Prefactor for Trampoline MPP accumulation by carlaKC · Pull Request #4510 · lightningdevkit/rust-lightning

carlaKC · 2026-03-24T17:16:16Z

This PR contains a set of refactors pulled out of #4493:

Extract common timeout / MPP logic into separate functions
Allow construction of blinded trampoline paths
Misc constructors + validation

ldk-reviews-bot · 2026-03-24T17:16:20Z

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ldk-claude-review-bot · 2026-03-24T17:27:51Z

I've completed a thorough review of every file and hunk in this PR diff, including reading source code for context around all major changes. I examined:

blinded_path/payment.rs: ForwardTlvsInfo trait, ForwardNode<F> generics, new_for_trampoline constructor, generic blinded_hops and compute_payinfo functions. All trait accessor methods correctly delegate to the corresponding struct fields.
blinded_payment_tests.rs: Replacement of create_blinded_tail with create_trampoline_forward_blinded_tail (verified the new function exists in functional_test_utils.rs with matching parameters).
channelmanager.rs (bulk of the PR):
- MppPart extraction and HasMppPart trait: Field mapping from ClaimableHTLC to MppPart is correct throughout.
- check_mpp_timeout: Completion condition (total_intended_recvd_value >= total_mpp_value) correctly mirrors check_claimable_incoming_htlc. Minor difference in timer tick increment order vs old any() short-circuiting, but has no observable effect since timed-out HTLCs are all drained.
- check_claimable_incoming_htlc: Logic matches old check_total_value! macro.
- handle_claimable_htlc: committed_to_claimable tracking preserved through Err(bool) return.
- fail_htlc! macro refactor: htlc_source is correctly pre-constructed before claimable_htlc is moved; all field mappings verified against old macro.
- claim_funds_from_hop signature change (HTLCPreviousHopData → &HTLCPreviousHopData): Method only reads fields (all Copy), never moves the struct. Both call sites already pass references.
- check_incoming_htlc_cltv: MIN_CLTV_EXPIRY_DELTA parameter correctly passed at both call sites; was previously hardcoded inside the function.
- previous_hop_data() method: Uses core::slice::from_ref correctly.
- Serialization: TLV tags unchanged (0, 1, 2, 3, 4, 5, 6, 8, 10). Read/write functions correctly map through mpp_part. Backwards compatibility maintained.
- PendingHTLCRouting::TrampolineForward: Field rename incoming_shared_secret → trampoline_shared_secret with TLV tag 0 unchanged. trampoline_shared_secret() accessor updated to handle this variant.

No issues found.

codecov · 2026-03-24T18:25:27Z

Codecov Report

❌ Patch coverage is 91.08527% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.19%. Comparing base (ab31f99) to head (f15271e).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/ln/channelmanager.rs	94.36%	12 Missing ⚠️
lightning/src/blinded_path/payment.rs	75.60%	9 Missing and 1 partial ⚠️
lightning/src/ln/onion_payment.rs	75.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #4510    +/-   ##
========================================
  Coverage   86.19%   86.19%            
========================================
  Files         160      160            
  Lines      107537   107713   +176     
  Branches   107537   107713   +176     
========================================
+ Hits        92693    92848   +155     
- Misses      12220    12238    +18     
- Partials     2624     2627     +3

Flag	Coverage Δ
tests	`86.19% <91.08%> (+<0.01%)`	⬆️

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ldk-reviews-bot · 2026-03-28T00:00:34Z

🔔 1st Reminder

Hey @valentinewallace! 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.

ldk-reviews-bot · 2026-03-30T00:01:16Z

🔔 2nd Reminder

Hey @valentinewallace! 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.

carlaKC · 2026-03-30T12:30:15Z

Some discussion in the dependent PR, taking @valentinewallace off as a reviewer for now to save some bot-spam.

Pull out all fields that are common to incoming claimable and trampoline MPP HTLCs. This will be used in future commits to accumulate MPP HTLCs that are part of trampoline forwards - we can't claim these, but need to accumulate them in the same way as receives before forwarding onwards.

We'll use this shared logic when we need to timeout trampoline HTLCs. Note that there's a slight behavior change in this commit. Previously, we'd do a first pass to check out total received value and return early if we'd reached it without applying a MPP tick to any HTLC. Now, we'll apply the MPP tick as we accumulate our total value received. This does not make any difference, because we never MPP-timeout fully accumulated MPP payments so it doesn't matter if we've applied the tick when we've reached our full amount.

We'll re-use this to check trampoline MPP timeout in future commits.

In the commit that follows we're going to need to take ownership of our htlc before this macro is used, so we pull out the information we need in advance.

We're going to use the same logic for trampoline and for incoming MPP payments, so we pull this out into a separate function.

To allow re-use with trampoline payments which won't use the ClaimablePayment type, make handling generic for anything with MPP parts.

For trampoline payments, we don't want to enforce a minimum cltv delta between our incoming and outer onion outgoing CLTV because we'll calculate our delta from the inner trampoline onion's value. However, we still want to check that we get at least the CLTV that the sending node intended for us and we still want to validate our incoming value. Refactor to allow setting a zero delta, for use for trampoline payments.

To use helper functions for either trampoline or regular paths.

To create trampoline forwarding and single hop receiving tails.

carlaKC force-pushed the 2299-mpp-prefactor branch from 40f75a4 to f15271e Compare March 24, 2026 17:19

ldk-reviews-bot requested a review from wpaulino March 24, 2026 17:26

valentinewallace requested review from valentinewallace and removed request for wpaulino March 25, 2026 15:57

carlaKC added this to Weekly Goals Mar 26, 2026

jkczyz assigned carlaKC Mar 26, 2026

carlaKC removed the request for review from valentinewallace March 30, 2026 12:30

carlaKC added 12 commits March 30, 2026 16:23

ln/refactor: add previous_hop_data helper for HTLCSource

1ddd91c

ln/refactor: rename shared secret and populate in HTLCPreviousHopData

bc2d6c7

ln/refactor: move on chain timeout check into claimable htlc

d025de8

We'll re-use this to check trampoline MPP timeout in future commits.

ln/refactor: remove claimable htlc from fail_htlc macro

04c5565

In the commit that follows we're going to need to take ownership of our htlc before this macro is used, so we pull out the information we need in advance.

ln/refactor: move checks on incoming mpp accumulation into method

cc5ea6e

We're going to use the same logic for trampoline and for incoming MPP payments, so we pull this out into a separate function.

ln/refactor: introduce HasMppPart generic to share incoming mpp

91a9066

To allow re-use with trampoline payments which won't use the ClaimablePayment type, make handling generic for anything with MPP parts.

blinded_path/refactor: make construction generic over forwarding type

1287117

To use helper functions for either trampoline or regular paths.

blinded_path: add constructor for trampoline blinded path

3360f8d

ln/test: add multi-purpose trampoline test helper

6aae0dd

To create trampoline forwarding and single hop receiving tails.

carlaKC force-pushed the 2299-mpp-prefactor branch from f15271e to 6aae0dd Compare April 1, 2026 21:24

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Prefactor for Trampoline MPP accumulation#4510

Prefactor for Trampoline MPP accumulation#4510
carlaKC wants to merge 12 commits intolightningdevkit:mainfrom
carlaKC:2299-mpp-prefactor

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