set relation data on leader elected so alert rules are properly set (#…

…277)

* set relation data on leader elected so alert rules are properly set

* Check type in unit test

* data will never be None

lib/charms/prometheus_k8s/v0/prometheus_scrape.py

@@ -1577,6 +1577,9 @@ def __init__(
 
         self.framework.observe(self._charm.on.upgrade_charm, self._set_scrape_job_spec)
 
+        # If there is no leader during relation_joined we will still need to set alert rules.
+        self.framework.observe(self._charm.on.leader_elected, self._set_scrape_job_spec)
+
     def _set_scrape_job_spec(self, event):
         """Ensure scrape target information is made available to prometheus.
 

tests/unit/test_endpoint_provider.py

@@ -615,3 +615,26 @@ def test_unit_label_is_retained_if_hard_coded(self):
             for rule in group["rules"]:
                 self.assertIn("juju_unit", rule["labels"])
                 self.assertIn("juju_unit=", rule["expr"])
+
+
+class TestNoLeader(unittest.TestCase):
+    """Tests the case where leader is not set immediately."""
+
+    def setUp(self):
+        self.harness = Harness(EndpointProviderCharm, meta=PROVIDER_META)
+        self.addCleanup(self.harness.cleanup)
+        self.harness.set_leader(False)
+        self.harness.begin_with_initial_hooks()
+
+    @patch("ops.testing._TestingModelBackend.network_get")
+    def test_alert_rules(self, _):
+        """Verify alert rules are added when leader is elected after the relation is created."""
+        rel_id = self.harness.add_relation(RELATION_NAME, "provider")
+        self.harness.add_relation_unit(rel_id, "provider/0")
+        self.harness.set_leader(True)
+
+        data = self.harness.get_relation_data(rel_id, self.harness.model.app.name).get(
+            "alert_rules"
+        )
+        self.assertIsNotNone(data)
+        self.assertGreater(len(data), 0)  # type: ignore[arg-type]