Improve prediction of commitment stats in validate_update_fee

`ChannelContext::get_pending_htlc_stats` predicts that the set of HTLCs
on the next commitment will be all the HTLCs in
`ChannelContext.pending_inbound_htlcs`, and
`ChannelContext.pending_outbound_htlcs`, as well as all the outbound
HTLC adds in the holding cell.

This is an overestimate:

* Outbound HTLC removals which have been ACK'ed by the counterparty will
  certainly not be present in any *next* commitment, even though they
  remain in `pending_outbound_htlcs`.

* Outbound HTLCs in the `RemoteRemoved` state, will not be present in
  the next *local* commitment.

* Outbound HTLCs in the `LocalAnnounced` state have no guarantee that
  they were received by the counterparty before she sent the
  `update_fee`.

* Outbound `update_add_htlc`'s in the holding cell are certainly not
  known by the counterparty, and we will reevaluate their addition to
  the channel when freeing the holding cell.

* Inbound HTLCs in the `LocalRemoved` state will not be present in the
  next *remote* commitment.

This commit stops using `get_pending_htlc_stats` in favor of the newly
added `ChannelContext::get_next_{local, remote}_commitment_stats`
methods, and fixes the issues described above.

We now always calculate dust exposure using a buffer from
`msg.feerate_per_kw`, and not from
`max(self.feerate_per_kw, msg.feerate_per_kw)`.

Author: tankyleo

lightning/src/ln/channel.rs

@@ -4341,16 +4341,21 @@ where
 		let dust_exposure_limiting_feerate = self.get_dust_exposure_limiting_feerate(
 			&fee_estimator, funding.get_channel_type(),
 		);
-		let htlc_stats = self.get_pending_htlc_stats(funding, None, dust_exposure_limiting_feerate);
+		// Do not include outbound update_add_htlc's in the holding cell, or those which haven't yet been ACK'ed by the counterparty (ie. LocalAnnounced HTLCs)
+		let include_counterparty_unknown_htlcs = false;
+		let next_local_commitment_stats = self.get_next_local_commitment_stats(funding, None, include_counterparty_unknown_htlcs, 0, msg.feerate_per_kw, dust_exposure_limiting_feerate);
+		let next_remote_commitment_stats = self.get_next_remote_commitment_stats(funding, None, include_counterparty_unknown_htlcs, 0, msg.feerate_per_kw, dust_exposure_limiting_feerate);
+
 		let max_dust_htlc_exposure_msat = self.get_max_dust_htlc_exposure_msat(dust_exposure_limiting_feerate);
-		if htlc_stats.on_holder_tx_dust_exposure_msat > max_dust_htlc_exposure_msat {
+		if next_local_commitment_stats.dust_exposure_msat > max_dust_htlc_exposure_msat {
 			return Err(ChannelError::close(format!("Peer sent update_fee with a feerate ({}) which may over-expose us to dust-in-flight on our own transactions (totaling {} msat)",
-				msg.feerate_per_kw, htlc_stats.on_holder_tx_dust_exposure_msat)));
+				msg.feerate_per_kw, next_local_commitment_stats.dust_exposure_msat)));
 		}
-		if htlc_stats.on_counterparty_tx_dust_exposure_msat > max_dust_htlc_exposure_msat {
+		if next_remote_commitment_stats.dust_exposure_msat > max_dust_htlc_exposure_msat {
 			return Err(ChannelError::close(format!("Peer sent update_fee with a feerate ({}) which may over-expose us to dust-in-flight on our counterparty's transactions (totaling {} msat)",
-				msg.feerate_per_kw, htlc_stats.on_counterparty_tx_dust_exposure_msat)));
+				msg.feerate_per_kw, next_remote_commitment_stats.dust_exposure_msat)));
 		}
+
 		Ok(())
 	}