Ensure minimum RBF feerate satisfies BIP125

[jkczyz]

The spec's 25/24 multiplier doesn't always satisfy BIP125's relay requirement of an absolute fee increase at low feerates, while a flat +25 sat/kwu increment falls below the spec's 25/24 rule above 600 sat/kwu. Use max(prev + 25, ceil(prev * 25/24)) for our own RBFs to satisfy both constraints, while still accepting the bare 25/24 rule from counterparties.

Potential spec change in lightning/bolts#1327.

[ldk-reviews-bot]

👋 Thanks for assigning @TheBlueMatt 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.

[jkczyz]

Will need for proper integration testing of LDK Node's rbf_channel API.

[ldk-claude-review-bot]

I have thoroughly reviewed the entire PR diff. All three issues from my prior review have been addressed in this revision:

1. min_rbf_feerate now uses max(prev + 25, ceil(prev * 25/24)) — correctly satisfying both BIP125 relay and the spec's 25/24 rule at all feerates.
2. Doc comments on FundingTemplate (lines 221-222, 236-238) accurately describe the dual-rule formula.
3. Test comment at splicing_tests.rs:6050-6051 now correctly shows the max() computation at 10,000 sat/kwu.

The new test test_splice_rbf_at_high_feerate properly exercises the crossover region (1000 sat/kwu, where 25/24 dominates). The new acceptor-side test block in test_splice_rbf_insufficient_feerate correctly validates that a counterparty feerate satisfying only the bare 25/24 rule (264) is accepted even though our own floor is higher (278).

No issues found.

[codecov]

Codecov Report

❌ Patch coverage is 94.44444% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 87.12%. Comparing base (8d00139) to head (1ff1bb4).
⚠️ Report is 117 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/ln/channel.rs	90.90%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4494      +/-   ##
==========================================
+ Coverage   86.24%   87.12%   +0.88%     
==========================================
  Files         160      163       +3     
  Lines      107909   108740     +831     
  Branches   107909   108740     +831     
==========================================
+ Hits        93061    94735    +1674     
+ Misses      12212    11520     -692     
+ Partials     2636     2485     -151     

Flag	Coverage Δ
fuzzing	40.21% <9.09%> (?)
tests	86.21% <94.44%> (-0.03%)	⬇️

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[wpaulino]

Needs a squash. Can we move forward with this yet or is there still more spec discussion to be had?

[jkczyz]

Needs a squash. Can we move forward with this yet or is there still more spec discussion to be had?

Squashed. Yeah, we can move forward. We still enforce the spec as it is currently written against our counterparty.