Make PaymentFailureReason upgradable

This allows downgrading to version 0.0.124 or later and using None for a
PaymentFailureReason that was added after.

Author: jkczyz

lightning/src/events/mod.rs

@@ -538,7 +538,7 @@ pub enum PaymentFailureReason {
 	InvoiceRequestRejected,
 }
 
-impl_writeable_tlv_based_enum!(PaymentFailureReason,
+impl_writeable_tlv_based_enum_upgradable!(PaymentFailureReason,
 	(0, RecipientRejected) => {},
 	(1, UnknownRequiredFeatures) => {},
 	(2, UserAbandoned) => {},
@@ -878,8 +878,9 @@ pub enum Event {
 		/// [`Offer`]: crate::offers::offer::Offer
 		payment_hash: Option<PaymentHash>,
 		/// The reason the payment failed. This is only `None` for events generated or serialized
-		/// by versions prior to 0.0.115 or when deserializing an `Event::InvoiceRequestFailed`,
-		/// which was removed in 0.0.124.
+		/// by versions prior to 0.0.115, when deserializing an `Event::InvoiceRequestFailed`, which
+		/// was removed in 0.0.124, or when downgrading to 0.0.124 or later with a reason that was
+		/// added after.
 		reason: Option<PaymentFailureReason>,
 	},
 	/// Indicates that a path for an outbound payment was successful.
@@ -1934,7 +1935,7 @@ impl MaybeReadable for Event {
 					Ok(Some(Event::PaymentFailed {
 						payment_id,
 						payment_hash: invoice_received.then(|| payment_hash),
-						reason,
+						reason: _init_tlv_based_struct_field!(reason, upgradable_option),
 					}))
 				};
 				f()

lightning/src/ln/outbound_payment.rs

@@ -96,7 +96,8 @@ pub(crate) enum PendingOutboundPayment {
 	Abandoned {
 		session_privs: HashSet<[u8; 32]>,
 		payment_hash: PaymentHash,
-		/// Will be `None` if the payment was serialized before 0.0.115.
+		/// Will be `None` if the payment was serialized before 0.0.115 or if downgrading to 0.0.124
+		/// or later with a reason that was added after.
 		reason: Option<PaymentFailureReason>,
 	},
 }
@@ -1940,7 +1941,7 @@ impl_writeable_tlv_based_enum_upgradable!(PendingOutboundPayment,
 	},
 	(3, Abandoned) => {
 		(0, session_privs, required),
-		(1, reason, option),
+		(1, reason, upgradable_option),
 		(2, payment_hash, required),
 	},
 	(5, AwaitingInvoice) => {