Adds dbcluster peer relation with logic to set db-bind-address in

relation data if we have a unique ingress address, or become blocked if
there is not a unique address.

metadata.yaml

@@ -16,3 +16,6 @@ provides:
     interface: http
   metrics-endpoint:
     interface: prometheus_scrape
+peers:
+  dbcluster:
+    interface: dbcluster

src/charm.py

@@ -21,12 +21,17 @@ class JujuControllerCharm(CharmBase):
 
     def __init__(self, *args):
         super().__init__(*args)
+
         self.framework.observe(self.on.config_changed, self._on_config_changed)
         self.framework.observe(self.on.start, self._on_start)
         self.framework.observe(
             self.on.dashboard_relation_joined, self._on_dashboard_relation_joined)
         self.framework.observe(
             self.on.website_relation_joined, self._on_website_relation_joined)
+        self.framework.observe(
+            self.on.dbcluster_relation_changed, self._on_dbcluster_relation_changed)
+
+        self._db_bind_address = ''
 
         self.control_socket = controlsocket.Client(
             socket_path="/var/lib/juju/control.socket")
@@ -39,7 +44,7 @@ def _on_start(self, _):
         self.unit.status = ActiveStatus()
 
     def _on_config_changed(self, _):
-        controller_url = self.config["controller-url"]
+        controller_url = self.config['controller-url']
         logger.info("got a new controller-url: %r", controller_url)
 
     def _on_dashboard_relation_joined(self, event):
@@ -55,10 +60,10 @@ def _on_dashboard_relation_joined(self, event):
 
     def _on_website_relation_joined(self, event):
         """Connect a website relation."""
-        logger.info("got a new website relation: %r", event)
+        logger.info('got a new website relation: %r', event)
         port = self.api_port()
         if port is None:
-            logger.error("machine does not appear to be a controller")
+            logger.error('machine does not appear to be a controller')
             self.unit.status = BlockedStatus('machine does not appear to be a controller')
             return
 
@@ -105,6 +110,22 @@ def _on_metrics_endpoint_relation_broken(self, event: RelationDepartedEvent):
         username = metrics_username(event.relation)
         self.control_socket.remove_metrics_user(username)
 
+    def _on_dbcluster_relation_changed(self, event):
+        ips = self.model.get_binding(event.relation).network.ingress_addresses
+
+        if len(ips) > 1:
+            logger.error('multiple possible DB bind addresses; set a dbcluster network binding')
+            self.unit.status = BlockedStatus(
+                'multiple possible DB bind addresses; set a dbcluster network binding')
+            return
+
+        ip = str(ips[0])
+        if self._db_bind_address == ip:
+            return
+
+        self._db_bind_address = ip
+        event.relation.data[self.unit].update({'db-bind-address': ip})
+
     def _agent_conf(self, key: str):
         """Read a value (by key) from the agent.conf file on disk."""
         unit_name = self.unit.name.replace('/', '-')

tests/test_charm.py

@@ -4,6 +4,7 @@
 import os
 import unittest
 from charm import JujuControllerCharm
+from ops.model import BlockedStatus
 from ops.testing import Harness
 from unittest.mock import mock_open, patch
 
@@ -14,10 +15,14 @@
 
 
 class TestCharm(unittest.TestCase):
-    def test_relation_joined(self):
-        harness = Harness(JujuControllerCharm)
-        self.addCleanup(harness.cleanup)
-        harness.begin()
+    def setUp(self):
+        self.harness = Harness(JujuControllerCharm)
+        self.addCleanup(self.harness.cleanup)
+        self.harness.begin()
+
+    def test_dashboard_relation_joined(self):
+        harness = self.harness
+
         harness.set_leader(True)
         harness.update_config({"controller-url": "wss://controller/api"})
         harness.update_config({"identity-provider-url": ""})
@@ -36,12 +41,10 @@ def test_relation_joined(self):
     })
     @patch("ops.model.Model.get_binding")
     @patch("builtins.open", new_callable=mock_open, read_data=agent_conf)
-    def test_website_relation_joined(self, _, ingress_address):
-        ingress_address.return_value = MockBinding("192.168.1.17")
+    def test_website_relation_joined(self, _, binding):
+        harness = self.harness
+        binding.return_value = mockBinding(["192.168.1.17"])
 
-        harness = Harness(JujuControllerCharm)
-        self.addCleanup(harness.cleanup)
-        harness.begin()
         harness.set_leader()
         relation_id = harness.add_relation('website', 'haproxy')
         harness.add_relation_unit(relation_id, 'haproxy/0')
@@ -58,10 +61,7 @@ def test_website_relation_joined(self, _, ingress_address):
     @patch("controlsocket.Client.remove_metrics_user")
     def test_metrics_endpoint_relation(self, mock_remove_user, mock_add_user,
                                        mock_metrics_provider, _):
-        harness = Harness(JujuControllerCharm)
-        self.addCleanup(harness.cleanup)
-        harness.begin()
-
+        harness = self.harness
         harness.add_network(address="192.168.1.17", endpoint="metrics-endpoint")
 
         relation_id = harness.add_relation('metrics-endpoint', 'prometheus-k8s')
@@ -88,12 +88,40 @@ def test_metrics_endpoint_relation(self, mock_remove_user, mock_add_user,
         harness.remove_relation(relation_id)
         mock_remove_user.assert_called_once_with(f'juju-metrics-r{relation_id}')
 
+    @patch("ops.model.Model.get_binding")
+    def test_dbcluster_relation_changed_single_addr(self, binding):
+        harness = self.harness
+        binding.return_value = mockBinding(["192.168.1.17"])
+
+        relation_id = harness.add_relation('dbcluster', 'controller')
+        harness.add_relation_unit(relation_id, 'juju-controller/1')
+
+        harness.charm._on_dbcluster_relation_changed(
+            harness.charm.model.get_relation('dbcluster').data[harness.charm.unit])
+
+        data = harness.get_relation_data(relation_id, 'juju-controller/0')
+        self.assertEqual(data["db-bind-address"], "192.168.1.17")
+
+    @patch("ops.model.Model.get_binding")
+    def test_dbcluster_relation_changed_multi_addr_error(self, binding):
+        harness = self.harness
+        binding.return_value = mockBinding(["192.168.1.17", "192.168.1.18"])
+
+        relation_id = harness.add_relation('dbcluster', 'controller')
+        harness.add_relation_unit(relation_id, 'juju-controller/1')
+
+        harness.charm._on_dbcluster_relation_changed(
+            harness.charm.model.get_relation('dbcluster').data[harness.charm.unit])
+
+        self.assertIsInstance(harness.charm.unit.status, BlockedStatus)
+
 
-class MockBinding:
-    def __init__(self, address):
-        self.network = MockNetwork(address)
+class mockNetwork:
+    def __init__(self, addresses):
+        self.ingress_addresses = addresses
+        self.ingress_address = addresses[0]
 
 
-class MockNetwork:
-    def __init__(self, address):
-        self.ingress_address = address
+class mockBinding:
+    def __init__(self, addresses):
+        self.network = mockNetwork(addresses)