Misc updates to tee up async ChannelMonitorUpdate persist for claims against closed channels

lightning/src/chain/channelmonitor.rs

@@ -3150,8 +3150,11 @@ impl<Signer: EcdsaChannelSigner> ChannelMonitorImpl<Signer> {
 					panic!("Attempted to apply post-force-close ChannelMonitorUpdate that wasn't providing a payment preimage");
 				},
 			}
-		} else if self.latest_update_id + 1 != updates.update_id {
-			panic!("Attempted to apply ChannelMonitorUpdates out of order, check the update_id before passing an update to update_monitor!");
+		}
+		if updates.update_id != LEGACY_CLOSED_CHANNEL_UPDATE_ID {
+			if self.latest_update_id + 1 != updates.update_id {
+				panic!("Attempted to apply ChannelMonitorUpdates out of order, check the update_id before passing an update to update_monitor!");
+			}
 		}
 		let mut ret = Ok(());
 		let bounded_fee_estimator = LowerBoundedFeeEstimator::new(&**fee_estimator);

lightning/src/ln/chanmon_update_fail_tests.rs

@@ -3288,7 +3288,7 @@ fn do_test_durable_preimages_on_closed_channel(close_chans_before_reload: bool,
 	// Finally, check that B created a payment preimage transaction and close out the payment.
 	let bs_txn = nodes[1].tx_broadcaster.txn_broadcasted.lock().unwrap().split_off(0);
 	assert_eq!(bs_txn.len(), if close_chans_before_reload && !close_only_a { 2 } else { 1 });
-	let bs_preimage_tx = &bs_txn[0];
+	let bs_preimage_tx = bs_txn.iter().find(|tx| tx.input[0].previous_output.txid == as_closing_tx[0].compute_txid()).unwrap();
 	check_spends!(bs_preimage_tx, as_closing_tx[0]);
 
 	if !close_chans_before_reload {