fix: correct SNSD cluster diagnostics (#5930)

crates/scanner/src/lib.rs

@@ -82,6 +82,7 @@ pub use storage_api::ScannerReplicationConfig as ReplicationConfig;
 pub use storage_api::scan::SCANNER_ACTIVITY_PROTOCOL_VERSION;
 
 static SCANNER_ACTIVE_WORK_UNITS: AtomicU64 = AtomicU64::new(0);
+static SCANNER_RUNTIME_INSTANCES: AtomicU64 = AtomicU64::new(0);
 static SCANNER_FOREGROUND_READ_ACTIVITY: AtomicU64 = AtomicU64::new(0);
 static SCANNER_FOREGROUND_STREAM_READS: AtomicU64 = AtomicU64::new(0);
 
@@ -89,6 +90,10 @@ pub fn current_scanner_activity() -> u64 {
     SCANNER_ACTIVE_WORK_UNITS.load(Ordering::Relaxed)
 }
 
+pub fn scanner_runtime_initialized() -> bool {
+    SCANNER_RUNTIME_INSTANCES.load(Ordering::Relaxed) > 0
+}
+
 pub fn set_foreground_read_activity(active: usize) {
     let active = u64::try_from(active).unwrap_or(u64::MAX);
     SCANNER_FOREGROUND_READ_ACTIVITY.store(active, Ordering::Relaxed);
@@ -138,6 +143,26 @@ impl ScannerActivityGuard {
     }
 }
 
+pub(crate) struct ScannerRuntimeGuard;
+
+impl ScannerRuntimeGuard {
+    pub(crate) fn new() -> Self {
+        SCANNER_RUNTIME_INSTANCES.fetch_add(1, Ordering::Relaxed);
+        Self
+    }
+}
+
+impl Drop for ScannerRuntimeGuard {
+    fn drop(&mut self) {
+        let _ = SCANNER_RUNTIME_INSTANCES.fetch_update(Ordering::Relaxed, Ordering::Relaxed, |current| current.checked_sub(1));
+    }
+}
+
+#[cfg(test)]
+fn reset_scanner_runtime_instances_for_test() {
+    SCANNER_RUNTIME_INSTANCES.store(0, Ordering::Relaxed);
+}
+
 impl Drop for ScannerActivityGuard {
     fn drop(&mut self) {
         let _ = SCANNER_ACTIVE_WORK_UNITS
@@ -559,4 +584,18 @@ mod tests {
         set_foreground_read_activity(0);
         assert_eq!(current_foreground_read_activity(), 1);
     }
+
+    #[test]
+    #[serial]
+    fn scanner_runtime_guard_tracks_runtime_lifetime() {
+        reset_scanner_runtime_instances_for_test();
+        assert!(!scanner_runtime_initialized());
+
+        {
+            let _guard = ScannerRuntimeGuard::new();
+            assert!(scanner_runtime_initialized());
+        }
+
+        assert!(!scanner_runtime_initialized());
+    }
 }