Use a message buffer abstraction in OnionMessenger

Onion messages are buffered for sending to the next node. Since the
network has limited adoption, connecting directly to a peer may be
necessary. Add an OnionMessageBuffer abstraction that can differentiate
between connected peers and those are pending a connection. This allows
for buffering messages before a connection is established and applying
different buffer policies for peers yet to be connected.

fuzz/src/onion_message.rs

@@ -269,7 +269,7 @@ mod tests {
 						"Received an onion message with path_id None and a reply_path: Custom(TestCustomMessage)"
 						.to_string())), Some(&1));
 			assert_eq!(log_entries.get(&("lightning::onion_message::messenger".to_string(),
-						"Sending onion message: TestCustomMessage".to_string())), Some(&1));
+						"Sending onion message when responding to Custom onion message with path_id None: TestCustomMessage".to_string())), Some(&1));
 		}
 
 		let two_unblinded_hops_om = "\

lightning/src/onion_message/messenger.rs

@@ -150,13 +150,70 @@ where
 	entropy_source: ES,
 	node_signer: NS,
 	logger: L,
-	pending_messages: Mutex<HashMap<PublicKey, VecDeque<OnionMessage>>>,
+	message_buffers: Mutex<HashMap<PublicKey, OnionMessageBuffer>>,
 	secp_ctx: Secp256k1<secp256k1::All>,
 	message_router: MR,
 	offers_handler: OMH,
 	custom_handler: CMH,
 }
 
+/// [`OnionMessage`]s buffered to be sent.
+enum OnionMessageBuffer {
+	/// Messages for a node connected as a peer.
+	ConnectedPeer(VecDeque<OnionMessage>),
+
+	/// Messages for a node that is not yet connected.
+	PendingConnection(VecDeque<OnionMessage>),
+}
+
+impl OnionMessageBuffer {
+	fn pending_messages(&self) -> &VecDeque<OnionMessage> {
+		match self {
+			OnionMessageBuffer::ConnectedPeer(pending_messages) => pending_messages,
+			OnionMessageBuffer::PendingConnection(pending_messages) => pending_messages,
+		}
+	}
+
+	fn enqueue_message(&mut self, message: OnionMessage) {
+		let pending_messages = match self {
+			OnionMessageBuffer::ConnectedPeer(pending_messages) => pending_messages,
+			OnionMessageBuffer::PendingConnection(pending_messages) => pending_messages,
+		};
+
+		pending_messages.push_back(message);
+	}
+
+	fn dequeue_message(&mut self) -> Option<OnionMessage> {
+		let pending_messages = match self {
+			OnionMessageBuffer::ConnectedPeer(pending_messages) => pending_messages,
+			OnionMessageBuffer::PendingConnection(pending_messages) => {
+				debug_assert!(false);
+				pending_messages
+			},
+		};
+
+		pending_messages.pop_front()
+	}
+
+	#[cfg(test)]
+	fn release_pending_messages(&mut self) -> VecDeque<OnionMessage> {
+		let pending_messages = match self {
+			OnionMessageBuffer::ConnectedPeer(pending_messages) => pending_messages,
+			OnionMessageBuffer::PendingConnection(pending_messages) => pending_messages,
+		};
+
+		core::mem::take(pending_messages)
+	}
+
+	fn mark_connected(&mut self) {
+		if let OnionMessageBuffer::PendingConnection(pending_messages) = self {
+			let mut new_pending_messages = VecDeque::new();
+			core::mem::swap(pending_messages, &mut new_pending_messages);
+			*self = OnionMessageBuffer::ConnectedPeer(new_pending_messages);
+		}
+	}
+}
+
 /// An [`OnionMessage`] for [`OnionMessenger`] to send.
 ///
 /// These are obtained when released from [`OnionMessenger`]'s handlers after which they are
@@ -502,7 +559,7 @@ where
 		OnionMessenger {
 			entropy_source,
 			node_signer,
-			pending_messages: Mutex::new(HashMap::new()),
+			message_buffers: Mutex::new(HashMap::new()),
 			secp_ctx,
 			logger,
 			message_router,
@@ -518,21 +575,23 @@ where
 	pub fn send_onion_message<T: OnionMessageContents>(
 		&self, path: OnionMessagePath, contents: T, reply_path: Option<BlindedPath>
 	) -> Result<(), SendError> {
-
 		log_trace!(self.logger, "Sending onion message: {:?}", contents);
-		
+
-		let (first_node_id, onion_msg) = create_onion_message(
+		let (first_node_id, onion_message) = create_onion_message(
 			&self.entropy_source, &self.node_signer, &self.secp_ctx, path, contents, reply_path
 		)?;
 
-		let mut pending_per_peer_msgs = self.pending_messages.lock().unwrap();
+		let mut message_buffers = self.message_buffers.lock().unwrap();
-		if outbound_buffer_full(&first_node_id, &pending_per_peer_msgs) { return Err(SendError::BufferFull) }
+		if outbound_buffer_full(&first_node_id, &message_buffers) {
-		match pending_per_peer_msgs.entry(first_node_id) {
+			return Err(SendError::BufferFull);
+		}
+
+		match message_buffers.entry(first_node_id) {
 			hash_map::Entry::Vacant(_) => Err(SendError::InvalidFirstHop),
 			hash_map::Entry::Occupied(mut e) => {
-				e.get_mut().push_back(onion_msg);
+				e.get_mut().enqueue_message(onion_message);
 				Ok(())
-			}
+			},
 		}
 	}
 
@@ -565,7 +624,7 @@ where
 			}
 		};
 
