Allow loading custom config files into Config Fixture.

This patch adds a set_config_files option that allows a tester to
force load a specific list of configuration files. It will override the
existing set of files, and trigger a reload of those configuration
files.

Note: This _does_ trigger the autodetection mechanism, as the
present state of oslo_config makes it so __call__ is the only thing
that sets self._args. After triggering, however, it immediately
overrides the detected file list and replaces it with the provided one.

Tests provided.

Change-Id: I0bcea8be9d2527c61bd8d150e9ed606fabdd290e

oslo_config/fixture.py

@@ -36,10 +36,18 @@ class Config(fixtures.Fixture):
         # NOTE(morganfainberg): unregister must be added to cleanup before
         # reset is because cleanup works in reverse order of registered items,
         # and a reset must occur before unregistering options can occur.
+        self.addCleanup(self._reset_default_config_files)
         self.addCleanup(self._unregister_config_opts)
         self.addCleanup(self.conf.reset)
         self._registered_config_opts = {}
 
+        # Grab an old copy of the default config files - if it exists - for
+        # subsequent cleanup.
+        if hasattr(self.conf, 'default_config_files'):
+            self._default_config_files = self.conf.default_config_files
+        else:
+            self._default_config_files = None
+
     def config(self, **kw):
         """Override configuration values.
 
@@ -61,6 +69,17 @@ class Config(fixtures.Fixture):
             self.conf.unregister_opts(self._registered_config_opts[group],
                                       group=group)
 
+    def _reset_default_config_files(self):
+        if not hasattr(self.conf, 'default_config_files'):
+            return
+
+        if self._default_config_files:
+            self.conf.default_config_files = self._default_config_files
+        else:
+            # Delete, because we could conceivably begin with the property
+            # being unset.
+            self.conf.default_config_files = None
+
     def register_opt(self, opt, group=None):
         """Register a single option for the test run.
 
@@ -140,3 +159,21 @@ class Config(fixtures.Fixture):
             raw_config[group][key] = [str(value)]
 
         self.conf._namespace._add_parsed_config_file(raw_config, raw_config)
+
+    def set_config_files(self, config_files):
+        """Specify a list of config files to read.
+
+        This method allows you to predefine the list of configuration files
+        that are loaded by oslo_config. It will ensure that your tests do not
+        attempt to autodetect, and accidentally pick up config files from
+        locally installed services.
+        """
+        if not isinstance(config_files, list):
+            raise AttributeError("Please pass a list() to set_config_files()")
+
+        # Make sure the namespace exists for our tests.
+        if not self.conf._namespace:
+            self.conf.__call__(args=[])
+
+        self.conf.default_config_files = config_files
+        self.conf.reload_config_files()

oslo_config/tests/test_fixture.conf

@@ -0,0 +1,3 @@
+[DEFAULT]
+first_test_opt=loaded_value_1
+second_test_opt=loaded_value_2

oslo_config/tests/test_fixture.py

@@ -115,3 +115,29 @@ class ConfigTestCase(base.BaseTestCase):
         self.config_fixture.register_opt(opt2)
         self.assertEqual(conf.first_test_opt, 'initial_value_1')
         self.assertEqual(conf.second_test_opt, 'initial_value_2')
+
+    def test_assert_default_files_cleanup(self):
+        """Assert that using the fixture forces a clean list."""
+        self.assertNotIn('default_config_files', conf)
+
+        config_files = ['./test_fixture.conf']
+        self.config_fixture.set_config_files(config_files)
+
+        self.assertEqual(conf.default_config_files, config_files)
+        self.config_fixture.cleanUp()
+
+        self.assertNotIn('default_config_files', conf)
+
+    def test_load_custom_files(self):
+        self.assertNotIn('default_config_files', conf)
+        config_files = ['./oslo_config/tests/test_fixture.conf']
+        self.config_fixture.set_config_files(config_files)
+
+        opt1 = cfg.StrOpt('first_test_opt', default='initial_value_1')
+        opt2 = cfg.StrOpt('second_test_opt', default='initial_value_2')
+
+        self.config_fixture.register_opt(opt1)
+        self.config_fixture.register_opt(opt2)
+
+        self.assertEqual('loaded_value_1', conf.get('first_test_opt'))
+        self.assertEqual('loaded_value_2', conf.get('second_test_opt'))