Use the PaymentParameter final CLTV delta over RouteParameters

`PaymentParams` is all about the parameters for a payment, i.e. the
parameters which are static across all the paths of a paymet.
`RouteParameters` is about the information specific to a given
`Route` (i.e. a set of paths, among multiple potential sets of
paths for a payment). The CLTV delta thus doesn't belong in
`RouterParameters` but instead in `PaymentParameters`.

Worse, because `RouteParameters` is built from the information in
the last hops of a `Route`, when we deliberately inflate the CLTV
delta in path-finding, retries of the payment will have the final
CLTV delta double-inflated as it inflates starting from the final
CLTV delta used in the last attempt.

When we calculate the `final_cltv_expiry_delta` to put in the
`RouteParameters` returned via events after a payment failure, we
should re-use the new one in the `PaymentParameters`, rather than
the one that was in the route itself.

Author: TheBlueMatt

lightning/src/ln/channelmanager.rs

@@ -245,6 +245,10 @@ pub(crate) enum HTLCSource {
 		first_hop_htlc_msat: u64,
 		payment_id: PaymentId,
 		payment_secret: Option<PaymentSecret>,
+		/// Note that this is now "deprecated" - we write it for forwards (and read it for
+		/// backwards) compatibility reasons, but prefer to use the data in the
+		/// [`super::outbound_payment`] module, which stores per-payment data once instead of in
+		/// each HTLC.
 		payment_params: Option<PaymentParameters>,
 	},
 }

lightning/src/ln/outbound_payment.rs

@@ -789,7 +789,9 @@ impl OutboundPayments {
 						Some(RouteParameters {
 							payment_params: payment_params.clone(),
 							final_value_msat: pending_amt_unsent,
-							final_cltv_expiry_delta: max_unsent_cltv_delta,
+							final_cltv_expiry_delta:
+								if let Some(delta) = payment_params.final_cltv_expiry_delta { delta }
+								else { max_unsent_cltv_delta },
 						})
 					} else { None }
 				} else { None },
@@ -987,7 +989,9 @@ impl OutboundPayments {
 			Some(RouteParameters {
 				payment_params: payment_params_data.clone(),
 				final_value_msat: path_last_hop.fee_msat,
-				final_cltv_expiry_delta: path_last_hop.cltv_expiry_delta,
+				final_cltv_expiry_delta:
+					if let Some(delta) = payment_params_data.final_cltv_expiry_delta { delta }
+					else { path_last_hop.cltv_expiry_delta },
 			})
 		} else { None };
 		log_trace!(logger, "Failing outbound payment HTLC with payment_hash {}", log_bytes!(payment_hash.0));