Merge pull request #4306 from tnull/2026-01-allow-empty-network-graph

`NetworkGraph`: Update chan/node estimation numbers, determine pre-allocation dynamically on read

Author: TheBlueMatt

lightning/src/routing/gossip.rs

@@ -1682,9 +1682,17 @@ where
 	fn read<R: io::Read>(reader: &mut R, logger: L) -> Result<NetworkGraph<L>, DecodeError> {
 		let _ver = read_ver_prefix!(reader, SERIALIZATION_VERSION);
 
+		const MAX_CHAN_COUNT_LIMIT: usize = 100_000_000;
+		const MAX_NODE_COUNT_LIMIT: usize = 10_000_000;
+
 		let chain_hash: ChainHash = Readable::read(reader)?;
 		let channels_count: u64 = Readable::read(reader)?;
-		let mut channels = IndexedMap::with_capacity(CHAN_COUNT_ESTIMATE);
+		// Pre-allocate 115% of the known channel count to avoid unnecessary reallocations.
+		let channels_map_capacity = (channels_count as u128 * 115 / 100)
+			.try_into()
+			.map(|v: usize| v.min(MAX_CHAN_COUNT_LIMIT))
+			.map_err(|_| DecodeError::InvalidValue)?;
+		let mut channels = IndexedMap::with_capacity(channels_map_capacity);
 		for _ in 0..channels_count {
 			let chan_id: u64 = Readable::read(reader)?;
 			let chan_info: ChannelInfo = Readable::read(reader)?;
@@ -1696,7 +1704,12 @@ where
 		if nodes_count > u32::max_value() as u64 / 2 {
 			return Err(DecodeError::InvalidValue);
 		}
-		let mut nodes = IndexedMap::with_capacity(NODE_COUNT_ESTIMATE);
+		// Pre-allocate 115% of the known channel count to avoid unnecessary reallocations.
+		let nodes_map_capacity: usize = (nodes_count as u128 * 115 / 100)
+			.try_into()
+			.map(|v: usize| v.min(MAX_NODE_COUNT_LIMIT))
+			.map_err(|_| DecodeError::InvalidValue)?;
+		let mut nodes = IndexedMap::with_capacity(nodes_map_capacity);
 		for i in 0..nodes_count {
 			let node_id = Readable::read(reader)?;
 			let mut node_info: NodeInfo = Readable::read(reader)?;
@@ -1772,13 +1785,15 @@ where
 	}
 }
 
-// In Jan, 2025 there were about 49K channels.
-// We over-allocate by a bit because 20% more is better than the double we get if we're slightly
-// too low
-const CHAN_COUNT_ESTIMATE: usize = 60_000;
-// In Jan, 2025 there were about 15K nodes
-// We over-allocate by a bit because 33% more is better than the double we get if we're slightly
-// too low
+/// In Jan, 2026 there were about 54K channels.
+///
+/// We over-allocate by a bit because ~15% more is better than the double we get if we're slightly
+/// too low.
+const CHAN_COUNT_ESTIMATE: usize = 63_000;
+/// In Jan, 2026 there were about 17K nodes
+///
+/// We over-allocate by a bit because 15% more is better than the double we get if we're slightly
+/// too low.
 const NODE_COUNT_ESTIMATE: usize = 20_000;
 
 impl<L: Deref> NetworkGraph<L>
@@ -1787,12 +1802,18 @@ where
 {
 	/// Creates a new, empty, network graph.
 	pub fn new(network: Network, logger: L) -> NetworkGraph<L> {
+		let (node_map_cap, chan_map_cap) = if matches!(network, Network::Bitcoin) {
+			(NODE_COUNT_ESTIMATE, CHAN_COUNT_ESTIMATE)
+		} else {
+			(0, 0)
+		};
+
 		Self {
 			secp_ctx: Secp256k1::verification_only(),
 			chain_hash: ChainHash::using_genesis_block(network),
 			logger,
-			channels: RwLock::new(IndexedMap::with_capacity(CHAN_COUNT_ESTIMATE)),
-			nodes: RwLock::new(IndexedMap::with_capacity(NODE_COUNT_ESTIMATE)),
+			channels: RwLock::new(IndexedMap::with_capacity(chan_map_cap)),
+			nodes: RwLock::new(IndexedMap::with_capacity(node_map_cap)),
 			next_node_counter: AtomicUsize::new(0),
 			removed_node_counters: Mutex::new(Vec::new()),
 			last_rapid_gossip_sync_timestamp: Mutex::new(None),