Store our skimmed fee in PendingHTLCRouting

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

This breaks compatibility with LDK versions prior to 0.0.116 due to the way the
field is serialized in the outgoing Channel on forward.

lightning/src/ln/channelmanager.rs

@@ -104,6 +104,9 @@ pub(super) enum PendingHTLCRouting {
 		/// The SCID from the onion that we should forward to. This could be a real SCID or a fake one
 		/// generated using `get_fake_scid` from the scid_utils::fake_scid module.
 		short_channel_id: u64, // This should be NonZero<u64> eventually when we bump MSRV
+		/// The fee we're skimming off the top of this HTLC. See
+		/// [`ChannelConfig::accept_underpaying_htlcs`].
+		skimmed_fee_msat: Option<u64>,
 	},
 	Receive {
 		payment_data: msgs::FinalOnionHopData,
@@ -2764,6 +2767,7 @@ where
 					routing: PendingHTLCRouting::Forward {
 						onion_packet: outgoing_packet,
 						short_channel_id,
+						skimmed_fee_msat: None,
 					},
 					payment_hash: msg.payment_hash.clone(),
 					incoming_shared_secret: shared_secret,
@@ -3402,8 +3406,16 @@ where
 			})?;
 
 		let routing = match payment.forward_info.routing {
-			PendingHTLCRouting::Forward { onion_packet, .. } => {
-				PendingHTLCRouting::Forward { onion_packet, short_channel_id: next_hop_scid }
+			PendingHTLCRouting::Forward { onion_packet, skimmed_fee_msat, .. } => {
+				debug_assert!(skimmed_fee_msat.is_none());
+				PendingHTLCRouting::Forward {
+					onion_packet,
+					short_channel_id: next_hop_scid,
+					skimmed_fee_msat:
+						// The minuend here must match the expected forward amount generated for the
+						// HTLCIntercepted event.
+						Some(payment.forward_info.outgoing_amt_msat.saturating_sub(amt_to_forward_msat)),
+				}
 			},
 			_ => unreachable!() // Only `PendingHTLCRouting::Forward`s are intercepted
 		};
@@ -7311,6 +7323,7 @@ impl_writeable_tlv_based!(PhantomRouteHints, {
 impl_writeable_tlv_based_enum!(PendingHTLCRouting,
 	(0, Forward) => {
 		(0, onion_packet, required),
+		(1, skimmed_fee_msat, option),
 		(2, short_channel_id, required),
 	},
 	(1, Receive) => {

lightning/src/util/config.rs

@@ -636,7 +636,7 @@ pub struct UserConfig {
 	///  fake short channel ids generated via [`ChannelManager::get_intercept_scid`]. Upon HTLC
 	///  intercept, LDK will generate an [`Event::HTLCIntercepted`] which MUST be handled by the user.
 	///
-	///  Setting this to true may break backwards compatibility with LDK versions < 0.0.113.
+	///  Setting this to true may break backwards compatibility with LDK versions < 0.0.116.
 	///
 	///  Default value: false.
 	///

pending_changelog/forward-underpaying-htlc.txt

@@ -0,0 +1,4 @@
+## Backwards Compat
+
+* Once upgraded, users with `UserConfig::accept_intercept_htlcs` set may not be able to downgrade to
+	LDK versions prior to 0.0.116.