Accept feerate increases even if they aren't high enough for us

lightning/src/ln/channel.rs

@@ -1075,8 +1075,9 @@ impl<Signer: Sign> Channel<Signer> {
 		})
 	}
 
-	fn check_remote_fee<F: Deref>(fee_estimator: &LowerBoundedFeeEstimator<F>, feerate_per_kw: u32) -> Result<(), ChannelError>
-		where F::Target: FeeEstimator
+	fn check_remote_fee<F: Deref, L: Deref>(fee_estimator: &LowerBoundedFeeEstimator<F>,
+		feerate_per_kw: u32, cur_feerate_per_kw: Option<u32>, logger: &L)
+		-> Result<(), ChannelError> where F::Target: FeeEstimator, L::Target: Logger,
 	{
 		// We only bound the fee updates on the upper side to prevent completely absurd feerates,
 		// always accepting up to 25 sat/vByte or 10x our fee estimator's "High Priority" fee.
@@ -1093,6 +1094,14 @@ impl<Signer: Sign> Channel<Signer> {
 		// of 1.1 sat/vbyte and a receiver that wants 1.1 rounded up to 2. Thus, we always add 250
 		// sat/kw before the comparison here.
 		if feerate_per_kw + 250 < lower_limit {
+			if let Some(cur_feerate) = cur_feerate_per_kw {
+				if feerate_per_kw > cur_feerate {
+					log_warn!(logger,
+						"Accepting feerate that may prevent us from closing this channel because it's higher than what we have now. Had {} s/kW, now {} s/kW.",
+						cur_feerate, feerate_per_kw);
+					return Ok(());
+				}
+			}
 			return Err(ChannelError::Close(format!("Peer's feerate much too low. Actual: {}. Our expected lower limit: {} (- 250)", feerate_per_kw, lower_limit)));
 		}
 		Ok(())
@@ -1179,7 +1188,7 @@ impl<Signer: Sign> Channel<Signer> {
 		if msg.htlc_minimum_msat >= full_channel_value_msat {
 			return Err(ChannelError::Close(format!("Minimum htlc value ({}) was larger than full channel value ({})", msg.htlc_minimum_msat, full_channel_value_msat)));
 		}
-		Channel::<Signer>::check_remote_fee(fee_estimator, msg.feerate_per_kw)?;
+		Channel::<Signer>::check_remote_fee(fee_estimator, msg.feerate_per_kw, None, logger)?;
 
 		let max_counterparty_selected_contest_delay = u16::min(config.channel_handshake_limits.their_to_self_delay, MAX_LOCAL_BREAKDOWN_TIMEOUT);
 		if msg.to_self_delay > max_counterparty_selected_contest_delay {
@@ -3762,16 +3771,16 @@ impl<Signer: Sign> Channel<Signer> {
 		}
 	}
 
-	pub fn update_fee<F: Deref>(&mut self, fee_estimator: &LowerBoundedFeeEstimator<F>, msg: &msgs::UpdateFee) -> Result<(), ChannelError>
-		where F::Target: FeeEstimator
+	pub fn update_fee<F: Deref, L: Deref>(&mut self, fee_estimator: &LowerBoundedFeeEstimator<F>, msg: &msgs::UpdateFee, logger: &L) -> Result<(), ChannelError>
+		where F::Target: FeeEstimator, L::Target: Logger
 	{
 		if self.is_outbound() {
 			return Err(ChannelError::Close("Non-funding remote tried to update channel fee".to_owned()));
 		}
 		if self.channel_state & (ChannelState::PeerDisconnected as u32) == ChannelState::PeerDisconnected as u32 {
 			return Err(ChannelError::Close("Peer sent update_fee when we needed a channel_reestablish".to_owned()));
 		}
-		Channel::<Signer>::check_remote_fee(fee_estimator, msg.feerate_per_kw)?;
+		Channel::<Signer>::check_remote_fee(fee_estimator, msg.feerate_per_kw, Some(self.feerate_per_kw), logger)?;
 		let feerate_over_dust_buffer = msg.feerate_per_kw > self.get_dust_buffer_feerate(None);
 
 		self.pending_update_fee = Some((msg.feerate_per_kw, FeeUpdateState::RemoteAnnounced));
@@ -6765,7 +6774,8 @@ mod tests {
 		// arithmetic, causing a panic with debug assertions enabled.
 		let fee_est = TestFeeEstimator { fee_est: 42 };
 		let bounded_fee_estimator = LowerBoundedFeeEstimator::new(&fee_est);
-		assert!(Channel::<InMemorySigner>::check_remote_fee(&bounded_fee_estimator, u32::max_value()).is_err());
+		assert!(Channel::<InMemorySigner>::check_remote_fee(&bounded_fee_estimator,
+			u32::max_value(), None, &&test_utils::TestLogger::new()).is_err());
 	}
 
