Use +25 sat/kwu increment for our minimum RBF feerate

The spec's 25/24 multiplier doesn't always satisfy BIP125's relay
requirement of an absolute fee increase. Use a flat +25 sat/kwu
(0.1 sat/vB) increment for our own RBFs instead, while still
accepting the 25/24 rule from counterparties.

Extract a `min_rbf_feerate` helper to consolidate the two call sites
and add a test that a counterparty feerate satisfying 25/24 (but not
+25) is accepted.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jkczyz

lightning/src/ln/channel.rs

@@ -6462,6 +6462,16 @@ fn get_v2_channel_reserve_satoshis(channel_value_satoshis: u64, dust_limit_satos
 	cmp::min(channel_value_satoshis, cmp::max(q, dust_limit_satoshis))
 }
 
+/// Returns the minimum feerate for our own RBF attempts given a previous feerate.
+///
+/// The spec (tx_init_rbf) requires the new feerate to be >= 25/24 of the previous feerate.
+/// However, that multiplier doesn't always satisfy BIP125's relay requirement of an absolute fee
+/// increase, so for our own RBFs we use a flat +25 sat/kwu (0.1 sat/vB) increment instead. We
+/// still accept the 25/24 rule from counterparties in [`FundedChannel::validate_tx_init_rbf`].
+fn min_rbf_feerate(prev_feerate: u32) -> FeeRate {
+	FeeRate::from_sat_per_kwu((prev_feerate as u64).saturating_add(25))
+}
+
 /// Context for negotiating channels (dual-funded V2 open, splicing)
 #[derive(Debug)]
 pub(super) struct FundingNegotiationContext {
@@ -11972,8 +11982,7 @@ where
 			//   slightly higher feerate than necessary. Call splice_channel again after
 			//   receiving SpliceFailed to get a fresh template without the RBF constraint.
 			let prev_feerate = negotiation.funding_feerate_sat_per_1000_weight();
-			let min_feerate_kwu = ((prev_feerate as u64) * 25).div_ceil(24);
-			(Some(FeeRate::from_sat_per_kwu(min_feerate_kwu)), None)
+			(Some(min_rbf_feerate(prev_feerate)), None)
 		} else {
 			// No RBF feerate to derive — either a fresh splice or a pending splice that
 			// can't be RBF'd (e.g., splice_locked already exchanged).
@@ -12057,10 +12066,7 @@ where
 		}
 
 		match pending_splice.last_funding_feerate_sat_per_1000_weight {
-			Some(prev_feerate) => {
-				let min_feerate_kwu = ((prev_feerate as u64) * 25).div_ceil(24);
-				Ok(FeeRate::from_sat_per_kwu(min_feerate_kwu))
-			},
+			Some(prev_feerate) => Ok(min_rbf_feerate(prev_feerate)),
 			None => Err(format!(
 				"Channel {} has no prior feerate to compute RBF minimum",
 				self.context.channel_id(),

lightning/src/ln/funding.rs

@@ -153,8 +153,8 @@ impl PriorContribution {
 /// prior contribution logic internally — reusing an adjusted prior when possible, re-running
 /// coin selection when needed, or creating a fee-bump-only contribution.
 ///
-/// Check [`FundingTemplate::min_rbf_feerate`] for the minimum feerate required (25/24 of
-/// the previous feerate). Use [`FundingTemplate::prior_contribution`] to inspect the prior
+/// Check [`FundingTemplate::min_rbf_feerate`] for the minimum feerate required (previous
+/// feerate + 25 sat/kwu). Use [`FundingTemplate::prior_contribution`] to inspect the prior
 /// contribution's parameters (e.g., [`FundingContribution::value_added`],
 /// [`FundingContribution::outputs`]) before deciding whether to reuse it via the RBF methods
 /// or build a fresh contribution with different parameters using the splice methods above.
@@ -167,7 +167,7 @@ pub struct FundingTemplate {
 	/// transaction.
 	shared_input: Option<Input>,
 
