policy: Don't persist default rule changes in tests

'oslo.policy' expects a list of default rules be provided when
initialising the enforcer. Unfortunately, it demonstrates a nasty habit
of modifying these defaults once it has them [1]. This can result in
configuration from one test bleeding through to the next.

Resolve this by initialising the enforcer with a copy of the default
rules, rather than the originals, allowing 'oslo.policy' to do whatever
it likes to the rules. Ultimately this should probably be fixed in
'oslo.policy', which should be either copying the defaults or creating
new rules based on these defaults, but that needs some thought and can
be done later.

[1] https://github.com/openstack/oslo.policy/blob/3.6.0/oslo_policy/policy.py#L762-L764

Change-Id: Ib864606d4cf4a8ecba95386bcc2b8a4bb68f2d17
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>
Story: 2008556
Task: 41687

placement/policies/__init__.py

@@ -25,7 +25,7 @@ from placement.policies import usage
 
 
 def list_rules():
-    return itertools.chain(
+    rules = itertools.chain(
         base.list_rules(),
         resource_provider.list_rules(),
         resource_class.list_rules(),
@@ -37,3 +37,4 @@ def list_rules():
         allocation_candidate.list_rules(),
         reshaper.list_rules(),
     )
+    return list(rules)

placement/policy.py

@@ -11,6 +11,8 @@
 #    under the License.
 """Policy Enforcement for placement API."""
 
+import typing as ty
+
 from oslo_config import cfg
 from oslo_log import log as logging
 from oslo_policy import opts as policy_opts
@@ -36,12 +38,14 @@ def reset():
 def init(
     conf: cfg.ConfigOpts,
     suppress_deprecation_warnings: bool = False,
+    rules: ty.List[policy.RuleDefault] = None,
 ):
     """Init an Enforcer class. Sets the _ENFORCER global.
 
     :param conf: A ConfigOpts object to load configuration from.
     :param suppress_deprecation_warnings: **Test only** Suppress policy
         deprecation warnings to avoid polluting logs.
+    :param rules: **Test only** The default rules to initialise.
     """
     global _ENFORCER
     if not _ENFORCER:
@@ -56,7 +60,7 @@ def init(
         _enforcer.suppress_default_change_warnings = True
         _enforcer.suppress_deprecation_warnings = suppress_deprecation_warnings
 
-        _enforcer.register_defaults(policies.list_rules())
+        _enforcer.register_defaults(rules or policies.list_rules())
         _enforcer.load_rules()
         _ENFORCER = _enforcer
 

placement/tests/unit/policy_fixture.py

@@ -12,11 +12,13 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
-import fixtures
+import copy
 
+import fixtures
 from oslo_policy import policy as oslo_policy
 
 from placement.conf import paths
+from placement import policies
 from placement import policy as placement_policy
 
 
@@ -32,9 +34,13 @@ class PolicyFixture(fixtures.Fixture):
         policy_file = paths.state_path_def('etc/placement/policy.yaml')
         self.conf_fixture.config(group='oslo_policy', policy_file=policy_file)
         placement_policy.reset()
+        # because oslo.policy has a nasty habit of modifying the default rules
+        # we provide, we must pass a copy of the rules rather then the rules
+        # themselves
         placement_policy.init(
             self.conf_fixture.conf,
-            suppress_deprecation_warnings=True)
+            suppress_deprecation_warnings=True,
+            rules=copy.deepcopy(policies.list_rules()))
         self.addCleanup(placement_policy.reset)
 
     @staticmethod