Track a counter for each node in our network graph

These counters are simply a unique number describing each node.
They have no specific meaning, but start at 0 and count up, with
counters being reused after a node has been deleted.

Author: TheBlueMatt

lightning/src/routing/gossip.rs

@@ -40,7 +40,6 @@ use crate::io_extras::{copy, sink};
 use crate::prelude::*;
 use core::{cmp, fmt};
 use crate::sync::{RwLock, RwLockReadGuard, LockTestExt};
-#[cfg(feature = "std")]
 use core::sync::atomic::{AtomicUsize, Ordering};
 use crate::sync::Mutex;
 use core::ops::{Bound, Deref};
@@ -184,6 +183,8 @@ pub struct NetworkGraph<L: Deref> where L::Target: Logger {
 	// Lock order: channels -> nodes
 	channels: RwLock<IndexedMap<u64, ChannelInfo>>,
 	nodes: RwLock<IndexedMap<NodeId, NodeInfo>>,
+	removed_node_counters: Mutex<Vec<u32>>,
+	next_node_counter: AtomicUsize,
 	// Lock order: removed_channels -> removed_nodes
 	//
 	// NOTE: In the following `removed_*` maps, we use seconds since UNIX epoch to track time instead
@@ -1333,15 +1334,27 @@ impl Readable for NodeAlias {
 	}
 }
 
-#[derive(Clone, Debug, PartialEq, Eq)]
+#[derive(Clone, Debug, Eq)]
 /// Details about a node in the network, known from the network announcement.
 pub struct NodeInfo {
 	/// All valid channels a node has announced
 	pub channels: Vec<u64>,
 	/// More information about a node from node_announcement.
 	/// Optional because we store a Node entry after learning about it from
 	/// a channel announcement, but before receiving a node announcement.
-	pub announcement_info: Option<NodeAnnouncementInfo>
+	pub announcement_info: Option<NodeAnnouncementInfo>,
+	/// In memory, each node is assigned a unique ID. They are eagerly reused, ensuring they remain
+	/// relatively dense.
+	///
+	/// These IDs allow the router to avoid a `HashMap` lookup by simply using this value as an
+	/// index in a `Vec`, skipping a big step in some of the hottest code when routing.
+	pub(crate) node_counter: u32,
+}
+
+impl PartialEq for NodeInfo {
+	fn eq(&self, o: &NodeInfo) -> bool {
+		self.channels == o.channels && self.announcement_info == o.announcement_info
+	}
 }
 
 impl NodeInfo {
@@ -1411,6 +1424,7 @@ impl Readable for NodeInfo {
 		Ok(NodeInfo {
 			announcement_info: announcement_info_wrap.map(|w| w.0),
 			channels,
+			node_counter: u32::max_value(),
 		})
 	}
 }
@@ -1420,6 +1434,8 @@ const MIN_SERIALIZATION_VERSION: u8 = 1;
 
 impl<L: Deref> Writeable for NetworkGraph<L> where L::Target: Logger {
 	fn write<W: Writer>(&self, writer: &mut W) -> Result<(), io::Error> {
+		self.test_node_counter_consistency();
+
 		write_ver_prefix!(writer, SERIALIZATION_VERSION, MIN_SERIALIZATION_VERSION);
 
 		self.chain_hash.write(writer)?;
@@ -1458,11 +1474,13 @@ impl<L: Deref> ReadableArgs<L> for NetworkGraph<L> where L::Target: Logger {
 			channels.insert(chan_id, chan_info);
 		}
 		let nodes_count: u64 = Readable::read(reader)?;
+		if nodes_count > u32::max_value() as u64 / 2 { return Err(DecodeError::InvalidValue); }
 		// In Nov, 2023 there were about 69K channels; we cap allocations to 1.5x that.
 		let mut nodes = IndexedMap::with_capacity(cmp::min(nodes_count as usize, 103500));
-		for _ in 0..nodes_count {
+		for i in 0..nodes_count {
 			let node_id = Readable::read(reader)?;
-			let node_info = Readable::read(reader)?;
+			let mut node_info: NodeInfo = Readable::read(reader)?;
+			node_info.node_counter = i as u32;
 			nodes.insert(node_id, node_info);
 		}
 
