Add CandidateRouteHop::Blinded variant

It's unclear what values 1-hop blinded paths should set their BlindedPayInfos
to, because those values are meant to refer to the fees/cltv delta on the path
*between* the intro node and the destination. We zero out these values in the
new variant's methods so they don't mess with path finding/construction.

Author: valentinewallace

lightning/src/routing/router.rs

@@ -942,9 +942,15 @@ enum CandidateRouteHop<'a> {
 		info: DirectedChannelInfo<'a>,
 		short_channel_id: u64,
 	},
-	/// A hop to the payee found in the payment invoice, though not necessarily a direct channel.
+	/// A hop to the payee found in the BOLT 11 payment invoice, though not necessarily a direct
+	/// channel.
 	PrivateHop {
 		hint: &'a RouteHintHop,
+	},
+	/// The payee's identity is concealed behind blinded paths provided in a BOLT 12 invoice.
+	Blinded {
+		hint: &'a (BlindedPayInfo, BlindedPath),
+		hint_idx: usize,
 	}
 }
 
@@ -954,6 +960,7 @@ impl<'a> CandidateRouteHop<'a> {
 			CandidateRouteHop::FirstHop { details } => Some(details.get_outbound_payment_scid().unwrap()),
 			CandidateRouteHop::PublicHop { short_channel_id, .. } => Some(*short_channel_id),
 			CandidateRouteHop::PrivateHop { hint } => Some(hint.short_channel_id),
+			CandidateRouteHop::Blinded { .. } => None,
 		}
 	}
 
@@ -963,6 +970,7 @@ impl<'a> CandidateRouteHop<'a> {
 			CandidateRouteHop::FirstHop { details } => details.counterparty.features.to_context(),
 			CandidateRouteHop::PublicHop { info, .. } => info.channel().features.clone(),
 			CandidateRouteHop::PrivateHop { .. } => ChannelFeatures::empty(),
+			CandidateRouteHop::Blinded { .. } => ChannelFeatures::empty(),
 		}
 	}
 
@@ -971,6 +979,8 @@ impl<'a> CandidateRouteHop<'a> {
 			CandidateRouteHop::FirstHop { .. } => 0,
 			CandidateRouteHop::PublicHop { info, .. } => info.direction().cltv_expiry_delta as u32,
 			CandidateRouteHop::PrivateHop { hint } => hint.cltv_expiry_delta as u32,
+			CandidateRouteHop::Blinded { hint, .. } =>
+				if hint.1.blinded_hops.len() == 1 { 0 } else { hint.0.cltv_expiry_delta as u32 }
 		}
 	}
 
@@ -979,6 +989,8 @@ impl<'a> CandidateRouteHop<'a> {
 			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),
+			CandidateRouteHop::Blinded { hint, .. } =>
+				if hint.1.blinded_hops.len() == 1 { 0 } else { hint.0.htlc_minimum_msat }
 		}
 	}
 
@@ -989,6 +1001,16 @@ impl<'a> CandidateRouteHop<'a> {
 			},
 			CandidateRouteHop::PublicHop { info, .. } => info.direction().fees,
 			CandidateRouteHop::PrivateHop { hint } => hint.fees,
+			CandidateRouteHop::Blinded { hint, .. } => {
+				if hint.1.blinded_hops.len() == 1 {
+					RoutingFees { base_msat: 0, proportional_millionths: 0 }
+				} else {
+					RoutingFees {
+						base_msat: hint.0.fee_base_msat,
+						proportional_millionths: hint.0.fee_proportional_millionths
+					}
+				}
+			}
 		}
 	}
 
@@ -1002,10 +1024,15 @@ impl<'a> CandidateRouteHop<'a> {
 				EffectiveCapacity::HintMaxHTLC { amount_msat: *max },
 			CandidateRouteHop::PrivateHop { hint: RouteHintHop { htlc_maximum_msat: None, .. }} =>
 				EffectiveCapacity::Infinite,
+			CandidateRouteHop::Blinded { hint, .. } =>
+				if hint.1.blinded_hops.len() == 1 { EffectiveCapacity::Infinite }
+				else { EffectiveCapacity::HintMaxHTLC { amount_msat: hint.0.htlc_maximum_msat } }
 		}
 	}
+
 	fn id(&self, channel_direction: bool /* src_node_id < target_node_id */) -> CandidateHopId {
 		match self {
+			CandidateRouteHop::Blinded { hint_idx, .. } => CandidateHopId::Blinded(*hint_idx),
 			_ => CandidateHopId::Clear((self.short_channel_id().unwrap(), channel_direction)),
 		}
 	}
@@ -1262,6 +1289,12 @@ struct LoggedCandidateHop<'a>(&'a CandidateRouteHop<'a>);
 impl<'a> fmt::Display for LoggedCandidateHop<'a> {
 	fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 		match self.0 {
+			CandidateRouteHop::Blinded { hint, .. } => {
+				"blinded route hint with introduction node id ".fmt(f)?;
+				hint.1.introduction_node_id.fmt(f)?;
+				" and blinding point ".fmt(f)?;
+				hint.1.blinding_point.fmt(f)
+			},
 			_ => {
 				"SCID ".fmt(f)?;
 				self.0.short_channel_id().unwrap().fmt(f)