ln/refactor: pass minimum delta into check_incoming_htlc_cltv

For trampoline payments, we don't want to enforce a minimum cltv delta
between our incoming and outer onion outgoing CLTV because we'll
calculate our delta from the inner trampoline onion's value. However,
we still want to check that we get at least the CLTV that the sending
node intended for us and we still want to validate our incoming value.
Refactor to allow setting a zero delta, for use for trampoline payments.

Author: carlaKC

lightning/src/ln/channelmanager.rs

@@ -5219,8 +5219,12 @@ impl<
 			};
 
 		let cur_height = self.best_block.read().unwrap().height + 1;
-		check_incoming_htlc_cltv(cur_height, next_hop.outgoing_cltv_value, msg.cltv_expiry)?;
-
+		check_incoming_htlc_cltv(
+			cur_height,
+			next_hop.outgoing_cltv_value,
+			msg.cltv_expiry,
+			MIN_CLTV_EXPIRY_DELTA,
+		)?;
 		Ok(intercept)
 	}
 

lightning/src/ln/onion_payment.rs

@@ -515,7 +515,7 @@ pub fn peel_payment_onion<NS: NodeSigner, L: Logger, T: secp256k1::Verification>
 			};
 
 			if let Err(reason) = check_incoming_htlc_cltv(
-				cur_height, outgoing_cltv_value, msg.cltv_expiry,
+				cur_height, outgoing_cltv_value, msg.cltv_expiry, MIN_CLTV_EXPIRY_DELTA,
 			) {
 				return Err(InboundHTLCErr {
 					msg: "incoming cltv check failed",
@@ -719,9 +719,9 @@ pub(super) fn decode_incoming_update_add_htlc_onion<NS: NodeSigner, L: Logger, T
 }
 
 pub(super) fn check_incoming_htlc_cltv(
-	cur_height: u32, outgoing_cltv_value: u32, cltv_expiry: u32,
+	cur_height: u32, outgoing_cltv_value: u32, cltv_expiry: u32, min_cltv_expiry_delta: u16,
 ) -> Result<(), LocalHTLCFailureReason> {
-	if (cltv_expiry as u64) < (outgoing_cltv_value) as u64 + MIN_CLTV_EXPIRY_DELTA as u64 {
+	if (cltv_expiry as u64) < (outgoing_cltv_value) as u64 + min_cltv_expiry_delta as u64 {
 		return Err(LocalHTLCFailureReason::IncorrectCLTVExpiry);
 	}
 	// Theoretically, channel counterparty shouldn't send us a HTLC expiring now,