Close a previously opened file in config generator

Proper handling of file lifecycle to avoid file-handler leakage.

The patch should cope with complains from Tempest:
/home/zuul/.../oslo_config/generator.py:770: ResourceWarning:
unclosed file <_io.TextIOWrapper name='/.../etc/tempest.conf.sample' mode='w' encoding='UTF-8'>
  generate(conf)

Also this patch should address unstable Tempest test case
that relies on presence of config file on file system -
tempest.tests.cmd.test_verify_tempest_config.TestDiscovery.test_get_config_file()
(http://git.openstack.org/cgit/openstack/tempest/tree/tempest/tests/cmd/test_verify_tempest_config.py#n590)

Change-Id: I02a4c6a9a6422075365afe6e29a877f3f8dfad32

oslo_config/generator.py

@@ -724,9 +724,14 @@ def generate(conf, output_file=None):
     """
     conf.register_opts(_generator_opts)
 
+    own_file = False
+
     if output_file is None:
-        output_file = (open(conf.output_file, 'w')
-                       if conf.output_file else sys.stdout)
+        if conf.output_file:
+            output_file = open(conf.output_file, 'w')
+            own_file = True
+        else:
+            output_file = sys.stdout
 
     groups = _get_groups(_list_opts(conf.namespace))
 
@@ -750,6 +755,9 @@ def generate(conf, output_file=None):
                                  output_file=output_file,
                                  conf=conf)
 
+    if own_file:
+        output_file.close()
+
 
 def main(args=None):
     """The main function of oslo-config-generator."""

oslo_config/tests/test_generator.py

@@ -1014,6 +1014,41 @@ class GeneratorTestCase(base.BaseTestCase):
             self.assertFalse(mock_log.warning.called)
 
 
+class GeneratorFileHandlingTestCase(base.BaseTestCase):
+
+    def setUp(self):
+        super(GeneratorFileHandlingTestCase, self).setUp()
+
+        self.conf = cfg.ConfigOpts()
+        self.config_fixture = config_fixture.Config(self.conf)
+        self.config = self.config_fixture.config
+
+    @mock.patch.object(generator, '_get_groups')
+    @mock.patch.object(generator, '_list_opts')
+    def test_close_generated_file(self, a, b):
+        generator.register_cli_opts(self.conf)
+        self.config(output_file='somefile')
+
+        m = mock.mock_open()
+        m.close = mock.Mock()
+
+        with mock.patch.object(generator, 'open', m, create=True):
+            generator.generate(self.conf, output_file=None)
+
+        m().close.assert_called_once()
+
+    @mock.patch.object(generator, '_get_groups')
+    @mock.patch.object(generator, '_list_opts')
+    def test_not_close_external_file(self, a, b):
+        generator.register_cli_opts(self.conf)
+        self.config()
+
+        m = mock.Mock()
+        generator.generate(self.conf, output_file=m)
+
+        m().close.assert_not_called()
+
+
 class DriverOptionTestCase(base.BaseTestCase):
 
     def setUp(self):