Add config knob for accepting underpaying HTLCs

See ChannelConfig::accept_underpaying_htlcs

Author: valentinewallace

lightning/src/util/config.rs

@@ -397,6 +397,36 @@ pub struct ChannelConfig {
 	/// [`Normal`]: crate::chain::chaininterface::ConfirmationTarget::Normal
 	/// [`Background`]: crate::chain::chaininterface::ConfirmationTarget::Background
 	pub force_close_avoidance_max_fee_satoshis: u64,
+	/// If set, allows this channel's counterparty to skim an additional fee off of this node's
+	/// inbound HTLCs. Useful for liquidity providers to offload on-chain channel costs to end users.
+	///
+	/// Usage:
+	/// - The payee will set this option and set its invoice route hints to use [`intercept scids`]
+	///   generated by this channel's counterparty.
+	/// - If this fee is being taken by a liquidity provider for a just-in-time channel, payees MUST
+	///   disable MPP in their invoices unless they have a way to let their counterparty know when all
+	///   MPP parts have been aggregated. Otherwise, excess channels may be opened for each MPP part.
+	/// - The counterparty will get an [`HTLCIntercepted`] event upon payment forward, and call
+	///   [`forward_intercepted_htlc`] with less than the amount provided in
+	///   [`HTLCIntercepted::expected_outbound_amount_msat`]. The difference between the expected and
+	///   actual forward amounts is their fee.
+	///
+	/// # Note
+	/// It's important for payee wallet software to verify that [`PaymentClaimable::amount_msat`] is
+	/// as-expected if this feature is activated, otherwise they may lose money!
+	///
+	/// # Note
+	/// Switching on this config flag may break compatibility with versions of LDK prior to 0.0.116.
+	///
+	/// Default value: false.
+	///
+	/// [`intercept scids`]: crate::ln::channelmanager::ChannelManager::get_intercept_scid
+	/// [`forward_intercepted_htlc`]: crate::ln::channelmanager::ChannelManager::forward_intercepted_htlc
+	/// [`HTLCIntercepted`]: crate::events::Event::HTLCIntercepted
+	/// [`HTLCIntercepted::expected_outbound_amount_msat`]: crate::events::Event::HTLCIntercepted::expected_outbound_amount_msat
+	/// [`PaymentClaimable::amount_msat`]: crate::events::Event::PaymentClaimable::amount_msat
+	//  TODO: link to bLIP when it's merged
+	pub accept_underpaying_htlcs: bool,
 }
 
 impl ChannelConfig {
@@ -429,12 +459,14 @@ impl Default for ChannelConfig {
 			cltv_expiry_delta: 6 * 12, // 6 blocks/hour * 12 hours
 			max_dust_htlc_exposure_msat: 5_000_000,
 			force_close_avoidance_max_fee_satoshis: 1000,
+			accept_underpaying_htlcs: false,
 		}
 	}
 }
 
 impl_writeable_tlv_based!(ChannelConfig, {
 	(0, forwarding_fee_proportional_millionths, required),
+	(1, accept_underpaying_htlcs, (default_value, false)),
 	(2, forwarding_fee_base_msat, required),
 	(4, cltv_expiry_delta, required),
 	(6, max_dust_htlc_exposure_msat, required),
@@ -543,6 +575,7 @@ impl crate::util::ser::Readable for LegacyChannelConfig {
 				cltv_expiry_delta,
 				force_close_avoidance_max_fee_satoshis,
 				forwarding_fee_base_msat,
+				accept_underpaying_htlcs: false,
 			},
 			announced_channel,
 			commit_upfront_shutdown_pubkey,