Merge pull request #4494 from jkczyz/2026-03-rbf-feerate

Ensure minimum RBF feerate satisfies BIP125

Author: TheBlueMatt

lightning/src/ln/channel.rs

@@ -6750,6 +6750,19 @@ fn get_v2_channel_reserve_satoshis(
 	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, at low feerates that multiplier doesn't always satisfy BIP125's relay requirement of
+/// an absolute fee increase, so we take the max of a flat +25 sat/kwu (0.1 sat/vB) increment
+/// and the spec's multiplicative rule. We still accept the bare 25/24 rule from counterparties
+/// in [`FundedChannel::validate_tx_init_rbf`].
+fn min_rbf_feerate(prev_feerate: u32) -> FeeRate {
+	let flat_increment = (prev_feerate as u64).saturating_add(25);
+	let spec_increment = ((prev_feerate as u64) * 25).div_ceil(24);
+	FeeRate::from_sat_per_kwu(cmp::max(flat_increment, spec_increment))
+}
+
 /// Context for negotiating channels (dual-funded V2 open, splicing)
 #[derive(Debug)]
 pub(super) struct FundingNegotiationContext {
@@ -12290,10 +12303,7 @@ where
 				prev_feerate.is_some(),
 				"pending_splice should have last_funding_feerate or funding_negotiation",
 			);
-			let min_rbf_feerate = prev_feerate.map(|f| {
-				let min_feerate_kwu = ((f as u64) * 25).div_ceil(24);
-				FeeRate::from_sat_per_kwu(min_feerate_kwu)
-			});
+			let min_rbf_feerate = prev_feerate.map(min_rbf_feerate);
 			let prior = if pending_splice.last_funding_feerate_sat_per_1000_weight.is_some() {
 				self.build_prior_contribution()
 			} else {
@@ -12385,10 +12395,7 @@ 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

@@ -218,8 +218,9 @@ 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 (the greater of
+/// the previous feerate + 25 sat/kwu and the spec's 25/24 rule). 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.
@@ -232,8 +233,9 @@ pub struct FundingTemplate {
 	/// transaction.
 	shared_input: Option<Input>,
 
-	/// The minimum RBF feerate (25/24 of the previous feerate), if this template is for an
-	/// RBF attempt. `None` for fresh splices with no pending splice candidates.
+	/// The minimum RBF feerate (the greater of previous feerate + 25 sat/kwu and the spec's
+	/// 25/24 rule), if this template is for an RBF attempt. `None` for fresh splices with no
+	/// pending splice candidates.
 	min_rbf_feerate: Option<FeeRate>,
 
 	/// The user's prior contribution from a previous splice negotiation, if available.
@@ -2262,8 +2264,8 @@ 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),
@@ -2276,7 +2278,7 @@ 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),
@@ -2359,8 +2361,8 @@ 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 {
@@ -2397,7 +2399,7 @@ 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);
@@ -2419,7 +2421,7 @@ 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);
@@ -2458,8 +2460,8 @@ 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 =