Add a next-outbound-HTLC minimum field to chan details and use it

In the coming commits, in order to ensure all routes we generate
are usable, we'll start calculating the next-HTLC minimum for our
channels and using it in the router. Here we set this up by adding
an always-0 field for it in `ChannelDetails` and use it when
routing.

fuzz/src/router.rs

@@ -265,6 +265,7 @@ pub fn do_test<Out: test_logger::Output>(data: &[u8], out: Out) {
 								balance_msat: 0,
 								outbound_capacity_msat: capacity.saturating_mul(1000),
 								next_outbound_htlc_limit_msat: capacity.saturating_mul(1000),
+								next_outbound_htlc_minimum_msat: 0,
 								inbound_htlc_minimum_msat: None,
 								inbound_htlc_maximum_msat: None,
 								config: None,

lightning/src/ln/channel.rs

@@ -73,6 +73,8 @@ pub struct AvailableBalances {
 	pub outbound_capacity_msat: u64,
 	/// The maximum value we can assign to the next outbound HTLC
 	pub next_outbound_htlc_limit_msat: u64,
+	/// The minimum value we can assign to the next outbound HTLC
+	pub next_outbound_htlc_minimum_msat: u64,
 }
 
 #[derive(Debug, Clone, Copy, PartialEq)]
@@ -2762,6 +2764,7 @@ impl<Signer: WriteableEcdsaChannelSigner> Channel<Signer> {
 				0) as u64,
 			outbound_capacity_msat,
 			next_outbound_htlc_limit_msat: available_capacity_msat,
+			next_outbound_htlc_minimum_msat: 0,
 			balance_msat,
 		}
 	}

lightning/src/ln/channelmanager.rs

@@ -1274,8 +1274,14 @@ pub struct ChannelDetails {
 	/// the current state and per-HTLC limit(s). This is intended for use when routing, allowing us
 	/// to use a limit as close as possible to the HTLC limit we can currently send.
 	///
-	/// See also [`ChannelDetails::balance_msat`] and [`ChannelDetails::outbound_capacity_msat`].
+	/// See also [`ChannelDetails::next_outbound_htlc_minimum_msat`],
+	/// [`ChannelDetails::balance_msat`], and [`ChannelDetails::outbound_capacity_msat`].
 	pub next_outbound_htlc_limit_msat: u64,
+	/// The minimum value for sending a single HTLC to the remote peer. This is the equivalent of
+	/// [`ChannelDetails::next_outbound_htlc_limit_msat`] but represents a lower-bound, rather than
+	/// an upper-bound. This is intended for use when routing, allowing us to ensure we pick a
+	/// route which is valid.
+	pub next_outbound_htlc_minimum_msat: u64,
 	/// The available inbound capacity for the remote peer to send HTLCs to us. This does not
 	/// include any pending HTLCs which are not yet fully resolved (and, thus, whose balance is not
 	/// available for inclusion in new inbound HTLCs).
@@ -1395,6 +1401,7 @@ impl ChannelDetails {
 			inbound_capacity_msat: balance.inbound_capacity_msat,
 			outbound_capacity_msat: balance.outbound_capacity_msat,
 			next_outbound_htlc_limit_msat: balance.next_outbound_htlc_limit_msat,
+			next_outbound_htlc_minimum_msat: balance.next_outbound_htlc_minimum_msat,
 			user_channel_id: channel.get_user_id(),
 			confirmations_required: channel.minimum_depth(),
 			confirmations: Some(channel.get_funding_tx_confirmations(best_block_height)),
@@ -6951,10 +6958,9 @@ impl Writeable for ChannelDetails {
 			(14, user_channel_id_low, required),
 			(16, self.balance_msat, required),
 			(18, self.outbound_capacity_msat, required),
-			// Note that by the time we get past the required read above, outbound_capacity_msat will be
-			// filled in, so we can safely unwrap it here.
-			(19, self.next_outbound_htlc_limit_msat, (default_value, outbound_capacity_msat.0.unwrap() as u64)),
+			(19, self.next_outbound_htlc_limit_msat, required),
 			(20, self.inbound_capacity_msat, required),
+			(21, self.next_outbound_htlc_minimum_msat, required),
 			(22, self.confirmations_required, option),
 			(24, self.force_close_spend_delay, option),
 			(26, self.is_outbound, required),
@@ -6991,6 +6997,7 @@ impl Readable for ChannelDetails {
 			// filled in, so we can safely unwrap it here.
 			(19, next_outbound_htlc_limit_msat, (default_value, outbound_capacity_msat.0.unwrap() as u64)),
 			(20, inbound_capacity_msat, required),
+			(21, next_outbound_htlc_minimum_msat, (default_value, 0)),
 			(22, confirmations_required, option),
 			(24, force_close_spend_delay, option),
 			(26, is_outbound, required),
@@ -7024,6 +7031,7 @@ impl Readable for ChannelDetails {
 			balance_msat: balance_msat.0.unwrap(),
 			outbound_capacity_msat: outbound_capacity_msat.0.unwrap(),
 			next_outbound_htlc_limit_msat: next_outbound_htlc_limit_msat.0.unwrap(),
+			next_outbound_htlc_minimum_msat: next_outbound_htlc_minimum_msat.0.unwrap(),
 			inbound_capacity_msat: inbound_capacity_msat.0.unwrap(),
 			confirmations_required,
 			confirmations,

lightning/src/routing/router.rs

@@ -929,7 +929,7 @@ impl<'a> CandidateRouteHop<'a> {
 
 	fn htlc_minimum_msat(&self) -> u64 {
 		match self {
-			CandidateRouteHop::FirstHop { .. } => 0,
+			CandidateRouteHop::FirstHop { details } => details.next_outbound_htlc_minimum_msat,
 			CandidateRouteHop::PublicHop { info, .. } => info.direction().htlc_minimum_msat,
 			CandidateRouteHop::PrivateHop { hint } => hint.htlc_minimum_msat.unwrap_or(0),
 		}
@@ -2460,6 +2460,7 @@ mod tests {
 			balance_msat: 0,
 			outbound_capacity_msat,
 			next_outbound_htlc_limit_msat: outbound_capacity_msat,
+			next_outbound_htlc_minimum_msat: 0,
 			inbound_capacity_msat: 42,
 			unspendable_punishment_reserve: None,
 			confirmations_required: None,
@@ -6121,6 +6122,7 @@ pub(crate) mod bench_utils {
 			user_channel_id: 0,
 			balance_msat: 10_000_000_000,
 			outbound_capacity_msat: 10_000_000_000,
+			next_outbound_htlc_minimum_msat: 0,
 			next_outbound_htlc_limit_msat: 10_000_000_000,
 			inbound_capacity_msat: 0,
 			unspendable_punishment_reserve: None,