XXX: Incl tx fee when calcing inbound HTLC limits on inbound channels

When we receive an inbound HTLC from a peer on an inbound channel,
the peer needs to be able to pay for the additional commitment tx
fees as well.

XXX: Needs tests + a similar limit on outbound. Probably also needs
new actions in chanmon_fail_consistency fuzztest for "send 1 msat
HTLC" or so.

diff --git a/lightning/src/ln/channel.rs b/lightning/src/ln/channel.rs
index 81762923fda224f83874ff4413dc3e4d7605ef3c..aea17ca9112a774cce00c8af5f8610ab1fc94253 100644 (file)
--- a/lightning/src/ln/channel.rs
+++ b/lightning/src/ln/channel.rs
@@ -1680,14 +1680,28 @@ impl<ChanSigner: ChannelKeys> Channel<ChanSigner> {
                // Channel state once they will not be present in the next received commitment
                // transaction).
                let mut removed_outbound_total_msat = 0;
+               // When calculating the set of HTLCs which will be included in their next
+               // commitment_signed, all inbound HTLCs are included (as all states imply it will be
+               // included) and only committed outbound HTLCs, see below.
+               let mut their_acked_htlcs = self.pending_inbound_htlcs.len() as u64;
                for ref htlc in self.pending_outbound_htlcs.iter() {
                        if let OutboundHTLCState::AwaitingRemoteRevokeToRemove(None) = htlc.state {
                                removed_outbound_total_msat += htlc.amount_msat;
                        } else if let OutboundHTLCState::AwaitingRemovedRemoteRevoke(None) = htlc.state {
                                removed_outbound_total_msat += htlc.amount_msat;
+                       } else if let OutboundHTLCState::Committed = htlc.state {
+                               // We only include outbound HTLCs if it will not be included in their next
+                               // commitment_signed, ie if they've responded to us with an RAA after announcement
+                               // and if they haven't yet started removal.
+                               their_acked_htlcs += 1;
                        }
                }
-               if htlc_inbound_value_msat + msg.amount_msat + self.value_to_self_msat > (self.channel_value_satoshis - Channel::<ChanSigner>::get_our_channel_reserve_satoshis(self.channel_value_satoshis)) * 1000 + removed_outbound_total_msat {
+               let remote_fee_cost = if !self.channel_outbound {
+                       // Note that we ignore the fact that feerate_per_kw can change at runtime here. That's
+                       // OK, soon enough it won't be the case.
+                       self.feerate_per_kw * (COMMITMENT_TX_BASE_WEIGHT + their_acked_htlcs * COMMITMENT_TX_WEIGHT_PER_HTLC) / 1000
+               } else { 0 };
+               if htlc_inbound_value_msat + msg.amount_msat + self.value_to_self_msat > (self.channel_value_satoshis - Channel::<ChanSigner>::get_our_channel_reserve_satoshis(self.channel_value_satoshis) - remote_fee_cost) * 1000 + removed_outbound_total_msat {
                        return Err(ChannelError::Close("Remote HTLC add would put them over their reserve value"));
                }
                if self.next_remote_htlc_id != msg.htlc_id {