@@ -1477,6 +1495,8 @@ impl<L: Deref> ReadableArgs<L> for NetworkGraph<L> where L::Target: Logger {
 			logger,
 			channels: RwLock::new(channels),
 			nodes: RwLock::new(nodes),
+			removed_node_counters: Mutex::new(Vec::new()),
+			next_node_counter: AtomicUsize::new(nodes_count as usize),
 			last_rapid_gossip_sync_timestamp: Mutex::new(last_rapid_gossip_sync_timestamp),
 			removed_nodes: Mutex::new(new_hash_map()),
 			removed_channels: Mutex::new(new_hash_map()),
@@ -1522,15 +1542,42 @@ impl<L: Deref> NetworkGraph<L> where L::Target: Logger {
 			logger,
 			channels: RwLock::new(IndexedMap::new()),
 			nodes: RwLock::new(IndexedMap::new()),
+			next_node_counter: AtomicUsize::new(0),
+			removed_node_counters: Mutex::new(Vec::new()),
 			last_rapid_gossip_sync_timestamp: Mutex::new(None),
 			removed_channels: Mutex::new(new_hash_map()),
 			removed_nodes: Mutex::new(new_hash_map()),
 			pending_checks: utxo::PendingChecks::new(),
 		}
 	}
 
+	fn test_node_counter_consistency(&self) {
+		#[cfg(debug_assertions)] {
+			let nodes = self.nodes.read().unwrap();
+			let removed_node_counters = self.removed_node_counters.lock().unwrap();
+			let next_counter = self.next_node_counter.load(Ordering::Acquire);
+			assert!(next_counter < (u32::max_value() as usize) / 2);
+			let mut used_node_counters = vec![0u8; next_counter / 8 + 1];
+
+			for counter in removed_node_counters.iter() {
+				let pos = (*counter as usize) / 8;
+				let bit = 1 << (counter % 8);
+				assert_eq!(used_node_counters[pos] & bit, 0);
+				used_node_counters[pos] |= bit;
+			}
+			for (_, node) in nodes.unordered_iter() {
+				assert!((node.node_counter as usize) < next_counter);
+				let pos = (node.node_counter as usize) / 8;
+				let bit = 1 << (node.node_counter % 8);
+				assert_eq!(used_node_counters[pos] & bit, 0);
+				used_node_counters[pos] |= bit;
+			}
+		}
+	}
+
 	/// Returns a read-only view of the network graph.
 	pub fn read_only(&'_ self) -> ReadOnlyNetworkGraph<'_> {
+		self.test_node_counter_consistency();
 		let channels = self.channels.read().unwrap();
 		let nodes = self.nodes.read().unwrap();
 		ReadOnlyNetworkGraph {
@@ -1717,7 +1764,7 @@ impl<L: Deref> NetworkGraph<L> where L::Target: Logger {
 					// b) we don't track UTXOs of channels we know about and remove them if they
 					//    get reorg'd out.
 					// c) it's unclear how to do so without exposing ourselves to massive DoS risk.
-					Self::remove_channel_in_nodes(&mut nodes, &entry.get(), short_channel_id);
+					self.remove_channel_in_nodes(&mut nodes, &entry.get(), short_channel_id);
 					*entry.get_mut() = channel_info;
 				} else {
 					return Err(LightningError{err: "Already have knowledge of channel".to_owned(), action: ErrorAction::IgnoreDuplicateGossip});
@@ -1734,9 +1781,13 @@ impl<L: Deref> NetworkGraph<L> where L::Target: Logger {
 					node_entry.into_mut().channels.push(short_channel_id);
 				},
 				IndexedMapEntry::Vacant(node_entry) => {
+					let mut removed_node_counters = self.removed_node_counters.lock().unwrap();
+					let node_counter = removed_node_counters.pop()
+						.unwrap_or(self.next_node_counter.fetch_add(1, Ordering::Relaxed) as u32);
 					node_entry.insert(NodeInfo {
 						channels: vec!(short_channel_id),
 						announcement_info: None,
+						node_counter,
 					});
 				}
 			};
@@ -1855,7 +1906,7 @@ impl<L: Deref> NetworkGraph<L> where L::Target: Logger {
 		if let Some(chan) = channels.remove(&short_channel_id) {
 			let mut nodes = self.nodes.write().unwrap();
 			self.removed_channels.lock().unwrap().insert(short_channel_id, current_time_unix);
-			Self::remove_channel_in_nodes(&mut nodes, &chan, short_channel_id);
+			self.remove_channel_in_nodes(&mut nodes, &chan, short_channel_id);
 		}
 	}
 
