Use consistent cltv_expiry_delta in ForwardTlvs

When converting from CounterpartyForwardingInfo to PaymentRelay, the
cltv_expiry_delta is copied. Then, when forming a blinded payment path,
the value is mutated so that esoteric values don't reveal information
about the path. However, the value was only used in computing
PaymentConstraints and wasn't actually updated in PaymentRelay. Move the
logic for modifying the cltv_expiry_delta to the conversion code to
avoid this inconsistency.

Author: jkczyz

lightning/src/blinded_path/payment.rs

@@ -97,12 +97,24 @@ pub struct PaymentConstraints {
 	pub htlc_minimum_msat: u64,
 }
 
-impl From<CounterpartyForwardingInfo> for PaymentRelay {
-	fn from(info: CounterpartyForwardingInfo) -> Self {
+impl TryFrom<CounterpartyForwardingInfo> for PaymentRelay {
+	type Error = ();
+
+	fn try_from(info: CounterpartyForwardingInfo) -> Result<Self, ()> {
 		let CounterpartyForwardingInfo {
 			fee_base_msat, fee_proportional_millionths, cltv_expiry_delta
 		} = info;
-		Self { cltv_expiry_delta, fee_proportional_millionths, fee_base_msat }
+
+		// Avoid exposing esoteric CLTV expiry deltas
+		let cltv_expiry_delta = match cltv_expiry_delta {
+			0..=40 => 40,
+			41..=80 => 80,
+			81..=144 => 144,
+			145..=216 => 216,
+			_ => return Err(()),
+		};
+
+		Ok(Self { cltv_expiry_delta, fee_proportional_millionths, fee_base_msat })
 	}
 }
 

lightning/src/routing/router.rs

@@ -114,19 +114,14 @@ impl<G: Deref<Target = NetworkGraph<L>> + Clone, L: Deref, S: Deref, SP: Sized,
 					None => return None,
 				};
 				let payment_relay: PaymentRelay = match details.counterparty.forwarding_info {
-					Some(forwarding_info) => forwarding_info.into(),
+					Some(forwarding_info) => match forwarding_info.try_into() {
+						Ok(payment_relay) => payment_relay,
+						Err(()) => return None,
+					},
 					None => return None,
 				};
 
-				// Avoid exposing esoteric CLTV expiry deltas
-				let cltv_expiry_delta = match payment_relay.cltv_expiry_delta {
-					0..=40 => 40u32,
-					41..=80 => 80u32,
-					81..=144 => 144u32,
-					145..=216 => 216u32,
-					_ => return None,
-				};
-
+				let cltv_expiry_delta = payment_relay.cltv_expiry_delta as u32;
 				let payment_constraints = PaymentConstraints {
 					max_cltv_expiry: tlvs.payment_constraints.max_cltv_expiry + cltv_expiry_delta,
 					htlc_minimum_msat: details.inbound_htlc_minimum_msat.unwrap_or(0),