Fix missing user_channel_id in PaymentForwarded

Previously, if a forwarding node reloaded mid-HTLC-forward with a preimage in
the outbound edge monitor and the outbound edge channel still open, and
subsequently reclaimed the inbound HTLC backwards, the PaymentForwarded event
would be missing the next_user_channel_id field.

Author: valentinewallace

lightning/src/ln/chanmon_update_fail_tests.rs

@@ -3938,7 +3938,12 @@ fn do_test_durable_preimages_on_closed_channel(
 	let evs = nodes[1].node.get_and_clear_pending_events();
 	assert_eq!(evs.len(), if close_chans_before_reload { 2 } else { 1 });
 	for ev in evs {
-		if let Event::PaymentForwarded { .. } = ev {
+		if let Event::PaymentForwarded { claim_from_onchain_tx, next_user_channel_id, .. } = ev {
+			if !claim_from_onchain_tx {
+				// If the outbound channel is still open, the `next_user_channel_id` should be available.
+				// This was previously broken.
+				assert!(next_user_channel_id.is_some())
+			}
 		} else {
 			panic!();
 		}

lightning/src/ln/channelmanager.rs

@@ -18707,14 +18707,16 @@ impl<
 				}
 			}
 			for (channel_id, monitor) in args.channel_monitors.iter() {
-				let mut is_channel_closed = true;
+				let (mut is_channel_closed, mut user_channel_id_opt) = (true, None);
 				let counterparty_node_id = monitor.get_counterparty_node_id();
 				if let Some(peer_state_mtx) = per_peer_state.get(&counterparty_node_id) {
 					let mut peer_state_lock = peer_state_mtx.lock().unwrap();
 					let peer_state = &mut *peer_state_lock;
-					is_channel_closed = !peer_state.channel_by_id.contains_key(channel_id);
-					if reconstruct_manager_from_monitors && !is_channel_closed {
-						if let Some(chan) = peer_state.channel_by_id.get(channel_id) {
+					if let Some(chan) = peer_state.channel_by_id.get(channel_id) {
+						is_channel_closed = false;
+						user_channel_id_opt = Some(chan.context().get_user_id());
+
+						if reconstruct_manager_from_monitors {
 							if let Some(funded_chan) = chan.as_funded() {
 								for (payment_hash, prev_hop) in funded_chan.outbound_htlc_forwards()
 								{
@@ -19014,7 +19016,7 @@ impl<
 
 								Some((htlc_source, payment_preimage, htlc.amount_msat,
 									is_channel_closed, monitor.get_counterparty_node_id(),
-									monitor.get_funding_txo(), monitor.channel_id(), None))
+									monitor.get_funding_txo(), monitor.channel_id(), user_channel_id_opt))
 							} else { None }
 						} else {
 							// If it was an outbound payment, we've handled it above - if a preimage