Replace EventsProvider on OnionMessageHandler with a single fn

`OnionMessageHandler`s are expected to regularly release a set of
nodes which we need to directly connect to to deliver onion
messages. In order to do so, they currently extend
`EventsProvider`, returning that set as `Event::ConnectionNeeded`s.

While this works fine in Rust, the `EventsProvider` interface
doesn't map well in bindings due to it taking a flexible trait impl
as a method argument.

Instead, here, we convert `OnionMessageHandler` to include a single
function which returns nodes in the form of a `node_id` and
`Vec<SocketAddress>`. This is a bit simpler, if less flexible, API,
and while largely equivalent, is easier to map in bindings.

lightning-background-processor/src/lib.rs

@@ -694,7 +694,10 @@ where
 		persister, chain_monitor,
 		chain_monitor.process_pending_events_async(async_event_handler).await,
 		channel_manager, channel_manager.process_pending_events_async(async_event_handler).await,
-		peer_manager, process_onion_message_handler_events_async(&peer_manager, async_event_handler).await,
+		peer_manager,
+		for event in onion_message_handler_events(peer_manager) {
+			handler(event).await
+		},
 		gossip_sync, logger, scorer, should_break, {
 			let fut = Selector {
 				a: channel_manager.get_event_or_persistence_needed_future(),
@@ -719,23 +722,11 @@ where
 	)
 }
 
-#[cfg(feature = "futures")]
-async fn process_onion_message_handler_events_async<
-	EventHandlerFuture: core::future::Future<Output = ()>,
-	EventHandler: Fn(Event) -> EventHandlerFuture,
-	PM: 'static + Deref + Send + Sync,
->(
-	peer_manager: &PM, handler: EventHandler
-)
-where
-	PM::Target: APeerManager + Send + Sync,
-{
-	let events = core::cell::RefCell::new(Vec::new());
-	peer_manager.onion_message_handler().process_pending_events(&|e| events.borrow_mut().push(e));
-
-	for event in events.into_inner() {
-		handler(event).await
-	}
+fn onion_message_handler_events<PM: 'static + Deref + Send + Sync>(
+	peer_manager: &PM
+) -> impl Iterator<Item=Event> where PM::Target: APeerManager + Send + Sync {
+	peer_manager.onion_message_handler().get_and_clear_connections_needed()
+		.into_iter().map(|(node_id, addresses)| Event::ConnectionNeeded { node_id, addresses })
 }
 
 #[cfg(feature = "std")]
@@ -851,7 +842,9 @@ impl BackgroundProcessor {
 				persister, chain_monitor, chain_monitor.process_pending_events(&event_handler),
 				channel_manager, channel_manager.process_pending_events(&event_handler),
 				peer_manager,
-				peer_manager.onion_message_handler().process_pending_events(&event_handler),
+				for event in onion_message_handler_events(&peer_manager) {
+					event_handler.handle_event(event);
+				},
 				gossip_sync, logger, scorer, stop_thread.load(Ordering::Acquire),
 				{ Sleeper::from_two_futures(
 					channel_manager.get_event_or_persistence_needed_future(),

lightning/src/ln/msgs.rs

@@ -1632,7 +1632,16 @@ pub trait RoutingMessageHandler : MessageSendEventsProvider {
 }
 
 /// A handler for received [`OnionMessage`]s and for providing generated ones to send.
-pub trait OnionMessageHandler: EventsProvider {
+pub trait OnionMessageHandler {
+	/// Because much of the lightning network does not yet support forwarding onion messages, we
+	/// may need to directly connect to a node which will forward a message for us. In such a case,
+	/// this method will return the set of nodes which need connection by node_id and the
+	/// corresponding socket addresses where they may accept incoming connections.
+	///
+	/// Thus, this method should be polled regularly to detect messages await such a direct
+	/// connection.
+	fn get_and_clear_connections_needed(&self) -> Vec<(PublicKey, Vec<SocketAddress>)>;
+
 	/// Handle an incoming `onion_message` message from the given peer.
 	fn handle_onion_message(&self, peer_node_id: &PublicKey, msg: &OnionMessage);
 

lightning/src/ln/peer_handler.rs

@@ -123,6 +123,7 @@ impl RoutingMessageHandler for IgnoringMessageHandler {
 	fn processing_queue_high(&self) -> bool { false }
 }
 impl OnionMessageHandler for IgnoringMessageHandler {
+	fn get_and_clear_connections_needed(&self) -> Vec<(PublicKey, Vec<SocketAddress>)> { Vec::new() }
 	fn handle_onion_message(&self, _their_node_id: &PublicKey, _msg: &msgs::OnionMessage) {}
 	fn next_onion_message_for_peer(&self, _peer_node_id: PublicKey) -> Option<msgs::OnionMessage> { None }
 	fn peer_connected(&self, _their_node_id: &PublicKey, _init: &msgs::Init, _inbound: bool) -> Result<(), ()> { Ok(()) }

lightning/src/onion_message/messenger.rs

@@ -890,7 +890,7 @@ fn outbound_buffer_full(peer_node_id: &PublicKey, buffer: &HashMap<PublicKey, On
 	false
 }
 
-impl<ES: Deref, NS: Deref, L: Deref, MR: Deref, OMH: Deref, CMH: Deref> EventsProvider
+impl<ES: Deref, NS: Deref, L: Deref, MR: Deref, OMH: Deref, CMH: Deref> OnionMessageHandler
 for OnionMessenger<ES, NS, L, MR, OMH, CMH>
 where
 	ES::Target: EntropySource,
@@ -900,27 +900,18 @@ where
 	OMH::Target: OffersMessageHandler,
 	CMH::Target: CustomOnionMessageHandler,
 {
-	fn process_pending_events<H: Deref>(&self, handler: H) where H::Target: EventHandler {
+	fn get_and_clear_connections_needed(&self) -> Vec<(PublicKey, Vec<SocketAddress>)> {
+		let mut res = Vec::new();
 		for (node_id, recipient) in self.message_recipients.lock().unwrap().iter_mut() {
 			if let OnionMessageRecipient::PendingConnection(_, addresses, _) = recipient {
 				if let Some(addresses) = addresses.take() {
-					handler.handle_event(Event::ConnectionNeeded { node_id: *node_id, addresses });
+					res.push((*node_id, addresses));
 				}
 			}
 		}
+		res
 	}
-}
 
-impl<ES: Deref, NS: Deref, L: Deref, MR: Deref, OMH: Deref, CMH: Deref> OnionMessageHandler
-for OnionMessenger<ES, NS, L, MR, OMH, CMH>
-where
-	ES::Target: EntropySource,
-	NS::Target: NodeSigner,
-	L::Target: Logger,
-	MR::Target: MessageRouter,
-	OMH::Target: OffersMessageHandler,
-	CMH::Target: CustomOnionMessageHandler,
-{
 	fn handle_onion_message(&self, _peer_node_id: &PublicKey, msg: &OnionMessage) {
 		match peel_onion_message(
 			msg, &self.secp_ctx, &*self.node_signer, &*self.logger, &*self.custom_handler