Drop buffered messages for timed out nodes

OnionMessenger buffers onion messages for nodes that are pending a
connection. To prevent DoS concerns, drop the buffered messages if the
node has not connected within one timer tick, which should correspond to
10 seconds for PeerManager when using lightning-background-processor.

Author: jkczyz

lightning/src/onion_message/messenger.rs

@@ -797,7 +797,19 @@ where
 	}
 
 	fn release_pending_connections(&self) -> Vec<NodeAnnouncement> {
-		self.message_buffers.lock().unwrap()
+		let mut message_buffers = self.message_buffers.lock().unwrap();
+
+		// Drop any pending recipients since the last call to avoid retaining buffered messages for
+		// too long.
+		message_buffers.retain(|_, recipient| match recipient {
+			OnionMessageBuffer::PendingConnection(_, None) => false,
+			OnionMessageBuffer::PendingConnection(_, Some(_)) => true,
+			_ => true,
+		});
+
+		// Release node announcements for pending recipients so that their buffered messages are
+		// dropped the next time this method is called.
+		message_buffers
 			.values_mut()
 			.filter_map(|recipient| match recipient {
 				OnionMessageBuffer::PendingConnection(_, node_announcement) => {