Add a new ClosureReason::PeerFeerateTooLow

Closure due to feerate disagreements are a specific closure reason
which admins can understand and tune their config (in the form of
their `FeeEstimator`) to avoid, so having a separate
`ClosureReason` for it is useful.

Author: TheBlueMatt

lightning/src/events/mod.rs

@@ -330,6 +330,21 @@ pub enum ClosureReason {
 	FundingBatchClosure,
 	/// One of our HTLCs timed out in a channel, causing us to force close the channel.
 	HTLCsTimedOut,
+	/// Our peer provided a feerate which violated our required minimum (fetched from our
+	/// [`FeeEstimator`] either as [`ConfirmationTarget::MinAllowedAnchorChannelRemoteFee`] or
+	/// [`ConfirmationTarget::MinAllowedNonAnchorChannelRemoteFee`]).
+	///
+	/// [`FeeEstimator`]: crate::chain::chaininterface::FeeEstimator
+	/// [`ConfirmationTarget::MinAllowedAnchorChannelRemoteFee`]: crate::chain::chaininterface::ConfirmationTarget::MinAllowedAnchorChannelRemoteFee
+	/// [`ConfirmationTarget::MinAllowedNonAnchorChannelRemoteFee`]: crate::chain::chaininterface::ConfirmationTarget::MinAllowedNonAnchorChannelRemoteFee
+	PeerFeerateTooLow {
+		/// The feerate on our channel set by our peer.
+		peer_feerate_sat_per_kw: u32,
+		/// The required feerate we enforce, from our [`FeeEstimator`].
+		///
+		/// [`FeeEstimator`]: crate::chain::chaininterface::FeeEstimator
+		required_feerate_sat_per_kw: u32,
+	},
 }
 
 impl core::fmt::Display for ClosureReason {
@@ -354,6 +369,11 @@ impl core::fmt::Display for ClosureReason {
 			ClosureReason::CounterpartyCoopClosedUnfundedChannel => f.write_str("the peer requested the unfunded channel be closed"),
 			ClosureReason::FundingBatchClosure => f.write_str("another channel in the same funding batch closed"),
 			ClosureReason::HTLCsTimedOut => f.write_str("htlcs on the channel timed out"),
+			ClosureReason::PeerFeerateTooLow { peer_feerate_sat_per_kw, required_feerate_sat_per_kw } =>
+				f.write_fmt(format_args!(
+					"peer provided a feerate ({} sat/kw) which was below our lower bound ({} sat/kw)",
+					peer_feerate_sat_per_kw, required_feerate_sat_per_kw,
+				)),
 		}
 	}
 }
@@ -372,6 +392,10 @@ impl_writeable_tlv_based_enum_upgradable!(ClosureReason,
 	(17, CounterpartyInitiatedCooperativeClosure) => {},
 	(19, LocallyInitiatedCooperativeClosure) => {},
 	(21, HTLCsTimedOut) => {},
+	(23, PeerFeerateTooLow) => {
+		(0, peer_feerate_sat_per_kw, required),
+		(2, required_feerate_sat_per_kw, required),
+	},
 );
 
 /// Intended destination of a failed HTLC as indicated in [`Event::HTLCHandlingFailed`].

lightning/src/ln/channel.rs

@@ -3698,7 +3698,12 @@ impl<SP: Deref> Channel<SP> where
 					return Ok(());
 				}
 			}
-			return Err(ChannelError::close(format!("Peer's feerate much too low. Actual: {}. Our expected lower limit: {}", feerate_per_kw, lower_limit)));
+			return Err(ChannelError::Close((format!(
+				"Peer's feerate much too low. Actual: {}. Our expected lower limit: {}", feerate_per_kw, lower_limit
+			), ClosureReason::PeerFeerateTooLow {
+				peer_feerate_sat_per_kw: feerate_per_kw,
+				required_feerate_sat_per_kw: lower_limit,
+			})));
 		}
 		Ok(())
 	}

lightning/src/ln/functional_tests.rs

@@ -10401,9 +10401,9 @@ fn accept_busted_but_better_fee() {
 	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".to_owned() },
-				[nodes[0].node.get_our_node_id()], 100000);
+			check_closed_event!(nodes[1], 1, ClosureReason::PeerFeerateTooLow {
+				peer_feerate_sat_per_kw: 1000, required_feerate_sat_per_kw: 5000,
+			}, [nodes[0].node.get_our_node_id()], 100000);
 			check_closed_broadcast!(nodes[1], true);
 			check_added_monitors!(nodes[1], 1);
 		},