Merge pull request lightningdevkit#4096 from TheBlueMatt/2025-09-limt-gossip-by-size

Author: TheBlueMatt

lightning/src/ln/peer_channel_encryptor.rs

@@ -641,6 +641,11 @@ impl PeerChannelEncryptor {
 /// padding to allow for future encryption/MACing.
 pub struct MessageBuf(Vec<u8>);
 impl MessageBuf {
+	/// The total allocated space for this message
+	pub fn capacity(&self) -> usize {
+		self.0.capacity()
+	}
+
 	/// Creates a new buffer from an encoded message (i.e. the two message-type bytes followed by
 	/// the message contents).
 	///

lightning/src/ln/peer_handler.rs

@@ -718,19 +718,11 @@ enum MessageBatchImpl {
 	CommitmentSigned(Vec<msgs::CommitmentSigned>),
 }
 
-/// The ratio between buffer sizes at which we stop sending initial sync messages vs when we stop
-/// forwarding gossip messages to peers altogether.
-const FORWARD_INIT_SYNC_BUFFER_LIMIT_RATIO: usize = 2;
-
 /// When the outbound buffer has this many messages, we'll stop reading bytes from the peer until
 /// we have fewer than this many messages in the outbound buffer again.
 /// We also use this as the target number of outbound gossip messages to keep in the write buffer,
 /// refilled as we send bytes.
 const OUTBOUND_BUFFER_LIMIT_READ_PAUSE: usize = 12;
-/// When the outbound buffer has this many messages, we'll simply skip relaying gossip messages to
-/// the peer.
-const OUTBOUND_BUFFER_LIMIT_DROP_GOSSIP: usize =
-	OUTBOUND_BUFFER_LIMIT_READ_PAUSE * FORWARD_INIT_SYNC_BUFFER_LIMIT_RATIO;
 
 /// If we've sent a ping, and are still awaiting a response, we may need to churn our way through
 /// the socket receive buffer before receiving the ping.
@@ -754,10 +746,20 @@ const MAX_BUFFER_DRAIN_TICK_INTERVALS_PER_PEER: i8 = 4;
 /// process before the next ping.
 ///
 /// Note that we continue responding to other messages even after we've sent this many messages, so
-/// it's more of a general guideline used for gossip backfill (and gossip forwarding, times
-/// [`FORWARD_INIT_SYNC_BUFFER_LIMIT_RATIO`]) than a hard limit.
+/// this really limits gossip broadcast, gossip backfill, and onion message relay.
 const BUFFER_DRAIN_MSGS_PER_TICK: usize = 32;
 
+/// The maximum number of bytes which we allow in a peer's outbound buffers before we start
+/// dropping outbound gossip forwards.
+///
+/// This is currently 128KiB, or two messages at the maximum message size (though in practice we
+/// refuse to forward gossip messages which are substantially larger than we expect, so this is
+/// closer to ~85 messages if all queued messages are maximum-sized channel announcements).
+///
+/// Note that as we always drain the gossip forwarding queue before continuing gossip backfill,
+/// the equivalent maximum buffer size for gossip backfill is zero.
+const OUTBOUND_BUFFER_SIZE_LIMIT_DROP_GOSSIP: usize = 64 * 1024 * 2;
+
 struct Peer {
 	channel_encryptor: PeerChannelEncryptor,
 	/// We cache a `NodeId` here to avoid serializing peers' keys every time we forward gossip
@@ -889,12 +891,11 @@ impl Peer {
 
 	/// Returns whether this peer's outbound buffers are full and we should drop gossip broadcasts.
 	fn buffer_full_drop_gossip_broadcast(&self) -> bool {
-		let total_outbound_buffered =
-			self.gossip_broadcast_buffer.len() + self.pending_outbound_buffer.len();
+		let total_outbound_buffered: usize =
+			self.gossip_broadcast_buffer.iter().map(|m| m.capacity()).sum::<usize>()
+				+ self.pending_outbound_buffer.iter().map(|m| m.capacity()).sum::<usize>();
 
