Implement mandatory configuration parameters

This patchset adds a class attribute that specifies which configuration
parameters are mandatory, setting the charm's status to 'blocked'
if any of them aren't present (and informing which ones are missing).

Change-Id: Ifcaea22a31f6f5af5edacd2ba5a3e901e7a37225

ops_openstack/core.py

@@ -64,6 +64,8 @@ class OSBaseCharm(CharmBase):
 
     REQUIRED_RELATIONS = []
 
+    MANDATORY_CONFIG = []
+
     def __init__(self, framework):
         super().__init__(framework)
         self.custom_status_checks = []
@@ -72,6 +74,7 @@ class OSBaseCharm(CharmBase):
         self._stored.set_default(series_upgrade=False)
         self.framework.observe(self.on.install, self.on_install)
         self.framework.observe(self.on.update_status, self.on_update_status)
+        self.framework.observe(self.on.config_changed, self._on_config)
         # A charm may not have pause/resume actions if it does not manage a
         # daemon.
         try:
@@ -234,6 +237,22 @@ class OSBaseCharm(CharmBase):
         self._stored.is_paused = False
         self.update_status()
 
+    def on_config(self, event):
+        """Main entry point for configuration changes."""
+        pass
+
+    def _on_config(self, event):
+        missing = []
+        config = self.framework.model.config
+        for param in self.MANDATORY_CONFIG:
+            if param not in config:
+                missing.append(param)
+        if missing:
+            self.unit.status = BlockedStatus(
+                'Missing option(s): ' + ','.join(missing))
+            return
+        self.on_config(event)
+
 
 def charm_class(cls):
     _releases[cls.release] = {'deb': cls}

unit_tests/test_ops_openstack.py

@@ -77,6 +77,9 @@ class OpenStackTestAPICharm(OpenStackTestPlugin1,
         else:
             return ActiveStatus()
 
+    def on_config(self, _):
+        self.unit.status = ActiveStatus()
+
 
 class CharmTestCase(unittest.TestCase):
 
@@ -150,6 +153,9 @@ class TestOSBaseCharm(CharmTestCase):
                         description: yet another failure to report
             ''')
 
+    def tearDown(self):
+        OpenStackTestAPICharm.MANDATORY_CONFIG = []
+
     def test_init(self):
         self.harness.begin()
         self.assertFalse(self.harness.charm._stored.is_started)
@@ -352,3 +358,13 @@ class TestOSBaseCharm(CharmTestCase):
             'resume',
             services=['apache2', 'ks-api'],
             charm_func=None)
+
+    def test_mandatory_config(self):
+        OpenStackTestAPICharm.MANDATORY_CONFIG = ['source']
+        self.harness.begin()
+        self.harness.update_config({})
+        self.assertTrue(
+            isinstance(self.harness.charm.unit.status, BlockedStatus))
+        self.harness.update_config({'source': 'value'})
+        self.assertTrue(
+            isinstance(self.harness.charm.unit.status, ActiveStatus))