f move check for fee update into channelmanager

The logic of "no feerate change has happened" belongs in channel
manager (previously it just didn't work for zero fees). Change
channel to just assert that we're not making these calls (but don't
panic, since this isn't going to put us at risk of lost funds).

Author: carlaKC

lightning/src/ln/channel.rs

@@ -4335,10 +4335,6 @@ where
 		F::Target: FeeEstimator,
 		L::Target: Logger,
 	{
-		if funding.get_channel_type().supports_anchor_zero_fee_commitments() {
-			return false;
-		}
-
 		// Before proposing a feerate update, check that we can actually afford the new fee.
 		let dust_exposure_limiting_feerate = self.get_dust_exposure_limiting_feerate(
 			&fee_estimator, funding.get_channel_type(),
@@ -7727,6 +7723,10 @@ where
 			panic!("Cannot update fee while peer is disconnected/we're awaiting a monitor update (ChannelManager should have caught this)");
 		}
 
+		// Sending a fee update for zero fee commitments will trigger a warning and disconnect
+		// from our peer, but does not result in a loss of funds so we do not panic here.
+		debug_assert!(!self.funding.get_channel_type().supports_anchor_zero_fee_commitments());
+
 		let can_send_update_fee = core::iter::once(&self.funding)
 			.chain(self.pending_funding.iter())
 			.all(|funding| self.context.can_send_update_fee(funding, feerate_per_kw, fee_estimator, logger));

lightning/src/ln/channelmanager.rs

@@ -6963,10 +6963,17 @@ where
 
 		let logger = WithChannelContext::from(&self.logger, &chan.context, None);
 
-		// If the feerate has decreased by less than half, don't bother
-		if new_feerate <= chan.context.get_feerate_sat_per_1000_weight() && new_feerate * 2 > chan.context.get_feerate_sat_per_1000_weight() {
-			return NotifyOption::SkipPersistNoEvents;
+		let current_feerate = chan.context.get_feerate_sat_per_1000_weight();
+		let update_fee_required = match new_feerate.cmp(&current_feerate) {
+			cmp::Ordering::Greater => true,
+			cmp::Ordering::Equal => false,
+			// Only bother with a fee update if feerate has decreased at least half.
+			cmp::Ordering::Less => new_feerate * 2 <= current_feerate,
+		};
+		if !update_fee_required {
+			return NotifyOption::SkipPersistNoEvents
 		}
+
 		if !chan.context.is_live() {
 			log_trace!(logger, "Channel {} does not qualify for a feerate change from {} to {} as it cannot currently be updated (probably the peer is disconnected).",
 				chan_id, chan.context.get_feerate_sat_per_1000_weight(), new_feerate);