Add MonitorUpdateCompletionAction::EmitForwardEvent

Indicates we should emit an Event::PaymentForwarded and possibly ack a monitor
event via Watch::ack_monitor_event.

This is generated when we've completed an inbound edge preimage update for an
HTLC forward, at which point it's safe to generate the forward event. If the
inbound edge is closed, a monitor event id may be included so we can tell the
outbound edge to stop telling us about the claim once the forward event is
processed by the user.

Author: valentinewallace

lightning/src/ln/channelmanager.rs

@@ -1485,6 +1485,17 @@ pub(crate) enum MonitorUpdateCompletionAction {
 	/// to avoid acking a monitor event until after an HTLC is fully removed via revoke_and_ack, to
 	/// ensure that the HTLC gets resolved even if we lose the holding cell.
 	AckMonitorEvents { event_ids: Vec<MonitorEventSource> },
+	/// Indicates we should emit an [`Event::PaymentForwarded`] and possibly ack a monitor event via
+	/// [`Watch::ack_monitor_event`].
+	///
+	/// This is generated when we've completed an inbound edge preimage update for an HTLC forward,
+	/// at which point it's safe to generate the forward event. If the inbound edge is closed, a
+	/// monitor event id may be included so we can tell the outbound edge to stop telling us about
+	/// the claim once the forward event is processed by the user.
+	EmitForwardEvent {
+		event: ForwardEventContents,
+		post_event_ackable_monitor_event: Option<MonitorEventSource>,
+	},
 }
 
 impl_writeable_tlv_based_enum_upgradable!(MonitorUpdateCompletionAction,
@@ -1508,6 +1519,10 @@ impl_writeable_tlv_based_enum_upgradable!(MonitorUpdateCompletionAction,
 	},
 	(3, AckMonitorEvents) => {
 		(1, event_ids, required_vec),
+	},
+	(5, EmitForwardEvent) => {
+		(1, event, required),
+		(3, post_event_ackable_monitor_event, option),
 	}
 );
 
@@ -10491,6 +10506,17 @@ This indicates a bug inside LDK. Please report this error at https://github.com/
 						self.chain_monitor.ack_monitor_event(id);
 					}
 				},
+				MonitorUpdateCompletionAction::EmitForwardEvent {
+					event,
+					post_event_ackable_monitor_event,
+				} => {
+					let post_event_action = post_event_ackable_monitor_event
+						.map(|event_id| EventCompletionAction::AckMonitorEvent { event_id });
+					self.pending_events
+						.lock()
+						.unwrap()
+						.push_back((event.into(), post_event_action));
+				},
 			}
 		}