ln: do not set or accept non-zero feerates in open_channel for v3

Sender: MUST set `feerate_per_kw` to zero
Receiver: MUST fail the channel if `feerate_per_kw` != 0

Co-authored-by: Matt Corallo <git@bluematt.me>

Author: carlaKC

lightning/src/ln/channel.rs

@@ -3036,12 +3036,18 @@ impl<SP: Deref> ChannelContext<SP> where SP::Target: SignerProvider {
 		debug_assert!(!channel_type.supports_any_optional_bits());
 		debug_assert!(!channel_type.requires_unknown_bits_from(&channelmanager::provided_channel_type_features(&config)));
 
-		let (commitment_conf_target, anchor_outputs_value_msat)  = if channel_type.supports_anchors_zero_fee_htlc_tx() {
-			(ConfirmationTarget::AnchorChannelFee, ANCHOR_OUTPUT_VALUE_SATOSHI * 2 * 1000)
-		} else {
-			(ConfirmationTarget::NonAnchorChannelFee, 0)
-		};
-		let commitment_feerate = fee_estimator.bounded_sat_per_1000_weight(commitment_conf_target);
+		let (commitment_feerate, anchor_outputs_value_msat) =
+			if channel_type.supports_anchor_zero_fee_commitments() {
+				(0, 0)
+			} else if channel_type.supports_anchors_zero_fee_htlc_tx() {
+				let feerate = fee_estimator
+					.bounded_sat_per_1000_weight(ConfirmationTarget::AnchorChannelFee);
+				(feerate, ANCHOR_OUTPUT_VALUE_SATOSHI * 2 * 1000)
+			} else {
+				let feerate = fee_estimator
+					.bounded_sat_per_1000_weight(ConfirmationTarget::NonAnchorChannelFee);
+				(feerate, 0)
+			};
 
 		let value_to_self_msat = channel_value_satoshis * 1000 - push_msat;
 		let commitment_tx_fee = commit_tx_fee_sat(commitment_feerate, MIN_AFFORDABLE_HTLC_COUNT, &channel_type) * 1000;
@@ -5232,6 +5238,15 @@ impl<SP: Deref> FundedChannel<SP> where
 		feerate_per_kw: u32, cur_feerate_per_kw: Option<u32>, logger: &L
 	) -> Result<(), ChannelError> where F::Target: FeeEstimator, L::Target: Logger,
 	{
+		if channel_type.supports_anchor_zero_fee_commitments() {
+			if feerate_per_kw != 0 {
+				let err = "Zero Fee Channels must never attempt to use a fee".to_owned();
+				return Err(ChannelError::close(err));
+			} else {
+				return Ok(());
+			}
+		}
+
 		let lower_limit_conf_target = if channel_type.supports_anchors_zero_fee_htlc_tx() {
 			ConfirmationTarget::MinAllowedAnchorChannelRemoteFee
 		} else {
@@ -13135,6 +13150,19 @@ mod tests {
 		do_test_supports_channel_type(config, expected_channel_type)
 	}
 
+	#[test]
+	fn test_supports_zero_fee_commitments() {
+		// Tests that if both sides support and negotiate `anchors_zero_fee_commitments`, it is
+		// the resulting `channel_type`.
+		let mut config = UserConfig::default();
+		config.channel_handshake_config.negotiate_anchor_zero_fee_commitments = true;
+
+		let mut expected_channel_type = ChannelTypeFeatures::empty();
+		expected_channel_type.set_anchor_zero_fee_commitments_required();
+
+		do_test_supports_channel_type(config, expected_channel_type)
+	}
+
 	fn do_test_supports_channel_type(config: UserConfig, expected_channel_type: ChannelTypeFeatures) {
 		let secp_ctx = Secp256k1::new();
 		let fee_estimator = LowerBoundedFeeEstimator::new(&TestFeeEstimator{fee_est: 15000});
@@ -13169,6 +13197,14 @@ mod tests {
 
 		assert_eq!(channel_a.funding.get_channel_type(), &expected_channel_type);
 		assert_eq!(channel_b.funding.get_channel_type(), &expected_channel_type);
+
+		if expected_channel_type.supports_anchor_zero_fee_commitments() {
+			assert_eq!(channel_a.context.feerate_per_kw, 0);
+			assert_eq!(channel_b.context.feerate_per_kw, 0);
+		} else {
+			assert_ne!(channel_a.context.feerate_per_kw, 0);
+			assert_ne!(channel_b.context.feerate_per_kw, 0);
+		}
 	}
 
 	#[test]