-	/// The minimum RBF feerate (25/24 of the previous feerate), if this template is for an
+	/// The minimum RBF feerate (previous feerate + 25 sat/kwu), if this template is for an
 	/// RBF attempt. `None` for fresh splices with no pending splice candidates.
 	min_rbf_feerate: Option<FeeRate>,
 
@@ -2161,8 +2161,8 @@ mod tests {
 		// When the caller's max_feerate is below the minimum RBF feerate, rbf_sync should
 		// return Err(()).
 		let prior_feerate = FeeRate::from_sat_per_kwu(2000);
-		let min_rbf_feerate = FeeRate::from_sat_per_kwu(5000);
-		let max_feerate = FeeRate::from_sat_per_kwu(3000);
+		let min_rbf_feerate = FeeRate::from_sat_per_kwu(2025);
+		let max_feerate = FeeRate::from_sat_per_kwu(2020);
 
 		let prior = FundingContribution {
 			value_added: Amount::from_sat(50_000),
@@ -2175,7 +2175,7 @@ mod tests {
 			is_splice: true,
 		};
 
-		// max_feerate (3000) < min_rbf_feerate (5000).
+		// max_feerate (2020) < min_rbf_feerate (2025).
 		let template = FundingTemplate::new(
 			None,
 			Some(min_rbf_feerate),
@@ -2255,8 +2255,8 @@ mod tests {
 		// When the prior contribution's feerate is below the minimum RBF feerate and no
 		// holder balance is available, rbf_sync should run coin selection to add inputs that
 		// cover the higher RBF fee.
-		let min_rbf_feerate = FeeRate::from_sat_per_kwu(5000);
 		let prior_feerate = FeeRate::from_sat_per_kwu(2000);
+		let min_rbf_feerate = FeeRate::from_sat_per_kwu(2025);
 		let withdrawal = funding_output_sats(20_000);
 
 		let prior = FundingContribution {
@@ -2293,7 +2293,7 @@ mod tests {
 	fn test_rbf_sync_no_prior_fee_bump_only_runs_coin_selection() {
 		// When there is no prior contribution (e.g., acceptor), rbf_sync should run coin
 		// selection to add inputs for a fee-bump-only contribution.
-		let min_rbf_feerate = FeeRate::from_sat_per_kwu(5000);
+		let min_rbf_feerate = FeeRate::from_sat_per_kwu(2025);
 
 		let template =
 			FundingTemplate::new(Some(shared_input(100_000)), Some(min_rbf_feerate), None);
@@ -2315,7 +2315,7 @@ mod tests {
 		// When the prior contribution's feerate is below the minimum RBF feerate and no
 		// holder balance is available, rbf_sync should use the caller's max_feerate (not the
 		// prior's) for the resulting contribution.
-		let min_rbf_feerate = FeeRate::from_sat_per_kwu(5000);
+		let min_rbf_feerate = FeeRate::from_sat_per_kwu(2025);
 		let prior_max_feerate = FeeRate::from_sat_per_kwu(50_000);
 		let callers_max_feerate = FeeRate::from_sat_per_kwu(10_000);
 		let withdrawal = funding_output_sats(20_000);
@@ -2354,8 +2354,8 @@ mod tests {
 		// When splice_out_sync is called on a template with min_rbf_feerate set (user
 		// choosing a fresh splice-out instead of rbf_sync), coin selection should NOT run.
 		// Fees come from the channel balance.
-		let min_rbf_feerate = FeeRate::from_sat_per_kwu(5000);
-		let feerate = FeeRate::from_sat_per_kwu(5000);
+		let min_rbf_feerate = FeeRate::from_sat_per_kwu(2025);
+		let feerate = FeeRate::from_sat_per_kwu(2025);
 		let withdrawal = funding_output_sats(20_000);
 
 		let template =