Read the RGS sync timestamp from the network graph

Previously, after each successful Rapid Gossip Sync update the
background task wrote `latest_rgs_snapshot_timestamp` to the persisted
`NodeMetrics` immediately, while the network graph itself is only
flushed to disk later by LDK's background processor. A crash in that
window left the on-disk metric ahead of the on-disk graph — on restart
we'd resume RGS from the newer timestamp and permanently skip the
updates that were never persisted together with the graph.

Instead, seed the RGS start timestamp from
`NetworkGraph::get_last_rapid_gossip_sync_timestamp`, which is part of
the graph's own serialized state and therefore lands on disk atomically
with the channel updates it describes. The same source now backs the
RGS timestamp reported via `NodeStatus::latest_rgs_snapshot_timestamp`,
so the reported value always matches what's reflected in the graph.
Worst case after a crash is that we refetch the snapshots since the
last persisted graph — an idempotent operation — rather than silently
losing them.

The `latest_rgs_snapshot_timestamp` field is retired from `NodeMetrics`,
and TLV slot 6 is kept readable for backwards compatibility via LDK's
`legacy` TLV grammar. Old persisted records still deserialize; new
records no longer carry slot 6. The dead "reset RGS timestamp on
gossip-source switch" block in the P2P builder branch also goes away,
since the graph's timestamp remains the correct resume point across a
P2P→RGS switch.

Fixes lightningdevkit#615. Disclosing the use of an AI coding assistant.

Co-Authored-By: HAL 9000

Author: tnull

src/builder.rs

@@ -57,7 +57,7 @@ use crate::io::sqlite_store::SqliteStore;
 use crate::io::utils::{
 	read_event_queue, read_external_pathfinding_scores_from_cache, read_network_graph,
 	read_node_metrics, read_output_sweeper, read_payments, read_peer_info, read_pending_payments,
-	read_scorer, update_and_persist_node_metrics,
+	read_scorer,
 };
 use crate::io::vss_store::VssStoreBuilder;
 use crate::io::{
@@ -1770,19 +1770,11 @@ fn build_with_store_internal(
 				Arc::clone(&logger),
 			));
 
-			// Reset the RGS sync timestamp in case we somehow switch gossip sources
-			update_and_persist_node_metrics(&node_metrics, &*kv_store, Arc::clone(&logger), |m| {
-				m.latest_rgs_snapshot_timestamp = None
-			})
-			.map_err(|e| {
-				log_error!(logger, "Failed writing to store: {}", e);
-				BuildError::WriteFailed
-			})?;
 			p2p_source
 		},
 		GossipSourceConfig::RapidGossipSync(rgs_server) => {
 			let latest_sync_timestamp =
-				node_metrics.read().expect("lock").latest_rgs_snapshot_timestamp.unwrap_or(0);
+				network_graph.get_last_rapid_gossip_sync_timestamp().unwrap_or(0);
 			Arc::new(GossipSource::new_rgs(
 				rgs_server.clone(),
 				latest_sync_timestamp,

