Re-claim forwarded HTLCs on startup

Now that we let `commitment_signed` `ChannelMonitorUpdate`s from a
downstream channel complete prior to the preimage
`ChannelMonitorUpdate` on the upstream channel, we may not get a
`update_fulfill_htlc` replay on startup. Thus, we have to ensure
any payment preimages contained in that downstream update are
re-claimed on startup.

Here we do this during the existing walk of the `ChannelMonitor`
preimages for closed channels.

Author: TheBlueMatt

lightning/src/ln/channelmanager.rs

@@ -8273,6 +8273,55 @@ where
 						}
 					}
 				}
+
+				// Whether the downstream channel was closed or not, try to re-apply any payment
+				// preimages from it which may be needed in upstream channels for forwarded
+				// payments.
+				for (htlc_source, (htlc, preimage_opt)) in monitor.get_all_current_outbound_htlcs() {
+					match htlc_source {
+						HTLCSource::PreviousHopData(prev_hop_data) => {
+							if let Some(payment_preimage) =  preimage_opt {
+								let mut is_chan_open = false;
+								if let Some((node_id, chan_id)) = short_to_chan_info.get(&prev_hop_data.short_channel_id) {
+									if let Some(mut peer) = per_peer_state.get_mut(node_id).map(|node| node.lock().unwrap()) {
+										if let Some(chan) = peer.channel_by_id.get_mut(chan_id) {
+											is_chan_open = true;
+											match chan.get_update_fulfill_htlc_and_commit(prev_hop_data.htlc_id, payment_preimage, &args.logger) {
+												UpdateFulfillCommitFetch::DuplicateClaim {} => {},
+												UpdateFulfillCommitFetch::NewClaim { monitor_update, .. } => {
+													// The ChannelMonitor that gave us this
+													// preimage is for a now-closed channel -
+													// no further updates to that channel can
+													// happen which would result in the
+													// preimage being removed, thus we're
+													// guaranteed to regenerate this claim on
+													// restart as long as the source monitor
+													// sticks around.
+													pending_background_events.push(
+														BackgroundEvent::MonitorUpdateRegeneratedOnStartup {
+															counterparty_node_id: *node_id,
+															funding_txo: prev_hop_data.outpoint,
+															update: monitor_update.clone()
+														});
+												},
+											}
+										}
+									}
+								}
+								if !is_chan_open {
+									let monitor_update = ChannelMonitorUpdate {
+										update_id: CLOSED_CHANNEL_UPDATE_ID,
+										updates: vec![ChannelMonitorUpdateStep::PaymentPreimage { payment_preimage }],
+									};
+									pending_background_events.push(BackgroundEvent::
+										ClosingMonitorUpdateRegeneratedOnStartup(
+											(prev_hop_data.outpoint, monitor_update)));
+								}
+							}
+						},
+						_ => {},
+					}
+				}
 			}
 		}