Advertise htlc_hold feature in ChannelManager

Now that we support the feature of sending payments as an often-offline sender
to an often-offline recipient, including the sender's LSP-side, we can start
conditionally advertising the feature bit to other nodes on the network.

Author: valentinewallace

lightning/src/ln/channelmanager.rs

@@ -15036,6 +15036,12 @@ pub fn provided_init_features(config: &UserConfig) -> InitFeatures {
 		features.set_anchor_zero_fee_commitments_optional();
 	}
 
+	// If we are configured to be an announced node, we are expected to be always-online and can
+	// advertise the htlc_hold feature.
+	if config.enable_htlc_hold {
+		features.set_htlc_hold_optional();
+	}
+
 	features
 }
 

lightning/src/util/config.rs

@@ -947,6 +947,17 @@ pub struct UserConfig {
 	///
 	/// [`StaticInvoice`]: crate::offers::static_invoice::StaticInvoice
 	pub hold_outbound_htlcs_at_next_hop: bool,
+	/// LDK supports a feature for always-online nodes such that these nodes can hold onto an HTLC
+	/// from an often-offline channel peer until the often-offline payment recipient sends an onion
+	/// message telling the always-online node to release the HTLC. If this is set to `true`, our node
+	/// will carry out this feature for channel peers that request it.
+	///
+	/// This should only be set to `true` for nodes which expect to be online reliably.
+	///
+	/// Setting this to `true` may break backwards compatibility with LDK versions <= 0.2.
+	///
+	/// Default value: `false`
+	pub enable_htlc_hold: bool,
 }
 
 impl Default for UserConfig {
@@ -962,6 +973,7 @@ impl Default for UserConfig {
 			manually_handle_bolt12_invoices: false,
 			enable_dual_funded_channels: false,
 			hold_outbound_htlcs_at_next_hop: true,
+			enable_htlc_hold: false,
 		}
 	}
 }
@@ -982,7 +994,8 @@ impl Readable for UserConfig {
 			accept_intercept_htlcs: Readable::read(reader)?,
 			manually_handle_bolt12_invoices: Readable::read(reader)?,
 			enable_dual_funded_channels: Readable::read(reader)?,
-			send_payments_async: Readable::read(reader)?,
+			hold_outbound_htlcs_at_next_hop: Readable::read(reader)?,
+			enable_htlc_hold: Readable::read(reader)?,
 		})
 	}
 }