Stream produced by watch_object will include an item when the object isn't in any initial list

- added a comment
- resolved a nightly clippy
- use a match guard instead of an nested if

Author: markdingram

kube-runtime/src/watcher.rs

@@ -845,7 +845,13 @@ pub fn watch_object<K: Resource + Clone + DeserializeOwned + Debug + Send + 'sta
     // footgun: Api::all may generate events from namespaced objects with the same name in different namespaces
     let fields = format!("metadata.name={name}");
     watcher(api, Config::default().fields(&fields))
-        // track whether the object was seen in each initial listing
+        // The `obj_seen` state is used to track whether the object exists in each Init / InitApply / InitDone
+        // sequence of events. If the object wasn't seen in any particular sequence it is treated as deleted and
+        // `None` is emitted when the InitDone event is received.
+        //
+        // The first check ensures `None` is emitted if the object was already gone (or not found), subsequent
+        // checks ensure `None` is emitted even if for some reason the Delete event wasn't received, which
+        // could happen given K8S events aren't guaranteed delivery.
         .scan(false, |obj_seen, event| {
             if matches!(event, Ok(Event::Init)) {
                 *obj_seen = false;
@@ -857,19 +863,13 @@ pub fn watch_object<K: Resource + Clone + DeserializeOwned + Debug + Send + 'sta
         .filter_map(|(obj_seen, event)| async move {
             match event {
                 // Pass up `Some` for Found / Updated
-                Ok(Event::Apply(obj)) | Ok(Event::InitApply(obj)) => Some(Ok(Some(obj))),
+                Ok(Event::Apply(obj) | Event::InitApply(obj)) => Some(Ok(Some(obj))),
                 // Pass up `None` for Deleted
                 Ok(Event::Delete(_)) => Some(Ok(None)),
-                // Ignore marker event
-                Ok(Event::Init) => None,
-                // Pass up `None` if the object wasn't seen in any initial list
-                Ok(Event::InitDone) => {
-                    if obj_seen {
-                        None
-                    } else {
-                        Some(Ok(None))
-                    }
-                }
+                // Pass up `None` if the object wasn't seen in the initial list
+                Ok(Event::InitDone) if !obj_seen => Some(Ok(None)),
+                // Ignore marker events
+                Ok(Event::Init | Event::InitDone) => None,
                 // Bubble up errors
                 Err(err) => Some(Err(err)),
             }