Resync now does a full re-list; add test_resync_period_triggers_full_list

Co-authored-by: brendandburns <5751682+brendandburns@users.noreply.github.com>

Author: Copilot

kubernetes/informer/informer.py

@@ -248,14 +248,22 @@ def _run_loop(self):
                         self._fire(BOOKMARK, event.get("raw_object", obj))
                     elif evt_type == ERROR:
                         self._fire(ERROR, obj)
-                    # Periodic resync: re-list and fire MODIFIED for all cached objects
+                    # Periodic resync: full re-list from the API server, then
+                    # fire MODIFIED for every cached object so reconciliation
+                    # loops receive a fresh notification.
                     if (
                         self._resync_period > 0
                         and (time.monotonic() - last_resync) >= self._resync_period
                     ):
                         logger.debug("Informer resync triggered")
-                        for cached_obj in self._cache.list():
-                            self._fire(MODIFIED, cached_obj)
+                        try:
+                            self._initial_list()
+                        except Exception as exc:
+                            logger.exception("Error during resync list; continuing")
+                            self._fire(ERROR, exc)
+                        else:
+                            for cached_obj in self._cache.list():
+                                self._fire(MODIFIED, cached_obj)
                         last_resync = time.monotonic()
             except ApiException as exc:
                 if exc.status == 410:

kubernetes/test/test_informer.py

@@ -359,6 +359,42 @@ def fake_stream(func, **kw):
         self.assertEqual(len(cached), 1)
         self.assertIs(cached[0], pod)
 
+    def test_resync_period_triggers_full_list(self):
+        """A full List call must be made to the API server on every resync_period."""
+        pod = _make_pod("default", "resync-pod")
+
+        list_func = MagicMock()
+        list_resp = MagicMock()
+        list_resp.items = [pod]
+        list_resp.metadata = MagicMock(resource_version="5")
+        list_func.return_value = list_resp
+
+        informer = SharedInformer(list_func=list_func, resync_period=60)
+
+        with patch("kubernetes.informer.informer.Watch") as MockWatch, \
+                patch("kubernetes.informer.informer.time") as mock_time:
+            # Sequence of time.monotonic() calls:
+            #   1. last_resync = time.monotonic()          → 0.0
+            #   2. (time.monotonic() - last_resync) check  → 61.0  (triggers resync)
+            #   3. last_resync = time.monotonic()          → 61.0  (reset after resync)
+            mock_time.monotonic.side_effect = [0.0, 61.0, 61.0]
+
+            mock_w = MagicMock()
+            mock_w.resource_version = "5"
+
+            def fake_stream(func, **kw):
+                yield {"type": "ADDED", "object": pod}
+                informer._stop_event.set()
+
+            mock_w.stream.side_effect = fake_stream
+            MockWatch.return_value = mock_w
+
+            informer.start()
+            informer._thread.join(timeout=3)
+
+        # list_func called once for the initial list + once for the resync = 2
+        self.assertEqual(list_func.call_count, 2)
+
     def test_resource_version_stored_from_watch(self):
         """After the watch stream ends the latest RV is preserved for reconnect."""
         pod = _make_pod("default", "rv-pod")