huskies: merge 1156 story Periodic liveness tick so runtime freezes have a precise timestamp

server/src/startup/tick_loop.rs

@@ -226,6 +226,29 @@ pub(crate) fn spawn_tick_loop(
     });
 }
 
+/// Spawn a permanent background task that emits a liveness heartbeat log line
+/// every `interval_secs` seconds on the main tokio runtime.
+///
+/// Running on the main runtime (not `spawn_blocking`) means the heartbeat stops
+/// the moment the runtime freezes, giving an exact timestamp of the freeze from
+/// the last `[liveness]` line in the logs.
+pub(crate) fn spawn_liveness_tick(interval_secs: u64) {
+    let interval_secs = interval_secs.max(1);
+    crate::slog!("[liveness] liveness tick started; cadence={interval_secs}s");
+    tokio::spawn(async move {
+        let duration = std::time::Duration::from_secs(interval_secs);
+        let mut ticker = tokio::time::interval(duration);
+        loop {
+            ticker.tick().await;
+            let ts = std::time::UNIX_EPOCH
+                .elapsed()
+                .unwrap_or_default()
+                .as_secs();
+            crate::slog!("[liveness] tokio runtime alive ts={ts}");
+        }
+    });
+}
+
 /// Fire any due generic scheduled timers and re-arm recurring ones.
 ///
 /// Called every second from the unified tick loop.  Catch-up semantics: timers