ensure we reset contexts when fixture is used

Previously using the ClearRequestContext fixture only reset contexts
on fixture teardown. If *all* tests in a test suite use this fixture,
this is fine. However if any tests do not, it means you might start
with a carry over context from previous tests.

Using this fixture means you really wanted a clean slate, so fix it so
that you get a clean state when you get started as well as when you
finish.

Change-Id: I5e6f7bc2866fe66f269c7d5e1f36c711151489c2
Closes-Bug: #1428728

oslo_context/fixture.py

@@ -16,10 +16,20 @@ from oslo_context import context
 
 
 class ClearRequestContext(fixtures.Fixture):
-    """Clears any cached RequestContext at the end of a test case."""
+    """Clears any cached RequestContext
+
+    This resets RequestContext at the beginning and end of tests that
+    use this fixture to ensure that we have a clean slate for running
+    tests, and that we leave a clean slate for other tests that might
+    run later in the same process.
+    """
 
     def setUp(self):
         super(ClearRequestContext, self).setUp()
+        # we need to clear both when we start, and when we finish,
+        # because there might be other tests running that don't handle
+        # this correctly.
+        self._remove_cached_context()
         self.addCleanup(self._remove_cached_context)
 
     def _remove_cached_context(self):

oslo_context/tests/test_fixture.py

@@ -28,3 +28,19 @@ class ClearRequestContextTest(test_base.BaseTestCase):
         self.assertIs(context.get_current(), ctx)
         fixture.ClearRequestContext()._remove_cached_context()
         self.assertIsNone(context.get_current())
+
+    def test_store_current_resets_correctly(self):
+        # By default a new context is stored.
+        ctx = context.RequestContext()
+
+        # the use of the fixture should put us in a reset state, not
+        # doing so is a bug because when this fixture is consumed by
+        # other test suites there is no guaruntee that all tests use
+        # this fixture.
+        self.useFixture(fixture.ClearRequestContext())
+        self.assertIsNone(context.get_current())
+
+        ctx = context.RequestContext()
+        self.assertIs(context.get_current(), ctx)
+        fixture.ClearRequestContext()._remove_cached_context()
+        self.assertIsNone(context.get_current())