Check that funder covers the fee spike buffer multiple after a splice

We do this for HTLCs, so we should also do this for splices. This
applies to `only_static_remote_key` channels alone.

Author: tankyleo

lightning/src/ln/channel.rs

@@ -5960,7 +5960,9 @@ impl<SP: SignerProvider> ChannelContext<SP> {
 				max_reserved_commit_tx_fee_msat as i64;
 			if capacity_minus_commitment_fee_msat < (real_dust_limit_timeout_sat as i64) * 1000 {
 				let one_htlc_difference_msat = max_reserved_commit_tx_fee_msat - min_reserved_commit_tx_fee_msat;
-				debug_assert!(one_htlc_difference_msat != 0);
+				if !funding.get_channel_type().supports_anchor_zero_fee_commitments() {
+					debug_assert!(one_htlc_difference_msat != 0);
+				}
 				capacity_minus_commitment_fee_msat += one_htlc_difference_msat as i64;
 				capacity_minus_commitment_fee_msat = cmp::min(real_dust_limit_timeout_sat as i64 * 1000 - 1, capacity_minus_commitment_fee_msat);
 				available_capacity_msat = cmp::max(0, cmp::min(capacity_minus_commitment_fee_msat, available_capacity_msat as i64)) as u64;
@@ -12720,20 +12722,34 @@ where
 		&self, funding: &FundingScope,
 	) -> Result<(Amount, Amount), String> {
 		let include_counterparty_unknown_htlcs = true;
-		// Make sure that that the funder of the channel can pay the transaction fees for an additional
-		// nondust HTLC on the channel.
-		let addl_nondust_htlc_count = 1;
 		// We are not interested in dust exposure
 		let dust_exposure_limiting_feerate = None;
 
+		let addl_nondust_htlc_count =
+			if funding.get_channel_type().supports_anchor_zero_fee_commitments() {
+				0
+			} else {
+				// Require the channel opener to reserve enough funds to pay the fees for an
+				// additional non-dust HTLC in the channel.
+				1
+			};
+
+		let feerate_per_kw = if !funding.get_channel_type().supports_anchors_zero_fee_htlc_tx() {
+			// Similar to HTLC additions, require the funder to have enough funds reserved for
+			// fees such that the feerate can jump without rendering the channel useless.
+			self.context.feerate_per_kw * FEE_SPIKE_BUFFER_FEE_INCREASE_MULTIPLE as u32
+		} else {
+			self.context.feerate_per_kw
+		};
+
 		let local_commitment_stats = self
 			.context
 			.get_next_local_commitment_stats(
 				funding,
 				None, // htlc_candidate
 				include_counterparty_unknown_htlcs,
 				addl_nondust_htlc_count,
-				self.context.feerate_per_kw,
+				feerate_per_kw,
 				dust_exposure_limiting_feerate,
 			)
 			.map_err(|()| "Balance after HTLCs and anchors exhausted on local commitment")?;
@@ -12749,7 +12765,7 @@ where
 				None, // htlc_candidate
 				include_counterparty_unknown_htlcs,
 				addl_nondust_htlc_count,
-				self.context.feerate_per_kw,
+				feerate_per_kw,
 				dust_exposure_limiting_feerate,
 			)
 			.map_err(|()| "Balance after HTLCs and anchors exhausted on remote commitment")?;