Add config knob for forwarding intercept payments

Author: valentinewallace

lightning/src/ln/channelmanager.rs

@@ -2226,8 +2226,9 @@ impl<M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelManager<M, T, K, F
 						None => { // unknown_next_peer
 							// Note that this is likely a timing oracle for detecting whether an scid is a
 							// phantom or an intercept.
-							if fake_scid::is_valid_phantom(&self.fake_scid_rand_bytes, *short_channel_id, &self.genesis_hash) ||
-							   fake_scid::is_valid_intercept(&self.fake_scid_rand_bytes, *short_channel_id, &self.genesis_hash)
+							if (self.default_configuration.accept_intercept_htlcs &&
+							   fake_scid::is_valid_intercept(&self.fake_scid_rand_bytes, *short_channel_id, &self.genesis_hash)) ||
+							   fake_scid::is_valid_phantom(&self.fake_scid_rand_bytes, *short_channel_id, &self.genesis_hash)
 							{
 								None
 							} else {
@@ -3050,14 +3051,16 @@ impl<M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelManager<M, T, K, F
 	/// Intercepted HTLCs can be useful for Lightning Service Providers (LSPs) to open a just-in-time
 	/// channel to a receiving node if the node lacks sufficient inbound liquidity.
 	///
-	/// To make use of intercepted HTLCs, use [`ChannelManager::get_intercept_scid`] to generate short
-	/// channel id(s) to put in the receiver's invoice route hints. These route hints will signal to
-	/// LDK to generate an [`HTLCIntercepted`] event when it receives the forwarded HTLC, and this
-	/// method or [`ChannelManager::fail_intercepted_htlc`] MUST be called in response to the event.
+	/// To make use of intercepted HTLCs, set [`UserConfig::accept_intercept_htlcs`] and use
+	/// [`ChannelManager::get_intercept_scid`] to generate short channel id(s) to put in the
+	/// receiver's invoice route hints. These route hints will signal to LDK to generate an
+	/// [`HTLCIntercepted`] event when it receives the forwarded HTLC, and this method or
+	/// [`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.
 	///
+	/// [`UserConfig::accept_intercept_htlcs`]: crate::util::config::UserConfig::accept_intercept_htlcs
 	/// [`HTLCIntercepted`]: events::Event::HTLCIntercepted
 	// TODO: when we move to deciding the best outbound channel at forward time, only take
 	// `next_node_id` and not `next_hop_channel_id`

lightning/src/ln/payment_tests.rs

@@ -1385,9 +1385,13 @@ fn intercepted_payment() {
 fn do_test_intercepted_payment(fail_intercept: bool) {
 	let chanmon_cfgs = create_chanmon_cfgs(3);
 	let node_cfgs = create_node_cfgs(3, &chanmon_cfgs);
-	let mut chan_config = test_default_channel_config();
-	chan_config.manually_accept_inbound_channels = true;
-	let node_chanmgrs = create_node_chanmgrs(3, &node_cfgs, &[None, None, Some(chan_config)]);
+
+	let mut zero_conf_chan_config = test_default_channel_config();
+	zero_conf_chan_config.manually_accept_inbound_channels = true;
+	let mut intercept_forwards_config = test_default_channel_config();
+	intercept_forwards_config.accept_intercept_htlcs = true;
+	let node_chanmgrs = create_node_chanmgrs(3, &node_cfgs, &[None, Some(intercept_forwards_config), Some(zero_conf_chan_config)]);
+
 	let nodes = create_network(3, &node_cfgs, &node_chanmgrs);
 	let scorer = test_utils::TestScorer::with_penalty(0);
 	let random_seed_bytes = chanmon_cfgs[0].keys_manager.get_secure_random_bytes();

lightning/src/util/config.rs

@@ -505,6 +505,17 @@ pub struct UserConfig {
 	/// [`msgs::OpenChannel`]: crate::ln::msgs::OpenChannel
 	/// [`msgs::AcceptChannel`]: crate::ln::msgs::AcceptChannel
 	pub manually_accept_inbound_channels: bool,
+	///  If this is set to true, LDK will intercept HTLCs that are attempting to be forwarded over
+	///  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.
+	///
+	///  Default value: false.
+	///
+	/// [`ChannelManager::get_intercept_scid`]: crate::ln::channelmanager::ChannelManager::get_intercept_scid
+	/// [`Event::HTLCIntercepted`]: crate::util::events::Event::HTLCIntercepted
+	pub accept_intercept_htlcs: bool,
 }
 
 impl Default for UserConfig {
@@ -516,6 +527,7 @@ impl Default for UserConfig {
 			accept_forwards_to_priv_channels: false,
 			accept_inbound_channels: true,
 			manually_accept_inbound_channels: false,
+			accept_intercept_htlcs: false,
 		}
 	}
 }

lightning/src/util/events.rs

@@ -608,13 +608,14 @@ pub enum Event {
 	},
 	/// Used to indicate that we've intercepted an HTLC forward. This event will only be generated if
 	/// you've encoded an intercept scid in the receiver's invoice route hints using
-	/// [`ChannelManager::get_intercept_scid`].
+	/// [`ChannelManager::get_intercept_scid`] and have set [`UserConfig::accept_intercept_htlcs`].
 	///
 	/// [`ChannelManager::forward_intercepted_htlc`] or
 	/// [`ChannelManager::fail_intercepted_htlc`] MUST be called in response to this event. See
 	/// their docs for more information.
 	///
 	/// [`ChannelManager::get_intercept_scid`]: crate::ln::channelmanager::ChannelManager::get_intercept_scid
+	/// [`UserConfig::accept_intercept_htlcs`]: crate::util::config::UserConfig::accept_intercept_htlcs
 	/// [`ChannelManager::forward_intercepted_htlc`]: crate::ln::channelmanager::ChannelManager::forward_intercepted_htlc
 	/// [`ChannelManager::fail_intercepted_htlc`]: crate::ln::channelmanager::ChannelManager::fail_intercepted_htlc
 	HTLCIntercepted {