Attempt a last-ditch ChannelManager persistence if the BP exits

If the BP exits because it failed to write our ChannelManager to
disk, we'll probably lose some channels on startup. However,
there's still some chance for us to write the `ChannelManager` in a
retry, which we should do before we exit.

Author: TheBlueMatt

src/main.rs

@@ -914,12 +914,11 @@ async fn start_ldk() {
 
 	// Exit if either CLI polling exits or the background processor exits (which shouldn't happen
 	// unless we fail to write to the filesystem).
+	let mut bg_res = Ok(Ok(()));
 	tokio::select! {
 		_ = cli_poll => {},
-		bg_res = &mut background_processor => {
-			stop_listen_connect.store(true, Ordering::Release);
-			peer_manager.disconnect_all_peers();
-			panic!("ERR: background processing stopped with result {:?}, exiting", bg_res);
+		bg_exit = &mut background_processor => {
+			bg_res = bg_exit;
 		},
 	}
 
@@ -928,6 +927,21 @@ async fn start_ldk() {
 	stop_listen_connect.store(true, Ordering::Release);
 	peer_manager.disconnect_all_peers();
 
+	if let Err(e) = bg_res {
+		let persist_res = persister.persist("manager", &*channel_manager).unwrap();
+		use lightning::util::logger::Logger;
+		lightning::log_error!(
+			&*logger,
+			"Last-ditch ChannelManager persistence result: {:?}",
+			persist_res
+		);
+		panic!(
+			"ERR: background processing stopped with result {:?}, exiting.\n\
+			Last-ditch ChannelManager persistence result {:?}",
+			e, persist_res
+		);
+	}
+
 	// Stop the background processor.
 	if !bp_exit.is_closed() {
 		bp_exit.send(()).unwrap();