Expose outbound SCID alias in ChannelDetails and use in routing

This supports routing outbound over 0-conf channels by utilizing
the outbound SCID alias that we assign to all channels to refer to
the selected channel when routing.

Author: TheBlueMatt

lightning/src/ln/channelmanager.rs

@@ -969,6 +969,13 @@ pub struct ChannelDetails {
 	/// [`inbound_scid_alias`]: Self::inbound_scid_alias
 	/// [`get_inbound_payment_scid`]: Self::get_inbound_payment_scid
 	pub short_channel_id: Option<u64>,
+	/// An optional [`short_channel_id`] alias for this channel, randomly generated by us and
+	/// usable in place of [`short_channel_id`] to reference the channel in outbound routes even if
+	/// the channel has not yet been confirmed (as long as [`confirmations_required`] is
+	/// `Some(0)`).
+	///
+	/// This will be `None` as long as the channel is not available for routing outbound payments.
+	pub outbound_scid_alias: Option<u64>,
 	/// An optional [`short_channel_id`] alias for this channel, randomly generated by our
 	/// counterparty and usable in place of [`short_channel_id`] in invoice route hints. Our
 	/// counterparty will recognize the alias provided here in place of the [`short_channel_id`]
@@ -1086,6 +1093,16 @@ impl ChannelDetails {
 	pub fn get_inbound_payment_scid(&self) -> Option<u64> {
 		self.inbound_scid_alias.or(self.short_channel_id)
 	}
+
+	/// Gets the current SCID which should be used to identify this channel for outbound payments.
+	/// This should be used in [`Route`]s to describe the first hop or in other contexts where
+	/// we're sending or forwarding a payment outbound over this channel.
+	///
+	/// This is either the [`ChannelDetails::short_channel_id`], if set, or the
+	/// [`ChannelDetails::outbound_scid_alias`]. See those for more information.
+	pub fn get_outbound_payment_scid(&self) -> Option<u64> {
+		self.short_channel_id.or(self.outbound_scid_alias)
+	}
 }
 
 /// If a payment fails to send, it can be in one of several states. This enum is returned as the
@@ -1712,6 +1729,7 @@ impl<Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelMana
 					// `have_received_message` indicates that type negotiation has completed.
 					channel_type: if channel.have_received_message() { Some(channel.get_channel_type().clone()) } else { None },
 					short_channel_id: channel.get_short_channel_id(),
+					outbound_scid_alias: if channel.is_live() { Some(channel.outbound_scid_alias()) } else { None },
 					inbound_scid_alias: channel.latest_inbound_scid_alias(),
 					channel_value_satoshis: channel.get_value_satoshis(),
 					unspendable_punishment_reserve: to_self_reserve_satoshis,
@@ -5982,6 +6000,7 @@ impl_writeable_tlv_based!(ChannelDetails, {
 	(2, channel_id, required),
 	(3, channel_type, option),
 	(4, counterparty, required),
+	(5, outbound_scid_alias, option),
 	(6, funding_txo, option),
 	(8, short_channel_id, option),
 	(10, channel_value_satoshis, required),

lightning/src/ln/priv_short_conf_tests.rs

@@ -639,4 +639,6 @@ fn test_simple_0conf_channel() {
 
 	assert_eq!(nodes[0].node.list_usable_channels().len(), 1);
 	assert_eq!(nodes[1].node.list_usable_channels().len(), 1);
+
+	send_payment(&nodes[0], &[&nodes[1]], 100_000);
 }

lightning/src/routing/router.rs

@@ -368,7 +368,7 @@ enum CandidateRouteHop<'a> {
 impl<'a> CandidateRouteHop<'a> {
 	fn short_channel_id(&self) -> u64 {
 		match self {
-			CandidateRouteHop::FirstHop { details } => details.short_channel_id.unwrap(),
+			CandidateRouteHop::FirstHop { details } => details.get_outbound_payment_scid().unwrap(),
 			CandidateRouteHop::PublicHop { short_channel_id, .. } => *short_channel_id,
 			CandidateRouteHop::PrivateHop { hint } => hint.short_channel_id,
 		}
@@ -777,7 +777,7 @@ where L::Target: Logger {
 		HashMap::with_capacity(if first_hops.is_some() { first_hops.as_ref().unwrap().len() } else { 0 });
 	if let Some(hops) = first_hops {
 		for chan in hops {
-			if chan.short_channel_id.is_none() {
+			if chan.get_outbound_payment_scid().is_none() {
 				panic!("first_hops should be filled in with usable channels, not pending ones");
 			}
 			if chan.counterparty.node_id == *our_node_pubkey {
@@ -1331,7 +1331,7 @@ where L::Target: Logger {
 					let mut features_set = false;
 					if let Some(first_channels) = first_hop_targets.get(&ordered_hops.last().unwrap().0.node_id) {
 						for details in first_channels {
-							if details.short_channel_id.unwrap() == ordered_hops.last().unwrap().0.candidate.short_channel_id() {
+							if details.get_outbound_payment_scid().unwrap() == ordered_hops.last().unwrap().0.candidate.short_channel_id() {
 								ordered_hops.last_mut().unwrap().1 = details.counterparty.features.to_context();
 								features_set = true;
 								break;
@@ -1742,6 +1742,7 @@ mod tests {
 			funding_txo: Some(OutPoint { txid: bitcoin::Txid::from_slice(&[0; 32]).unwrap(), index: 0 }),
 			channel_type: None,
 			short_channel_id,
+			outbound_scid_alias: None,
 			inbound_scid_alias: None,
 			channel_value_satoshis: 0,
 			user_channel_id: 0,