Set extra skimmed fee on intercepted forward

Receivers need to use this value to verify incoming payments if
ChannelConfig::accept_underpaying_htlcs is set.

diff --git a/lightning/src/events/mod.rs b/lightning/src/events/mod.rs
index 76a7f884ad27ceb3a1550e1b7235bf7dc8795f6d..b4b5bf838ee1a7ab001506a958564861549cf8f6 100644 (file)
--- a/lightning/src/events/mod.rs
+++ b/lightning/src/events/mod.rs
@@ -623,6 +623,7 @@ pub enum Event {
                inbound_amount_msat: u64,
                /// How many msats the payer intended to route to the next node. Depending on the reason you are
                /// intercepting this payment, you might take a fee by forwarding less than this amount.
+               /// Forwarding less than this amount may break compatibility with LDK versions prior to 0.0.116.
                ///
                /// Note that LDK will NOT check that expected fees were factored into this value. You MUST
                /// check that whatever fee you want has been included here or subtract it as required. Further,

diff --git a/lightning/src/ln/channelmanager.rs b/lightning/src/ln/channelmanager.rs
index 7883e255f37acf2b10a6abf5bf176cd18672367c..0776bd56f0f4af1bbf60a98e757ba94ea25893d3 100644 (file)
--- a/lightning/src/ln/channelmanager.rs
+++ b/lightning/src/ln/channelmanager.rs
@@ -131,6 +131,9 @@ pub(super) struct PendingHTLCInfo {
        /// may overshoot this in either case)
        pub(super) outgoing_amt_msat: u64,
        pub(super) outgoing_cltv_value: u32,
+       /// The fee being skimmed off the top of this HTLC. If this is a forward, it'll be the fee we are
+       /// skimming. If we're receiving this HTLC, it's the fee that our counterparty skimmed.
+       pub(super) skimmed_fee_msat: Option<u64>,
 }
 
 #[derive(Clone)] // See Channel::revoke_and_ack for why, tl;dr: Rust bug
@@ -2616,6 +2619,7 @@ where
                        incoming_amt_msat: Some(amt_msat),
                        outgoing_amt_msat: hop_data.amt_to_forward,
                        outgoing_cltv_value: hop_data.outgoing_cltv_value,
+                       skimmed_fee_msat: None,
                })
        }
 
@@ -2890,6 +2894,7 @@ where
                                        incoming_amt_msat: Some(msg.amount_msat),
                                        outgoing_amt_msat: next_hop_data.amt_to_forward,
                                        outgoing_cltv_value: next_hop_data.outgoing_cltv_value,
+                                       skimmed_fee_msat: None,
                                })
                        }
                }
@@ -3485,13 +3490,16 @@ where
        /// [`ChannelManager::fail_intercepted_htlc`] MUST be called in response to the event.
        ///
        /// Note that LDK does not enforce fee requirements in `amt_to_forward_msat`, and will not stop
-       /// you from forwarding more than you received.
+       /// you from forwarding more than you received. See
+       /// [`HTLCIntercepted::expected_outbound_amount_msat`] for more on forwarding a different amount
+       /// than expected.
        ///
        /// Errors if the event was not handled in time, in which case the HTLC was automatically failed
        /// backwards.
        ///
        /// [`UserConfig::accept_intercept_htlcs`]: crate::util::config::UserConfig::accept_intercept_htlcs
        /// [`HTLCIntercepted`]: events::Event::HTLCIntercepted
+       /// [`HTLCIntercepted::expected_outbound_amount_msat`]: events::Event::HTLCIntercepted::expected_outbound_amount_msat
        // TODO: when we move to deciding the best outbound channel at forward time, only take
        // `next_node_id` and not `next_hop_channel_id`
        pub fn forward_intercepted_htlc(&self, intercept_id: InterceptId, next_hop_channel_id: &[u8; 32], next_node_id: PublicKey, amt_to_forward_msat: u64) -> Result<(), APIError> {
@@ -3530,7 +3538,10 @@ where
                        },
                        _ => unreachable!() // Only `PendingHTLCRouting::Forward`s are intercepted
                };
+               let skimmed_fee_msat =
+                       payment.forward_info.outgoing_amt_msat.saturating_sub(amt_to_forward_msat);
                let pending_htlc_info = PendingHTLCInfo {
+                       skimmed_fee_msat: if skimmed_fee_msat == 0 { None } else { Some(skimmed_fee_msat) },
                        outgoing_amt_msat: amt_to_forward_msat, routing, ..payment.forward_info
                };
 
@@ -3600,7 +3611,7 @@ where
                                                                                prev_short_channel_id, prev_htlc_id, prev_funding_outpoint, prev_user_channel_id,
                                                                                forward_info: PendingHTLCInfo {
                                                                                        routing, incoming_shared_secret, payment_hash, outgoing_amt_msat,
-                                                                                       outgoing_cltv_value, incoming_amt_msat: _
+                                                                                       outgoing_cltv_value, ..
                                                                                }
                                                                        }) => {
                                                                                macro_rules! failure_handler {
@@ -3713,7 +3724,7 @@ where
                                                                                prev_short_channel_id, prev_htlc_id, prev_funding_outpoint, prev_user_channel_id: _,
                                                                                forward_info: PendingHTLCInfo {
                                                                                        incoming_shared_secret, payment_hash, outgoing_amt_msat, outgoing_cltv_value,
-                                                                                       routing: PendingHTLCRouting::Forward { onion_packet, .. }, incoming_amt_msat: _,
+                                                                                       routing: PendingHTLCRouting::Forward { onion_packet, .. }, ..
                                                                                },
                                                                        }) => {
                                                                                log_trace!(self.logger, "Adding HTLC from short id {} with payment_hash {} to channel with short id {} after delay", prev_short_channel_id, log_bytes!(payment_hash.0), short_chan_id);
@@ -7448,6 +7459,7 @@ impl_writeable_tlv_based!(PendingHTLCInfo, {
        (6, outgoing_amt_msat, required),
        (8, outgoing_cltv_value, required),
        (9, incoming_amt_msat, option),
+       (10, skimmed_fee_msat, option),
 });
 
 

diff --git a/pending_changelog/forward-underpaying-htlc.txt b/pending_changelog/forward-underpaying-htlc.txt
new file mode 100644 (file)
index 0000000..21f4a14
--- /dev/null
+++ b/pending_changelog/forward-underpaying-htlc.txt
@@ -0,0 +1,4 @@
+## Backwards Compat
+
+* Forwarding less than the expected amount in `ChannelManager::forward_intercepted_htlc` may break
+       compatibility with versions of LDK prior to 0.0.116