-		let peers = self.pending_messages.lock().unwrap().keys().copied().collect();
+		let peers = self.message_buffers.lock().unwrap().keys().copied().collect();
 		let path = match self.message_router.find_path(sender, peers, destination) {
 			Ok(path) => path,
 			Err(()) => {
@@ -578,30 +637,29 @@ where
 
 		if let Err(e) = self.send_onion_message(path, contents, reply_path) {
 			log_trace!(self.logger, "Failed sending onion message {}: {:?}", log_suffix, e);
-			return;
 		}
 	}
 
 	#[cfg(test)]
 	pub(super) fn release_pending_msgs(&self) -> HashMap<PublicKey, VecDeque<OnionMessage>> {
-		let mut pending_msgs = self.pending_messages.lock().unwrap();
+		let mut message_buffers = self.message_buffers.lock().unwrap();
 		let mut msgs = HashMap::new();
 		// We don't want to disconnect the peers by removing them entirely from the original map, so we
-		// swap the pending message buffers individually.
+		// release the pending message buffers individually.
-		for (peer_node_id, pending_messages) in &mut *pending_msgs {
+		for (peer_node_id, buffer) in &mut *message_buffers {
-			msgs.insert(*peer_node_id, core::mem::take(pending_messages));
+			msgs.insert(*peer_node_id, buffer.release_pending_messages());
 		}
 		msgs
 	}
 }
 
-fn outbound_buffer_full(peer_node_id: &PublicKey, buffer: &HashMap<PublicKey, VecDeque<OnionMessage>>) -> bool {
+fn outbound_buffer_full(peer_node_id: &PublicKey, buffer: &HashMap<PublicKey, OnionMessageBuffer>) -> bool {
 	const MAX_TOTAL_BUFFER_SIZE: usize = (1 << 20) * 128;
 	const MAX_PER_PEER_BUFFER_SIZE: usize = (1 << 10) * 256;
 	let mut total_buffered_bytes = 0;
 	let mut peer_buffered_bytes = 0;
 	for (pk, peer_buf) in buffer {
-		for om in peer_buf {
+		for om in peer_buf.pending_messages() {
 			let om_len = om.serialized_length();
 			if pk == peer_node_id {
 				peer_buffered_bytes += om_len;
@@ -660,24 +718,28 @@ where
 				}
 			},
 			Ok(PeeledOnion::Forward(next_node_id, onion_message)) => {
-				let mut pending_per_peer_msgs = self.pending_messages.lock().unwrap();
+				let mut message_buffers = self.message_buffers.lock().unwrap();
-				if outbound_buffer_full(&next_node_id, &pending_per_peer_msgs) {
+				if outbound_buffer_full(&next_node_id, &message_buffers) {
 					log_trace!(self.logger, "Dropping forwarded onion message to peer {:?}: outbound buffer full", next_node_id);
 					return
 				}
 
 				#[cfg(fuzzing)]
-				pending_per_peer_msgs.entry(next_node_id).or_insert_with(VecDeque::new);
+				message_buffers
+					.entry(next_node_id)
+					.or_insert_with(|| OnionMessageBuffer::ConnectedPeer(VecDeque::new()));
 
-				match pending_per_peer_msgs.entry(next_node_id) {
+				match message_buffers.entry(next_node_id) {
-					hash_map::Entry::Vacant(_) => {
+					hash_map::Entry::Occupied(mut e) if matches!(
+						e.get(), OnionMessageBuffer::ConnectedPeer(..)
+					) => {
+						e.get_mut().enqueue_message(onion_message);
+						log_trace!(self.logger, "Forwarding an onion message to peer {}", next_node_id);
+					},
+					_ => {
 						log_trace!(self.logger, "Dropping forwarded onion message to disconnected peer {:?}", next_node_id);
 						return
 					},
-					hash_map::Entry::Occupied(mut e) => {
-						e.get_mut().push_back(onion_message);
-						log_trace!(self.logger, "Forwarding an onion message to peer {}", next_node_id);
-					}
 				}
 			},
 			Err(e) => {
@@ -688,15 +750,22 @@ where
 
 	fn peer_connected(&self, their_node_id: &PublicKey, init: &msgs::Init, _inbound: bool) -> Result<(), ()> {
 		if init.features.supports_onion_messages() {
-			let mut peers = self.pending_messages.lock().unwrap();
+			self.message_buffers.lock().unwrap()
-			peers.insert(their_node_id.clone(), VecDeque::new());
+				.entry(*their_node_id)
+				.or_insert_with(|| OnionMessageBuffer::ConnectedPeer(VecDeque::new()))
+				.mark_connected();
+		} else {
+			self.message_buffers.lock().unwrap().remove(their_node_id);
 		}
+
 		Ok(())
 	}
 
 	fn peer_disconnected(&self, their_node_id: &PublicKey) {
-		let mut pending_msgs = self.pending_messages.lock().unwrap();
+		match self.message_buffers.lock().unwrap().remove(their_node_id) {
-		pending_msgs.remove(their_node_id);
+			Some(OnionMessageBuffer::ConnectedPeer(..)) => {},
+			_ => debug_assert!(false),
+		}
 	}
 
 	fn provided_node_features(&self) -> NodeFeatures {
@@ -737,11 +806,9 @@ where
 			);
 		}
 
-		let mut pending_msgs = self.pending_messages.lock().unwrap();
+		self.message_buffers.lock().unwrap()
-		if let Some(msgs) = pending_msgs.get_mut(&peer_node_id) {
+			.get_mut(&peer_node_id)
-			return msgs.pop_front()
+			.and_then(|buffer| buffer.dequeue_message())
-		}
-		None
 	}
 }