactions (initiating + accepting) RBF splice funding transactions #4427
• Unify splice and RBF APIs Unify splice and RBF APIs #4486
• Use +25 sat/kwu increment for our minimum RBF feerate Ensure minimum RBF feerate satisfies BIP125 #4494
• Cap number of RBFs (commitment_signed batch only allows for 20 anyway) ref
• Filter out contributed inputs that also exist in previous negotiated-but-not-yet-locked splice attempts ref

Cancellation

• Allow cancellation of pending splice funding negotiations Allow cancellation of pending splice funding negotiations #4490
• Exit quiescence when splice_init and tx_init_rbf are rejected Exit quiescence when splice_init and tx_init_rbf are rejected #4495

Async Signing

• Support async signing of interactive-tx initial commitment signatures Support async signing of interactive-tx initial commitment signatures #4355
• Hold in-flight monitor updates until background event processing Hold in-flight monitor updates until background event processing #4377
• Release tx_signatures after async monitor update completes Release tx_signatures after async monitor update completes #4472

On-chain Monitoring

• Closed channel balance API ref
• Produce FundingInfo::Contribution variants in ChannelMonitor Produce FundingInfo::Contribution variants in ChannelMonitor #4498

Feature Flags

• Clarify splicing feature flag requirements Clarify splicing feature flag requirements #4274
• Switch SplicePrototype feature flag to the prod feature bit Switch SplicePrototype feature flag to the prod feature bit #4387

Bug Fixes & Correctness

• Use SignedAmount::unsigned_abs to avoid overflow Use SignedAmount::unsigned_abs to avoid overflow #4383
• Account for missing balance in splice max commitment output tracking Account for missing balance in splice max commitment output tracking #4417
• Check that funder covers the fee spike buffer multiple after a splice Check that funder covers the fee spike buffer multiple after a splice #4433
• Avoid sending stfu for quiescent splice action while pending splice Avoid sending stfu for quiescent splice action while pending splice #4426
• SCID preference for outbound payments on spliced channel fix: SCID preference for outbound payments on spliced channel #4376

Quiescence

• Free holding cell in remaining quiescence-exit code paths Free holding cell in remaining quiescence-exit code paths #4415
• Merge initial and retry stfu send paths Merge initial and retry stfu send paths #4432

Testing & Fuzzing

• Extend full_stack fuzz test to cover splicing Extend full_stack fuzz test to cover splicing #4361
• Drive splices to completion in chanmon_consistency Drive splices to completion in chanmon_consistency #4411
• Re-enable signer op for one channel at a time in chanmon_consistency Re-enable signer op for one channel at a time in chanmon_consistency #4489
• Handle missing SendTx* message events in chanmon_consistency fuzz: handle missing SendTx* message events in chanmon_consistency #4496

Infrastructure & Cleanup

• Rustfmt ChannelManager splice message handlers Rustfmt ChannelManager splice message handlers #4412
• Refactor CoinSelection-related types to wallet_utils module Refactor CoinSelection-related types to wallet_utils module #4382
• Refactor BroadcasterInterface to include TransactionType Refactor BroadcasterInterface to include TransactionType #4353

Remaining

• 0-conf — track past funding scopes until mature
• Integrate dual-funding Contribute funding inputs on accepting dual-funded channel #3735
• Sweep repo for TODO(splicing) and TODO(dual_funding)