From 4855ff63536430b1cd26734a0eeeaf0c25df553e Mon Sep 17 00:00:00 2001
From: tv <tv@krebsco.de>
Date: Sat, 31 Oct 2026 06:51:17 +0100
Subject: calculate peer clock drift

---
 src/api/peers.rs            |  6 ++++++
 src/api/stats/prometheus.rs | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 38 insertions(+)

(limited to 'src/api')

diff --git a/src/api/peers.rs b/src/api/peers.rs
index 020221b..3fe4c39 100644
--- a/src/api/peers.rs
+++ b/src/api/peers.rs
@@ -18,6 +18,7 @@ use crate::locks::MutexExt;
 use crate::state::{AppState, HelloState, Incidents, Liveness};
 use crate::swarm::peer::forgotten_path;
 use crate::transport::Destination;
+use crate::transport::paths::ClockOffset;
 use crate::transport::traffic::PeerTraffic;
 
 #[derive(Debug, Serialize)]
@@ -66,6 +67,9 @@ pub(crate) struct PeerStatus {
     /// Smoothed sync throughput (`swarm::throughput`); absent before the first large
     /// exchange.
     pub(crate) sync_throughput: Option<ThroughputInfo>,
+    /// How far its clock is ahead of ours, give or take (`Transport::clock_offset`);
+    /// absent before a PONG from it.
+    pub(crate) clock: Option<ClockOffset>,
 }
 
 #[derive(Debug, Serialize)]
@@ -141,6 +145,7 @@ pub(crate) async fn peer_list(state: &AppState) -> PeerListResponse {
         let alias = aliases.remove(&peer);
         let addresses = state.transport.addresses_of(&peer);
         let destination = state.transport.destination(&peer);
+        let clock = state.transport.clock_offset(&peer);
         peers.insert(
             peer,
             PeerStatus {
@@ -168,6 +173,7 @@ pub(crate) async fn peer_list(state: &AppState) -> PeerListResponse {
                 traffic,
                 incidents: ps.incidents,
                 sync_throughput,
+                clock,
             },
         );
     }
diff --git a/src/api/stats/prometheus.rs b/src/api/stats/prometheus.rs
index 8f7538f..2f33bef 100644
--- a/src/api/stats/prometheus.rs
+++ b/src/api/stats/prometheus.rs
@@ -9,6 +9,7 @@ use crate::api::media_type;
 use crate::api::peers::{PeerListResponse, PeerLivenessStatus};
 use crate::state::Incidents;
 use crate::transport::Destination;
+use crate::transport::paths::ClockOffset;
 use crate::transport::stats::{Reachability, TransportStats};
 use crate::transport::traffic::{ByKind, Counter, PeerTraffic};
 
@@ -151,6 +152,7 @@ struct Peer {
     incidents: Incidents,
     throughput: Option<f64>,
     destination: Option<Destination>,
+    clock: Option<ClockOffset>,
 }
 
 /// The rows of `/peers` (`peers::peer_list`), as the series need them.
@@ -171,6 +173,7 @@ fn peers(list: &PeerListResponse) -> Vec<Peer> {
             incidents: peer.incidents,
             throughput: peer.sync_throughput.as_ref().map(|t| t.bytes_per_sec),
             destination: peer.destination.clone(),
+            clock: peer.clock,
         })
         .collect()
 }
@@ -927,6 +930,35 @@ pub(super) fn render(stats: &StatsResponse, list: &PeerListResponse) -> String {
         }
     }
 
+    out.family(
+        "peer_clock_offset_seconds",
+        "gauge",
+        "How far the peer's clock is ahead of ours (behind: negative), from its PONGs.",
+    );
+    for peer in &peers {
+        if let Some(clock) = peer.clock {
+            out.sample(
+                "peer_clock_offset_seconds",
+                &[("peer", &peer.key)],
+                clock.offset_ms as f64 / 1000.0,
+            );
+        }
+    }
+    out.family(
+        "peer_clock_offset_within_seconds",
+        "gauge",
+        "Give or take this much: half the round trip it was measured over.",
+    );
+    for peer in &peers {
+        if let Some(clock) = peer.clock {
+            out.sample(
+                "peer_clock_offset_within_seconds",
+                &[("peer", &peer.key)],
+                clock.within_ms as f64 / 1000.0,
+            );
+        }
+    }
+
     out.family(
         "peer_messages_total",
         "counter",
-- 
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