Fix alerts assignment when event is None (#103)

src/charm.py

@@ -446,7 +446,7 @@ def _downstream_prometheus_scrape_relation_broken(self, _):
         self._stored.have_prometheus = False
         self._set_status()
 
-    def _on_nrpe_targets_changed(self, event):
+    def _on_nrpe_targets_changed(self, event: Optional[NrpeTargetsChangedEvent]):
         """Send NRPE jobs over to MetricsEndpointAggregator."""
         if event and isinstance(event, NrpeTargetsChangedEvent):
             removed_targets = event.removed_targets
@@ -461,9 +461,11 @@ def _on_nrpe_targets_changed(self, event):
                 )
 
             nrpes = cast(List[Dict[str, Any]], event.current_targets)
+            current_alerts = event.current_alerts
         else:
             # If the event arg is None, then the stored state value is already up-to-date.
             nrpes = self.nrpe_exporter.endpoints()
+            current_alerts = self.nrpe_exporter.alerts()
 
         self._modify_enrichment_file(endpoints=nrpes)
 
@@ -472,7 +474,7 @@ def _on_nrpe_targets_changed(self, event):
                 nrpe["target"], nrpe["app_name"], **nrpe["additional_fields"]
             )
 
-        for alert in event.current_alerts:
+        for alert in current_alerts:
             self.metrics_aggregator.set_alert_rule_data(
                 re.sub(r"/", "_", alert["labels"]["juju_unit"]),
                 alert,

tests/unit/test_relation_monitors.py

@@ -81,3 +81,19 @@ def test_monitors_changed(self):
             ]
         )
         self.assertEqual(expected, self.mock_enrichment_file.read_text())
+
+    def test_prometheus(self):
+        # GIVEN a post-startup charm
+        self.harness.begin_with_initial_hooks()
+
+        # WHEN "monitors" and "downstream-prometheus-scrape" relations join
+        rel_id_nrpe = self.harness.add_relation("monitors", "nrpe")
+        self.harness.add_relation_unit(rel_id_nrpe, "nrpe/0")
+        self.harness.update_relation_data(rel_id_nrpe, "nrpe/0", self.default_unit_data)
+
+        rel_id_prom = self.harness.add_relation("downstream-prometheus-scrape", "prom")
+        self.harness.add_relation_unit(rel_id_prom, "prom/0")
+
+        # THEN alert rules are transferred to prometheus over relation data
+        app_data = self.harness.get_relation_data(rel_id_prom, "cos-proxy")
+        self.assertIn("alert_rules", app_data)