Make PaymentFailureReason downgradable

The PaymentFailureReason variants for invoice request failures will
cause downgrades to break. Instead, use a new TLV for the reason and
continue to write the old TLV, only use None for the new reasons.

Author: jkczyz

lightning/src/events/mod.rs

@@ -1555,10 +1555,17 @@ impl Writeable for Event {
 					Some(payment_hash) => payment_hash,
 					None => &PaymentHash([0; 32]),
 				};
+				let legacy_reason = match reason {
+					Some(PaymentFailureReason::UnknownRequiredFeatures)
+						| Some(PaymentFailureReason::InvoiceRequestExpired)
+						| Some(PaymentFailureReason::InvoiceRequestRejected) => &None,
+					reason => reason,
+				};
 				write_tlv_fields!(writer, {
 					(0, payment_id, required),
-					(1, reason, option),
+					(1, legacy_reason, option),
 					(2, payment_hash, required),
+					(3, reason, option),
 				})
 			},
 			&Event::OpenChannelRequest { .. } => {
@@ -1926,12 +1933,15 @@ impl MaybeReadable for Event {
 					let mut payment_hash = PaymentHash([0; 32]);
 					let mut payment_id = PaymentId([0; 32]);
 					let mut reason = None;
+					let mut legacy_reason = None;
 					read_tlv_fields!(reader, {
 						(0, payment_id, required),
-						(1, reason, upgradable_option),
+						(1, legacy_reason, upgradable_option),
 						(2, payment_hash, required),
+						(3, reason, upgradable_option),
 					});
 					let invoice_received = payment_hash != PaymentHash([0; 32]);
+					let reason = reason.or(legacy_reason);
 					Ok(Some(Event::PaymentFailed {
 						payment_id,
 						payment_hash: invoice_received.then(|| payment_hash),

pending_changelog/3192-invoice-request-failed-event.txt

@@ -10,3 +10,7 @@
  * Any `Event::PaymentFailed` generated without a payment hash will deserialize
    with `PaymentHash([0; 32])` when downgrading. This can be treated like an
    `Event::InvoiceRequestFailed` (#3192).
+ * An `Event::PaymentFailed` generated with one of the following
+   `PaymentFailureReason`s will deserialize with reason `None` after downgrading
+   to a version prior to 0.0.124: `UnknownRequiredFeatures`,
+   `InvoiceRequestExpired`, or `InvoiceRequestRejected` (#3192).