Add config flag for held outbound HTLCs

If the flag is set to true, then if we as an often-offline payer receive a
static invoice to pay, we will attempt to hold the corresponding outbound HTLCs
with our next-hop channel counterparty(s) that support the  feature. This
allows our node to go offline once the HTLCs are locked in even though the
recipient may not yet be online to receive them.

Author: valentinewallace

fuzz/src/full_stack.rs

@@ -1048,7 +1048,7 @@ fn two_peer_forwarding_seed() -> Vec<u8> {
 	// our network key
 	ext_from_hex("0100000000000000000000000000000000000000000000000000000000000000", &mut test);
 	// config
-	ext_from_hex("000000000090000000000000000064000100000000000100ffff0000000000000000ffffffffffffffffffffffffffffffff0000000000000000ffffffffffffffff000000ffffffff00ffff1a000400010000020400000000040200000a08ffffffffffffffff000100000000", &mut test);
+	ext_from_hex("000000000090000000000000000064000100000000000100ffff0000000000000000ffffffffffffffffffffffffffffffff0000000000000000ffffffffffffffff000000ffffffff00ffff1a000400010000020400000000040200000a08ffffffffffffffff00010000000000", &mut test);
 
 	// new outbound connection with id 0
 	ext_from_hex("00", &mut test);
@@ -1502,7 +1502,7 @@ fn gossip_exchange_seed() -> Vec<u8> {
 	// our network key
 	ext_from_hex("0100000000000000000000000000000000000000000000000000000000000000", &mut test);
 	// config
-	ext_from_hex("000000000090000000000000000064000100000000000100ffff0000000000000000ffffffffffffffffffffffffffffffff0000000000000000ffffffffffffffff000000ffffffff00ffff1a000400010000020400000000040200000a08ffffffffffffffff000100000000", &mut test);
+	ext_from_hex("000000000090000000000000000064000100000000000100ffff0000000000000000ffffffffffffffffffffffffffffffff0000000000000000ffffffffffffffff000000ffffffff00ffff1a000400010000020400000000040200000a08ffffffffffffffff00010000000000", &mut test);
 
 	// new outbound connection with id 0
 	ext_from_hex("00", &mut test);

lightning/src/util/config.rs

@@ -943,6 +943,20 @@ pub struct UserConfig {
 	/// Default value: `false`
 	#[cfg(test)]
 	pub enable_htlc_hold: bool,
+	/// If this is set to true, then if we as an often-offline payer receive a [`StaticInvoice`] to
+	/// pay, we will attempt to hold the corresponding outbound HTLCs with our next-hop channel
+	/// counterparty(s) that support the `htlc_hold` feature. This allows our node to go offline once
+	/// the HTLCs are locked in even though the recipient may not yet be online to receive them.
+	///
+	/// This option is intended for usage by private nodes, and should NOT be set if we are an
+	/// announced node that is expected to be online at all times.
+	///
+	/// Setting this to `true` may lead to HTLC failures if downgrading to LDK versions < 0.2.
+	///
+	/// Default value: `false`
+	///
+	/// [`StaticInvoice`]: crate::offers::static_invoice::StaticInvoice
+	pub hold_outbound_htlcs_at_next_hop: bool,
 }
 
 impl Default for UserConfig {
@@ -959,6 +973,7 @@ impl Default for UserConfig {
 			enable_dual_funded_channels: false,
 			#[cfg(test)]
 			enable_htlc_hold: false,
+			hold_outbound_htlcs_at_next_hop: false,
 		}
 	}
 }
@@ -979,6 +994,7 @@ impl Readable for UserConfig {
 			accept_intercept_htlcs: Readable::read(reader)?,
 			manually_handle_bolt12_invoices: Readable::read(reader)?,
 			enable_dual_funded_channels: Readable::read(reader)?,
+			hold_outbound_htlcs_at_next_hop: Readable::read(reader)?,
 		})
 	}
 }