-		total_outbound_buffered > OUTBOUND_BUFFER_LIMIT_DROP_GOSSIP
-			|| self.msgs_sent_since_pong
-				> BUFFER_DRAIN_MSGS_PER_TICK * FORWARD_INIT_SYNC_BUFFER_LIMIT_RATIO
+		total_outbound_buffered > OUTBOUND_BUFFER_SIZE_LIMIT_DROP_GOSSIP
 	}
 
 	fn set_their_node_id(&mut self, node_id: PublicKey) {
@@ -4693,22 +4694,27 @@ mod tests {
 		let secp_ctx = Secp256k1::new();
 		let key = SecretKey::from_slice(&[1; 32]).unwrap();
 		let msg = channel_announcement(&key, &key, ChannelFeatures::empty(), 42, &secp_ctx);
+		// The message bufer size is the message length plus two 16-byte MACs plus a 2-byte length
+		// and 2-byte type.
+		let encoded_size = msg.serialized_length() + 16 * 2 + 2 + 2;
 		let msg_ev = MessageSendEvent::BroadcastChannelAnnouncement { msg, update_msg: None };
 
 		fd_a.hang_writes.store(true, Ordering::Relaxed);
 
 		// Now push an arbitrarily large number of messages and check that only
-		// `OUTBOUND_BUFFER_LIMIT_DROP_GOSSIP` messages end up in the queue.
-		for _ in 0..OUTBOUND_BUFFER_LIMIT_DROP_GOSSIP * 2 {
+		// `OUTBOUND_BUFFER_SIZE_LIMIT_DROP_GOSSIP` message bytes end up in the queue.
+		for _ in 0..OUTBOUND_BUFFER_SIZE_LIMIT_DROP_GOSSIP / encoded_size {
 			cfgs[0].routing_handler.pending_events.lock().unwrap().push(msg_ev.clone());
 			peers[0].process_events();
 		}
 
 		{
 			let peer_a_lock = peers[0].peers.read().unwrap();
-			let buf_len =
-				peer_a_lock.get(&fd_a).unwrap().lock().unwrap().gossip_broadcast_buffer.len();
-			assert_eq!(buf_len, OUTBOUND_BUFFER_LIMIT_DROP_GOSSIP);
+			let peer = peer_a_lock.get(&fd_a).unwrap().lock().unwrap();
+			let buf_len = peer.pending_outbound_buffer.iter().map(|m| m.capacity()).sum::<usize>()
+				+ peer.gossip_broadcast_buffer.iter().map(|m| m.capacity()).sum::<usize>();
+			assert!(buf_len > OUTBOUND_BUFFER_SIZE_LIMIT_DROP_GOSSIP - encoded_size);
+			assert!(buf_len < OUTBOUND_BUFFER_SIZE_LIMIT_DROP_GOSSIP);
 		}
 
 		// Check that if a broadcast message comes in from the channel handler (i.e. it is an
@@ -4718,14 +4724,17 @@ mod tests {
 
 		{
 			let peer_a_lock = peers[0].peers.read().unwrap();
-			let buf_len =
-				peer_a_lock.get(&fd_a).unwrap().lock().unwrap().gossip_broadcast_buffer.len();
-			assert_eq!(buf_len, OUTBOUND_BUFFER_LIMIT_DROP_GOSSIP + 1);
+			let peer = peer_a_lock.get(&fd_a).unwrap().lock().unwrap();
+			let buf_len = peer.pending_outbound_buffer.iter().map(|m| m.capacity()).sum::<usize>()
+				+ peer.gossip_broadcast_buffer.iter().map(|m| m.capacity()).sum::<usize>();
+			assert!(buf_len > OUTBOUND_BUFFER_SIZE_LIMIT_DROP_GOSSIP);
+			assert!(buf_len < OUTBOUND_BUFFER_SIZE_LIMIT_DROP_GOSSIP + encoded_size);
 		}
 
 		// Finally, deliver all the messages and make sure we got the right count. Note that there
 		// was an extra message that had already moved from the broadcast queue to the encrypted
-		// message queue so we actually receive `OUTBOUND_BUFFER_LIMIT_DROP_GOSSIP + 2` messages.
+		// message queue so we actually receive `OUTBOUND_BUFFER_SIZE_LIMIT_DROP_GOSSIP + 2`
+		// message bytes.
 		fd_a.hang_writes.store(false, Ordering::Relaxed);
 		cfgs[1].routing_handler.chan_anns_recvd.store(0, Ordering::Relaxed);
 		peers[0].write_buffer_space_avail(&mut fd_a).unwrap();
@@ -4740,7 +4749,7 @@ mod tests {
 
 		assert_eq!(
 			cfgs[1].routing_handler.chan_anns_recvd.load(Ordering::Relaxed),
-			OUTBOUND_BUFFER_LIMIT_DROP_GOSSIP + 2
+			OUTBOUND_BUFFER_SIZE_LIMIT_DROP_GOSSIP / encoded_size + 1
 		);
 	}