Test for disabling greendns

In commit 7c1d964faa we fixed how we disable greendns. This patch adds
a test for this. It also lays down the groundwork for future tests
of how we manage eventlet's monkeypatching.

How and what eventlet monkeypatches can be controlled by environment
variables that are processed by eventlet at import-time (for exmaple,
EVENTLET_NO_GREENDNS). Nova manages all of this in nova.monkey_patch.
Therefore, nova.monkey_patch must be the first thing to import
eventlet. As nova.tests.functional.__init__ imports nova.monkey_patch,
our new test can go in the functional tree.

Related-bug: 1895322
Change-Id: I5b6c45b7b9a9eca3c13ecfaa5f50942922b69270

nova/tests/functional/test_monkey_patch.py

@@ -0,0 +1,45 @@
+# Copyright 2020 Red Hat, Inc.  All rights reserved.
+#
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+# NOTE(artom) This file exists to test eventlet monkeypatching. How and what
+# eventlet monkeypatches can be controlled by environment variables that
+# are processed by eventlet at import-time (for exmaple, EVENTLET_NO_GREENDNS).
+# Nova manages all of this in nova.monkey_patch. Therefore, nova.monkey_patch
+# must be the first thing to import eventlet. As nova.tests.functional.__init__
+# imports nova.monkey_patch, we're OK here.
+
+import socket
+import traceback
+
+from nova import test
+
+
+class TestMonkeyPatch(test.TestCase):
+
+    def test_greendns_is_disabled(self):
+        """Try to resolve a fake fqdn. If we see greendns mentioned in the
+        traceback of the raised exception, it means we've not actually disabled
+        greendns. See the TODO and NOTE in nova.monkey_patch to understand why
+        greendns needs to be disabled.
+        """
+        raised = False
+        try:
+            socket.gethostbyname('goat.fake')
+        except Exception:
+            tb = traceback.format_exc()
+            # NOTE(artom) If we've correctly disabled greendns, we expect the
+            # traceback to not contain any reference to it.
+            self.assertNotIn('greendns.py', tb)
+            raised = True
+        self.assertTrue(raised)