MetricsProvider sets unit address on updated status (#268)

This commit ensures that the MetricsEndpointProvider sets
unit address on update status and config changed events also.
This change is required because podspec charms do not have
a pebble ready event and uptil now the MetricsEndpointProvider
was setting IP address only in response to pebble ready event
This change also ensures that the MetricsEndpointProvider
does not set invalid IP addresses.

lib/charms/prometheus_k8s/v0/prometheus_scrape.py

@@ -311,6 +311,8 @@ def _on_scrape_targets_changed(self, event):
 
 """  # noqa: W505
 
+import contextlib
+import ipaddress
 import json
 import logging
 import os
@@ -334,7 +336,7 @@ def _on_scrape_targets_changed(self, event):
 
 # Increment this PATCH version before using `charmcraft publish-lib` or reset
 # to 0 if you are raising the major API version
-LIBPATCH = 18
+LIBPATCH = 19
 
 logger = logging.getLogger(__name__)
 
@@ -1569,6 +1571,10 @@ def __init__(
                 self._set_unit_ip,
             )
 
+        # podspec charms do not have a pebble ready event so unit ips need to be set these events
+        self.framework.observe(self._charm.on.update_status, self._set_unit_ip)
+        self.framework.observe(self._charm.on.config_changed, self._set_unit_ip)
+
         self.framework.observe(self._charm.on.upgrade_charm, self._set_scrape_job_spec)
 
     def _set_scrape_job_spec(self, event):
@@ -1611,13 +1617,36 @@ def _set_unit_ip(self, _):
         event is actually needed.
         """
         for relation in self._charm.model.relations[self._relation_name]:
-            relation.data[self._charm.unit]["prometheus_scrape_unit_address"] = str(
-                self._charm.model.get_binding(relation).network.bind_address
-            )
+            unit_ip = str(self._charm.model.get_binding(relation).network.bind_address)
+
+            if not self._is_valid_unit_address(unit_ip):
+                # relation data will be updated later when a valid address becomes available
+                with contextlib.suppress(KeyError):
+                    del relation.data[self._charm.unit]["prometheus_scrape_unit_address"]
+                    del relation.data[self._charm.unit]["prometheus_scrape_unit_name"]
+                continue
+
+            relation.data[self._charm.unit]["prometheus_scrape_unit_address"] = unit_ip
             relation.data[self._charm.unit]["prometheus_scrape_unit_name"] = str(
                 self._charm.model.unit.name
             )
 
+    def _is_valid_unit_address(self, address: str) -> bool:
+        """Validate a unit address.
+
+        At present only IP address validation is supported, but
+        this may be extended to DNS addresses also, as needed.
+
+        Args:
+            address: a string representing a unit address
+        """
+        try:
+            _ = ipaddress.ip_address(address)
+        except ValueError:
+            return False
+
+        return True
+
     @property
     def _scrape_jobs(self) -> list:
         """Fetch list of scrape jobs.

tests/unit/test_endpoint_provider.py

@@ -152,6 +152,22 @@ def test_provider_unit_sets_bind_address_on_pebble_ready(self, *unused):
         self.assertIn("prometheus_scrape_unit_address", data)
         self.assertEqual(data["prometheus_scrape_unit_address"], "192.0.8.2")
 
+    @patch_network_get(private_address="192.0.8.2")
+    def test_provider_unit_sets_bind_address_on_update_status(self, *unused):
+        rel_id = self.harness.add_relation(RELATION_NAME, "provider")
+        self.harness.charm.on.update_status.emit()
+        data = self.harness.get_relation_data(rel_id, self.harness.charm.unit.name)
+        self.assertIn("prometheus_scrape_unit_address", data)
+        self.assertEqual(data["prometheus_scrape_unit_address"], "192.0.8.2")
+
+    @patch_network_get(private_address=None)
+    def test_provider_does_not_set_invalid_unit_address(self, *unused):
+        rel_id = self.harness.add_relation(RELATION_NAME, "provider")
+        self.harness.container_pebble_ready("prometheus-tester")
+        self.harness.charm.on.update_status.emit()
+        data = self.harness.get_relation_data(rel_id, self.harness.charm.unit.name)
+        self.assertNotIn("prometheus_scrape_unit_address", data)
+
     @patch_network_get(private_address="192.0.8.2")
     def test_provider_unit_sets_bind_address_on_relation_joined(self, *unused):
         rel_id = self.harness.add_relation(RELATION_NAME, "provider")