f Fix configurable in-flight limit percentage

Author: ViktorTigerstrom

lightning/src/ln/channel.rs

@@ -745,6 +745,10 @@ pub const COMMITMENT_TX_WEIGHT_PER_HTLC: u64 = 172;
 
 pub const ANCHOR_OUTPUT_VALUE_SATOSHI: u64 = 330;
 
+/// The percentage of the channel value `holder_max_htlc_value_in_flight_msat` used to be set to,
+/// before this was made configurable.
+pub const OLD_LDK_MAX_IN_FLIGHT_PERCENT: u8 = 10;
+
 /// Maximum `funding_satoshis` value according to the BOLT #2 specification, if
 /// `option_support_large_channel` (aka wumbo channels) is not supported.
 /// It's 2^24 - 1.
@@ -5893,13 +5897,16 @@ impl<Signer: Sign> Writeable for Channel<Signer> {
 
 		// The same logic applies for `holder_selected_channel_reserve_satoshis` values other than
 		// the default, and when `holder_max_htlc_value_in_flight_msat` is configured to be set to
-		// a different percentage of the channel value then the default value of
-		// `ChannelHandshakeConfig::max_inbound_htlc_value_in_flight_percent_of_channel`.
+		// a different percentage of the channel value then 10%, which older versions of LDK used
+		// to set it to before the percentage was made configurable.
 		let serialized_holder_selected_reserve =
 			if self.holder_selected_channel_reserve_satoshis != Self::get_holder_selected_channel_reserve_satoshis(self.channel_value_satoshis)
 			{ Some(self.holder_selected_channel_reserve_satoshis) } else { None };
+
+		let mut old_max_in_flight_percent_config = UserConfig::default().own_channel_config;
+		old_max_in_flight_percent_config.max_inbound_htlc_value_in_flight_percent_of_channel = OLD_LDK_MAX_IN_FLIGHT_PERCENT;
 		let serialized_holder_htlc_max_in_flight =
-			if self.holder_max_htlc_value_in_flight_msat != Self::get_holder_max_htlc_value_in_flight_msat(self.channel_value_satoshis, &UserConfig::default().own_channel_config)
+			if self.holder_max_htlc_value_in_flight_msat != Self::get_holder_max_htlc_value_in_flight_msat(self.channel_value_satoshis, &old_max_in_flight_percent_config)
 			{ Some(self.holder_max_htlc_value_in_flight_msat) } else { None };
 
 		write_tlv_fields!(writer, {

lightning/src/util/config.rs

@@ -48,23 +48,25 @@ pub struct ChannelHandshakeConfig {
 	/// Default value: 1. If the value is less than 1, it is ignored and set to 1, as is required
 	/// by the protocol.
 	pub our_htlc_minimum_msat: u64,
-	/// Sets how many the percent of the channel value we will cap the total value of outstanding
-	/// inbound HTLCs to.
+	/// Sets the percentage of the channel value we will cap the total value of outstanding inbound
+	/// HTLCs to.
 	///
 	/// This can be set to a value between 1-100, where the value corresponds to the percent of the
 	/// channel value in whole percentages.
 	///
-	/// Note that if configured to another value than the default value 10, any new channels
-	/// created with the non default value won't be backwards compatible with LDK versions prior
-	/// to 0.0.100.
+	/// Note that:
+	/// * If configured to another value than the default value 10, any new channels
+	/// created with the non default value will cause versions of LDK prior to 0.0.100 to refuse to
+	/// read the ChannelManager.
 	///
-	/// Note that this caps the total value for inbound HTLCs in-flight only, and there's currently
-	/// no availability to configure the cap for the total value of outbound HTLCs in-flight.
-	/// This effects the in-flight HTLC balance, which [`ChannelConfig::cltv_expiry_delta`] is
-	/// applied to.
+	/// * This caps the total value for inbound HTLCs in-flight only, and there's currently
+	/// no way to configure the cap for the total value of outbound HTLCs in-flight.
 	///
-	/// This does not cap the total value of the non-HTLC-encumbered balance which
-	/// [`ChannelHandshakeConfig::our_to_self_delay`] is applied to.
+	/// * The online requirements for ensuring the safety of HTLC-encumbered funds are
+	/// different from the non-HTLC-encumbered funds. This makes this an important knob to restrict
+	/// exposure to loss due to being offline for too long.
+	/// See [`ChannelHandshakeConfig::our_to_self_delay`] and [`ChannelConfig::cltv_expiry_delta`]
+	/// for more information.
 	///
 	/// Default value: 10.
 	/// Minimum value: 1, any values less than 1 will be treated as 1 instead.