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Use HTLC CLTV instead of onion CLTV values for payment claim timer - #4402

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TheBlueMatt merged 6 commits into
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TheBlueMatt:2026-02-trampoline-cltv-consistency
Feb 26, 2026
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Use HTLC CLTV instead of onion CLTV values for payment claim timer#4402
TheBlueMatt merged 6 commits into
lightningdevkit:mainfrom
TheBlueMatt:2026-02-trampoline-cltv-consistency

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@TheBlueMatt

@TheBlueMattTheBlueMatt commented Feb 9, 2026

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When we receive an HTLC as a part of a claim, we validate that the
CLTV on the HTLC is >= the CLTV that the sender requested we
receive, but then we use the CLTV value that the sender requested
we receive as the deadline to claim the HTLC anyway.
This isn't generally all that interesting (they're always the same
unless the previous-hop node gave us "free CLTV"), but for
trampoline payments where we're both a trampoline hop and the
blinded intro point and the recipient, it means we end up allowing
ourselves less claim time than we actually have.
Instead, here, we just use the actual HTLC CLTV deadline.

Somewhat annoyingly built on #4373 because I needed to use some of the test infra that was built there and also now its hard to rebase :(

cc @carlaKC

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carlaKC reviewed Feb 10, 2026
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Rebased trampoline work on this and all's good except one assert that's in conflict with the instructions in TrampolineHop's docs. test_update_add_htlc_bolt2_sender_cltv_expiry_too_high also unhappy.

Removing that, everything I have passes with these changes and it saves me a few commits fixing up onion building 🧹

Comment threadlightning/src/ln/onion_utils.rs Outdated
} else {
cur_cltv
};
let cltv = hop.cltv_expiry_delta().saturating_add(cur_cltv);

@carlaKCcarlaKCFeb 9, 2026

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Can move this inside the if idx == 0 branch because we're only using it for final hops.
Perhaps also rename to outgoing_cltv_value?

@TheBlueMattTheBlueMattFeb 10, 2026

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Went with declared_incoming_cltv since that felt much closer to what its actually being used for.

Comment threadlightning/src/ln/onion_utils.rs
Comment threadlightning/src/routing/router.rs Outdated
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@TheBlueMattTheBlueMatt mentioned this pull request Feb 10, 2026
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@TheBlueMatt
TheBlueMattforce-pushed the 2026-02-trampoline-cltv-consistency branch from 5d10f6b to 610dc33CompareFebruary 10, 2026 13:08
@TheBlueMatt

TheBlueMatt commented Feb 10, 2026

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Fixed the broken assertion and one broken test and rebased, thanks!

carlaKC
carlaKC previously approved these changes Feb 10, 2026
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fixups lgtm!

@carlaKCcarlaKC mentioned this pull request Feb 12, 2026
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@TheBlueMatt
Use HTLC CLTV instead of onion CLTV values for payment claim timer
37e75e7
When we receive an HTLC as a part of a claim, we validate that the
CLTV on the HTLC is >= the CLTV that the sender requested we
receive, but then we use the CLTV value that the sender requested
we receive as the deadline to claim the HTLC anyway.
This isn't generally all that interesting (they're always the same
unless the previous-hop node gave us "free CLTV"), but for
trampoline payments where we're both a trampoline hop and the
blinded intro point and the recipient, it means we end up allowing
ourselves less claim time than we actually have.
Instead, here, we just use the actual HTLC CLTV deadline.
@TheBlueMatt
Fix trampoline onion encoding to match doc-declared CLTV rules
4867c30
The docs for `RouteHop::cltv_expiry_delta` claim that it includes
any trampoline hops, but the way we actually implemented onion
building it did not.
Because the docs described a simpler and more backwards-compatible
API, we update the onion-building logic to match rather than
updating the docs.
@TheBlueMatt
Clarify CLTV value selection in the first blinded hop marginally
ec8580b
@TheBlueMatt
Add a Path::total_cltv_expiry_delta accessor
5ce6e42
@TheBlueMatt
Validate CLTV somewhat in Route::debug_assert_route_meets_params
54be6ef
Now that we've cleaned up trampoline CLTV building and added
`Path::total_cltv_expiry_delta`, we can use both to do some basic
validation of CLTV values on blinded tails in
`Route::debug_assert_route_meets_params`
@TheBlueMatt
Rename starting_htlc_offsetcur_block_height in onion building
e39437d
Now that we are consistently using the
`RouteHop::cltv_expiry_delta` as the last hop's starting CLTV
rather than summing trampoline hops, `starting_htlc_offset` is a
bit confusing - its actually always the current block height. Thus,
here we rename it.
@TheBlueMatt
TheBlueMattforce-pushed the 2026-02-trampoline-cltv-consistency branch from 610dc33 to e39437dCompareFebruary 24, 2026 15:02
@TheBlueMatt

TheBlueMatt commented Feb 24, 2026

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Rebased, fixed rustfmt, and squashed.

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✅ Added second reviewer: @joostjager

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Codecov Report

❌ Patch coverage is 80.26316% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.88%. Comparing base (df57978) to head (e39437d).
⚠️ Report is 37 commits behind head on main.

Files with missing linesPatch %Lines
lightning/src/routing/router.rs60.60%13 Missing ⚠️
lightning/src/ln/onion_utils.rs93.10%2 Missing ⚠️
Additional details and impacted files
@@ Coverage Diff @@## main #4402 +/- ##
==========================================
- Coverage 85.89% 85.88% -0.02% 
==========================================
Files 159 159 Lines 104402 104448 +46 Branches 104402 104448 +46 ==========================================
+ Hits 89681 89706 +25 - Misses 12216 12235 +19 - Partials 2505 2507 +2
FlagCoverage Δ
tests85.88% <80.26%> (-0.02%)⬇️

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carlaKC
carlaKC approved these changes Feb 25, 2026
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Thanks for cleaning this up, saves me some commits down the line!

joostjager
joostjager approved these changes Feb 26, 2026
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Nothing blocking

Comment threadlightning/src/routing/router.rs
Comment threadlightning/src/routing/router.rs
Comment threadlightning/src/routing/router.rs
@TheBlueMatt

TheBlueMatt commented Feb 26, 2026

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Will address followups in a followup.

@TheBlueMatt
TheBlueMatt merged commit a6e6676 into lightningdevkit:mainFeb 26, 2026
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@TheBlueMattTheBlueMatt mentioned this pull request Feb 26, 2026
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TheBlueMatt added a commit that referenced this pull request Mar 2, 2026
@TheBlueMatt
Merge pull request#4448from TheBlueMatt/2026-02-trampoline-cltv-con…
b1e2be0
…sistency-followups
#4402 Followups
@bolty-from-atl-bitlabbolty-from-atl-bitlabBot mentioned this pull request Mar 7, 2026
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@joostjagerjoostjager mentioned this pull request Mar 23, 2026
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