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-tests/src/upgrade_downgrade_tests.rs

@@ -457,7 +457,8 @@ fn do_test_0_1_htlc_forward_after_splice(fail_htlc: bool) {
 		script_pubkey: nodes[0].wallet_source.get_change_script().unwrap(),
 	}];
 	let channel_id = ChannelId(chan_id_bytes_a);
-	let funding_contribution = initiate_splice_out(&nodes[0], &nodes[1], channel_id, outputs);
+	let funding_contribution =
+		initiate_splice_out(&nodes[0], &nodes[1], channel_id, outputs).unwrap();
 	let (splice_tx, _) = splice_channel(&nodes[0], &nodes[1], channel_id, funding_contribution);
 	for node in nodes.iter() {
 		mine_transaction(node, &splice_tx);

lightning/src/ln/async_signer_tests.rs

@@ -1576,7 +1576,7 @@ fn test_async_splice_initial_commit_sig() {
 		value: Amount::from_sat(1_000),
 		script_pubkey: nodes[0].wallet_source.get_change_script().unwrap(),
 	}];
-	let contribution = initiate_splice_out(initiator, acceptor, channel_id, outputs);
+	let contribution = initiate_splice_out(initiator, acceptor, channel_id, outputs).unwrap();
 	negotiate_splice_tx(initiator, acceptor, channel_id, contribution);
 
 	assert!(initiator.node.get_and_clear_pending_msg_events().is_empty());

lightning/src/ln/channel.rs

@@ -5469,6 +5469,7 @@ impl<SP: SignerProvider> ChannelContext<SP> {
 				} else {
 					1
 				};
+			// Note that the feerate is 0 in zero-fee commitment channels, so this statement is a noop
 			let spiked_feerate = feerate * fee_spike_multiple;
 			let (remote_stats, _remote_htlcs) = self
 				.get_next_remote_commitment_stats(
@@ -12401,14 +12402,23 @@ where
 		// We are not interested in dust exposure
 		let dust_exposure_limiting_feerate = None;
 
+		// Note that the feerate is 0 in zero-fee commitment channels, so this statement is a noop
+		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_stats, _local_htlcs) = 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 exhausted on local commitment")?;
@@ -12420,7 +12430,7 @@ 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 exhausted on remote commitment")?;