Add 'WarningsFixture'

This duplicates what exists in nova and various other projects. The
important difference between this and what we're doing currently is that
it *restores*, rather than reset, the warning filters. There are more
various warning filters pre-configured in a typical Python environment,
including a few from third-party libraries such as requests [1][2] and
urllib3 [3] as well as stdlib [4]. By calling 'warnings.resetwarnings', we
*reset* all the warning filters [5]. This is clearly not something we want to
do, and resulted in tests puking warnings after the initial test run.

[1] https://github.com/psf/requests/blob/v2.26.0/requests/__init__.py#L127
[2] https://github.com/psf/requests/blob/v2.26.0/requests/__init__.py#L152
[3] https://github.com/urllib3/urllib3/blob/1.26.7/src/urllib3/__init__.py#L68-L78
[4] https://docs.python.org/3.8/library/warnings.html#default-warning-filter
[5] https://docs.python.org/3.8/library/warnings.html#warnings.resetwarnings

Change-Id: Ia2046dc32e3ac270b1dbcf2fe540104c1a8d95d8
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>

keystone/tests/unit/core.py

@@ -11,6 +11,7 @@
 # WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 # License for the specific language governing permissions and limitations
 # under the License.
+
 import atexit
 import base64
 import contextlib
@@ -24,7 +25,6 @@ import shutil
 import socket
 import sys
 import uuid
-import warnings
 
 import fixtures
 import flask
@@ -36,7 +36,6 @@ from oslo_context import fixture as oslo_ctx_fixture
 from oslo_log import fixture as log_fixture
 from oslo_log import log
 from oslo_utils import timeutils
-from sqlalchemy import exc
 import testtools
 from testtools import testcase
 
@@ -678,17 +677,8 @@ class BaseTestCase(testtools.TestCase):
         self.useFixture(fixtures.MockPatchObject(sys, 'exit',
                                                  side_effect=UnexpectedExit))
         self.useFixture(log_fixture.get_logging_handle_error_fixture())
+        self.useFixture(ksfixtures.WarningsFixture())
 
-        warnings.filterwarnings('error', category=DeprecationWarning,
-                                module='^keystone\\.')
-        warnings.filterwarnings(
-            'ignore', category=DeprecationWarning,
-            message=r"Using function/method 'db_version\(\)' is deprecated")
-        warnings.simplefilter('error', exc.SAWarning)
-        if hasattr(exc, "RemovedIn20Warning"):
-            warnings.simplefilter('ignore', exc.RemovedIn20Warning)
-
-        self.addCleanup(warnings.resetwarnings)
         # Ensure we have an empty threadlocal context at the start of each
         # test.
         self.assertIsNone(oslo_context.get_current())

keystone/tests/unit/ksfixtures/__init__.py

@@ -17,3 +17,4 @@ from keystone.tests.unit.ksfixtures.cache import Cache  # noqa
 from keystone.tests.unit.ksfixtures.jws_key_repository import JWSKeyRepository  # noqa
 from keystone.tests.unit.ksfixtures.key_repository import KeyRepository  # noqa
 from keystone.tests.unit.ksfixtures.policy import Policy  # noqa
+from keystone.tests.unit.ksfixtures.warnings import WarningsFixture  # noqa

keystone/tests/unit/ksfixtures/warnings.py

@@ -0,0 +1,54 @@
+# 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.
+
+import warnings
+
+import fixtures
+from sqlalchemy import exc as sqla_exc
+
+
+class WarningsFixture(fixtures.Fixture):
+    """Filters out warnings during test runs."""
+
+    def setUp(self):
+        super().setUp()
+
+        self._original_warning_filters = warnings.filters[:]
+
+        # NOTE(stephenfin): Make deprecation warnings only happen once.
+        # Otherwise this gets kind of crazy given the way that upstream python
+        # libs use this.
+        warnings.simplefilter('once', DeprecationWarning)
+
+        warnings.filterwarnings(
+            'error',
+            category=DeprecationWarning,
+            module='^keystone\\.',
+        )
+
+        # TODO(stephenfin): This will be fixed once we drop sqlalchemy-migrate
+        warnings.filterwarnings(
+            'ignore',
+            category=DeprecationWarning,
+            message=r"Using function/method 'db_version\(\)' is deprecated",
+        )
+
+        # TODO(stephenfin): We should filter on the specific RemovedIn20Warning
+        # warnings that affect us, so that we can slowly start addressing them
+        warnings.simplefilter('error', sqla_exc.SAWarning)
+        if hasattr(sqla_exc, 'RemovedIn20Warning'):
+            warnings.simplefilter('ignore', sqla_exc.RemovedIn20Warning)
+
+        self.addCleanup(self._reset_warning_filters)
+
+    def _reset_warning_filters(self):
+        warnings.filters[:] = self._original_warning_filters