 	struct Keys {

lightning/src/ln/channelmanager.rs

@@ -5189,7 +5189,7 @@ impl<M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelManager<M, T, K, F
 				if chan.get().get_counterparty_node_id() != *counterparty_node_id {
 					return Err(MsgHandleErrInternal::send_err_msg_no_close("Got a message for a channel from the wrong node!".to_owned(), msg.channel_id));
 				}
-				try_chan_entry!(self, chan.get_mut().update_fee(&self.fee_estimator, &msg), chan);
+				try_chan_entry!(self, chan.get_mut().update_fee(&self.fee_estimator, &msg, &self.logger), chan);
 			},
 			hash_map::Entry::Vacant(_) => return Err(MsgHandleErrInternal::send_err_msg_no_close("Failed to find corresponding channel".to_owned(), msg.channel_id))
 		}

lightning/src/ln/functional_tests.rs

@@ -10490,3 +10490,81 @@ fn test_non_final_funding_tx() {
 	assert!(nodes[0].node.funding_transaction_generated(&temp_channel_id, &nodes[1].node.get_our_node_id(), tx.clone()).is_ok());
 	get_event_msg!(nodes[0], MessageSendEvent::SendFundingCreated, nodes[1].node.get_our_node_id());
 }
+
+#[test]
+fn accept_busted_but_better_fee() {
+	// If a peer sends us a fee update that is too low, but higher than our previous channel
+	// feerate, we should accept it. In the future we may want to consider closing the channel
+	// later, but for now we only accept the update.
+	let mut chanmon_cfgs = create_chanmon_cfgs(2);
+	let node_cfgs = create_node_cfgs(2, &chanmon_cfgs);
+	let node_chanmgrs = create_node_chanmgrs(2, &node_cfgs, &[None, None]);
+	let nodes = create_network(2, &node_cfgs, &node_chanmgrs);
+
+	create_chan_between_nodes(&nodes[0], &nodes[1], channelmanager::provided_init_features(), channelmanager::provided_init_features());
+
+	// Set nodes[1] to expect 5,000 sat/kW.
+	{
+		let mut feerate_lock = chanmon_cfgs[1].fee_estimator.sat_per_kw.lock().unwrap();
+		*feerate_lock = 5000;
+	}
+
+	// If nodes[0] increases their feerate, even if its not enough, nodes[1] should accept it.
+	{
+		let mut feerate_lock = chanmon_cfgs[0].fee_estimator.sat_per_kw.lock().unwrap();
+		*feerate_lock = 1000;
+	}
+	nodes[0].node.timer_tick_occurred();
+	check_added_monitors!(nodes[0], 1);
+
+	let events = nodes[0].node.get_and_clear_pending_msg_events();
+	assert_eq!(events.len(), 1);
+	match events[0] {
+		MessageSendEvent::UpdateHTLCs { updates: msgs::CommitmentUpdate { ref update_fee, ref commitment_signed, .. }, .. } => {
+			nodes[1].node.handle_update_fee(&nodes[0].node.get_our_node_id(), update_fee.as_ref().unwrap());
+			commitment_signed_dance!(nodes[1], nodes[0], commitment_signed, false);
+		},
+		_ => panic!("Unexpected event"),
+	};
+
+	// If nodes[0] increases their feerate further, even if its not enough, nodes[1] should accept
+	// it.
+	{
+		let mut feerate_lock = chanmon_cfgs[0].fee_estimator.sat_per_kw.lock().unwrap();
+		*feerate_lock = 2000;
+	}
+	nodes[0].node.timer_tick_occurred();
+	check_added_monitors!(nodes[0], 1);
+
+	let events = nodes[0].node.get_and_clear_pending_msg_events();
+	assert_eq!(events.len(), 1);
+	match events[0] {
+		MessageSendEvent::UpdateHTLCs { updates: msgs::CommitmentUpdate { ref update_fee, ref commitment_signed, .. }, .. } => {
+			nodes[1].node.handle_update_fee(&nodes[0].node.get_our_node_id(), update_fee.as_ref().unwrap());
+			commitment_signed_dance!(nodes[1], nodes[0], commitment_signed, false);
+		},
+		_ => panic!("Unexpected event"),
+	};
+
+	// However, if nodes[0] decreases their feerate, nodes[1] should reject it and close the
+	// channel.
+	{
+		let mut feerate_lock = chanmon_cfgs[0].fee_estimator.sat_per_kw.lock().unwrap();
+		*feerate_lock = 1000;
+	}
+	nodes[0].node.timer_tick_occurred();
+	check_added_monitors!(nodes[0], 1);
+
+	let events = nodes[0].node.get_and_clear_pending_msg_events();
+	assert_eq!(events.len(), 1);
+	match events[0] {
+		MessageSendEvent::UpdateHTLCs { updates: msgs::CommitmentUpdate { ref update_fee, .. }, .. } => {
+			nodes[1].node.handle_update_fee(&nodes[0].node.get_our_node_id(), update_fee.as_ref().unwrap());
+			check_closed_event!(nodes[1], 1, ClosureReason::ProcessingError {
+				err: "Peer's feerate much too low. Actual: 1000. Our expected lower limit: 5000 (- 250)".to_owned() });
+			check_closed_broadcast!(nodes[1], true);
+			check_added_monitors!(nodes[1], 1);
+		},
+		_ => panic!("Unexpected event"),
+	};
+}