src/lib.rs

@@ -297,9 +297,7 @@ impl Node {
 
 		if self.gossip_source.is_rgs() {
 			let gossip_source = Arc::clone(&self.gossip_source);
-			let gossip_sync_store = Arc::clone(&self.kv_store);
 			let gossip_sync_logger = Arc::clone(&self.logger);
-			let gossip_node_metrics = Arc::clone(&self.node_metrics);
 			let mut stop_gossip_sync = self.stop_sender.subscribe();
 			self.runtime.spawn_cancellable_background_task(async move {
 				let mut interval = tokio::time::interval(RGS_SYNC_INTERVAL);
@@ -315,21 +313,12 @@ impl Node {
 						_ = interval.tick() => {
 							let now = Instant::now();
 							match gossip_source.update_rgs_snapshot().await {
-								Ok(updated_timestamp) => {
+								Ok(_updated_timestamp) => {
 									log_info!(
 										gossip_sync_logger,
 										"Background sync of RGS gossip data finished in {}ms.",
 										now.elapsed().as_millis()
 									);
-									update_and_persist_node_metrics(
-										&gossip_node_metrics,
-										&*gossip_sync_store,
-										Arc::clone(&gossip_sync_logger),
-										|m| m.latest_rgs_snapshot_timestamp = Some(updated_timestamp),
-									)
-									.unwrap_or_else(|e| {
-										log_error!(gossip_sync_logger, "Persistence failed: {}", e);
-									});
 								}
 								Err(e) => {
 									log_error!(
@@ -780,7 +769,7 @@ impl Node {
 		let latest_fee_rate_cache_update_timestamp =
 			locked_node_metrics.latest_fee_rate_cache_update_timestamp;
 		let latest_rgs_snapshot_timestamp =
-			locked_node_metrics.latest_rgs_snapshot_timestamp.map(|val| val as u64);
+			self.network_graph.get_last_rapid_gossip_sync_timestamp().map(|val| val as u64);
 		let latest_pathfinding_scores_sync_timestamp =
 			locked_node_metrics.latest_pathfinding_scores_sync_timestamp;
 		let latest_node_announcement_broadcast_timestamp =
@@ -2110,7 +2099,6 @@ pub(crate) struct NodeMetrics {
 	latest_lightning_wallet_sync_timestamp: Option<u64>,
 	latest_onchain_wallet_sync_timestamp: Option<u64>,
 	latest_fee_rate_cache_update_timestamp: Option<u64>,
-	latest_rgs_snapshot_timestamp: Option<u32>,
 	latest_pathfinding_scores_sync_timestamp: Option<u64>,
 	latest_node_announcement_broadcast_timestamp: Option<u64>,
 }
@@ -2121,7 +2109,6 @@ impl Default for NodeMetrics {
 			latest_lightning_wallet_sync_timestamp: None,
 			latest_onchain_wallet_sync_timestamp: None,
 			latest_fee_rate_cache_update_timestamp: None,
-			latest_rgs_snapshot_timestamp: None,
 			latest_pathfinding_scores_sync_timestamp: None,
 			latest_node_announcement_broadcast_timestamp: None,
 		}
@@ -2133,7 +2120,8 @@ impl_writeable_tlv_based!(NodeMetrics, {
 	(1, latest_pathfinding_scores_sync_timestamp, option),
 	(2, latest_onchain_wallet_sync_timestamp, option),
 	(4, latest_fee_rate_cache_update_timestamp, option),
-	(6, latest_rgs_snapshot_timestamp, option),
+	// 6 used to be latest_rgs_snapshot_timestamp
+	(6, _legacy_latest_rgs_snapshot_timestamp, (legacy, u32, |_| Ok(()), |_: &NodeMetrics| None::<Option<u32>> )),
 	(8, latest_node_announcement_broadcast_timestamp, option),
 	// 10 used to be latest_channel_monitor_archival_height
 	(10, _legacy_latest_channel_monitor_archival_height, (legacy, u32, |_| Ok(()), |_: &NodeMetrics| None::<Option<u32>> )),
@@ -2174,3 +2162,54 @@ pub(crate) fn new_channel_anchor_reserve_sats(
 		}
 	})
 }
+
+#[cfg(test)]
+mod tests {
+	use super::*;
+	use lightning::util::ser::{Readable, Writeable};
+
+	#[test]
+	fn node_metrics_reads_legacy_rgs_snapshot_timestamp() {
+		// Pre-#615, `NodeMetrics` persisted `latest_rgs_snapshot_timestamp` as an optional
+		// `u32` at TLV slot 6. The field has since been retired, but we must still read
+		// records written by older versions without failing. The shadow struct below
+		// mirrors main's `NodeMetrics` layout 1:1 so the byte stream we decode matches
+		// what an older on-disk record actually looked like.
+		#[derive(Debug)]
+		struct OldNodeMetrics {
+			latest_lightning_wallet_sync_timestamp: Option<u64>,
+			latest_onchain_wallet_sync_timestamp: Option<u64>,
+			latest_fee_rate_cache_update_timestamp: Option<u64>,
+			latest_rgs_snapshot_timestamp: Option<u32>,
+			latest_pathfinding_scores_sync_timestamp: Option<u64>,
+			latest_node_announcement_broadcast_timestamp: Option<u64>,
+		}
+		impl_writeable_tlv_based!(OldNodeMetrics, {
+			(0, latest_lightning_wallet_sync_timestamp, option),
+			(1, latest_pathfinding_scores_sync_timestamp, option),
+			(2, latest_onchain_wallet_sync_timestamp, option),
+			(4, latest_fee_rate_cache_update_timestamp, option),
+			(6, latest_rgs_snapshot_timestamp, option),
+			(8, latest_node_announcement_broadcast_timestamp, option),
+			// 10 used to be latest_channel_monitor_archival_height
+			(10, _legacy_latest_channel_monitor_archival_height, (legacy, u32, |_| Ok(()), |_: &OldNodeMetrics| None::<Option<u32>> )),
+		});
+
+		let old = OldNodeMetrics {
+			latest_lightning_wallet_sync_timestamp: Some(1_000),
+			latest_onchain_wallet_sync_timestamp: Some(1_100),
+			latest_fee_rate_cache_update_timestamp: Some(1_200),
+			latest_rgs_snapshot_timestamp: Some(1_700_000_000),
+			latest_pathfinding_scores_sync_timestamp: Some(1_300),
+			latest_node_announcement_broadcast_timestamp: Some(2_000),
+		};
+		let bytes = old.encode();
+
+		let new = NodeMetrics::read(&mut &bytes[..]).unwrap();
+		assert_eq!(new.latest_lightning_wallet_sync_timestamp, Some(1_000));
+		assert_eq!(new.latest_onchain_wallet_sync_timestamp, Some(1_100));
+		assert_eq!(new.latest_fee_rate_cache_update_timestamp, Some(1_200));
+		assert_eq!(new.latest_pathfinding_scores_sync_timestamp, Some(1_300));
+		assert_eq!(new.latest_node_announcement_broadcast_timestamp, Some(2_000));
+	}
+}