Drop buffered messages for timed out nodes

OnionMessenger buffers onion messages for nodes that are pending a
connection. To prevent DoS concerns, add a timer_tick_occurred method to
OnionMessageHandler so that buffered messages can be dropped. This will
be called in lightning-background-processor every 10 seconds.

Author: jkczyz

lightning/src/ln/msgs.rs

@@ -1580,6 +1580,9 @@ pub trait OnionMessageHandler: EventsProvider {
 	/// drop and refuse to forward onion messages to this peer.
 	fn peer_disconnected(&self, their_node_id: &PublicKey);
 
+	/// Performs actions that should happen on startup and roughly every ten seconds thereafter.
+	fn timer_tick_occurred(&self);
+
 	// Handler information:
 	/// Gets the node feature flags which this handler itself supports. All available handlers are
 	/// queried similarly and their feature flags are OR'd together to form the [`NodeFeatures`]

lightning/src/ln/peer_handler.rs

@@ -118,6 +118,7 @@ impl OnionMessageHandler for IgnoringMessageHandler {
 	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(()) }
 	fn peer_disconnected(&self, _their_node_id: &PublicKey) {}
+	fn timer_tick_occurred(&self) {}
 	fn provided_node_features(&self) -> NodeFeatures { NodeFeatures::empty() }
 	fn provided_init_features(&self, _their_node_id: &PublicKey) -> InitFeatures {
 		InitFeatures::empty()

lightning/src/onion_message/messenger.rs

@@ -871,6 +871,16 @@ where
 		self.message_buffers.lock().unwrap().remove(their_node_id);
 	}
 
+	fn timer_tick_occurred(&self) {
+		// Drop any pending recipients since the last call to avoid retaining buffered messages for
+		// too long.
+		self.message_buffers.lock().unwrap().retain(|_, recipient| match recipient {
+			OnionMessageBuffer::PendingConnection(_, None) => false,
+			OnionMessageBuffer::PendingConnection(_, Some(_)) => true,
+			_ => true,
+		});
+	}
+
 	fn provided_node_features(&self) -> NodeFeatures {
 		let mut features = NodeFeatures::empty();
 		features.set_onion_messages_optional();