Ack monitor events when failing-to-fail-backwards

Author: valentinewallace

lightning/src/ln/channelmanager.rs

@@ -8092,6 +8092,7 @@ impl<
 								&&logger,
 							),
 							htlc_id,
+							monitor_event_id,
 						))
 					} else {
 						self.forwarding_channel_not_found(
@@ -8124,7 +8125,7 @@ impl<
 							monitor_event_id,
 							&&logger,
 						);
-						Some((res, htlc_id))
+						Some((res, htlc_id, monitor_event_id))
 					} else {
 						self.forwarding_channel_not_found(
 							core::iter::once(forward_info).chain(draining_pending_forwards),
@@ -8137,7 +8138,7 @@ impl<
 					}
 				},
 			};
-			if let Some((queue_fail_htlc_res, htlc_id)) = queue_fail_htlc_res {
+			if let Some((queue_fail_htlc_res, htlc_id, monitor_event_id)) = queue_fail_htlc_res {
 				if let Err(e) = queue_fail_htlc_res {
 					if let ChannelError::Ignore(msg) = e {
 						if let Some(chan) = peer_state
@@ -8161,6 +8162,11 @@ impl<
 					// fail-backs are best-effort, we probably already have one
 					// pending, and if not that's OK, if not, the channel is on
 					// the chain and sending the HTLC-Timeout is their problem.
+					// Ack the monitor event since the fail is already pending or
+					// the HTLC no longer exists.
+					if let Some((event_id, channel_id)) = monitor_event_id {
+						self.chain_monitor.ack_monitor_event(channel_id, event_id);
+					}
 				}
 			}
 		}