@@ -1874,6 +1925,7 @@ impl<L: Deref> NetworkGraph<L> where L::Target: Logger {
 		let mut removed_nodes = self.removed_nodes.lock().unwrap();
 
 		if let Some(node) = nodes.remove(&node_id) {
+			let mut removed_node_counters = self.removed_node_counters.lock().unwrap();
 			for scid in node.channels.iter() {
 				if let Some(chan_info) = channels.remove(scid) {
 					let other_node_id = if node_id == chan_info.node_one { chan_info.node_two } else { chan_info.node_one };
@@ -1882,12 +1934,14 @@ impl<L: Deref> NetworkGraph<L> where L::Target: Logger {
 							*scid != *chan_id
 						});
 						if other_node_entry.get().channels.is_empty() {
+							removed_node_counters.push(other_node_entry.get().node_counter);
 							other_node_entry.remove_entry();
 						}
 					}
 					removed_channels.insert(*scid, current_time_unix);
 				}
 			}
+			removed_node_counters.push(node.node_counter);
 			removed_nodes.insert(node_id, current_time_unix);
 		}
 	}
@@ -1963,7 +2017,7 @@ impl<L: Deref> NetworkGraph<L> where L::Target: Logger {
 			let mut nodes = self.nodes.write().unwrap();
 			for scid in scids_to_remove {
 				let info = channels.remove(&scid).expect("We just accessed this scid, it should be present");
-				Self::remove_channel_in_nodes(&mut nodes, &info, scid);
+				self.remove_channel_in_nodes(&mut nodes, &info, scid);
 				self.removed_channels.lock().unwrap().insert(scid, Some(current_time_unix));
 			}
 		}
@@ -2145,14 +2199,15 @@ impl<L: Deref> NetworkGraph<L> where L::Target: Logger {
 		Ok(())
 	}
 
-	fn remove_channel_in_nodes(nodes: &mut IndexedMap<NodeId, NodeInfo>, chan: &ChannelInfo, short_channel_id: u64) {
+	fn remove_channel_in_nodes(&self, nodes: &mut IndexedMap<NodeId, NodeInfo>, chan: &ChannelInfo, short_channel_id: u64) {
 		macro_rules! remove_from_node {
 			($node_id: expr) => {
 				if let IndexedMapEntry::Occupied(mut entry) = nodes.entry($node_id) {
 					entry.get_mut().channels.retain(|chan_id| {
 						short_channel_id != *chan_id
 					});
 					if entry.get().channels.is_empty() {
+						self.removed_node_counters.lock().unwrap().push(entry.get().node_counter);
 						entry.remove_entry();
 					}
 				} else {
@@ -3574,6 +3629,7 @@ pub(crate) mod tests {
 		let valid_node_info = NodeInfo {
 			channels: Vec::new(),
 			announcement_info: Some(valid_node_ann_info),
+			node_counter: 0,
 		};
 
 		let mut encoded_valid_node_info = Vec::new();