fix usage of opportunistic test cases with enginefacade

This creates an in tree fixture that lets you rebuild the engine
facade for doing individual tests against MySQL. It is needed until
this is supported in upstream oslo.db, which might be some time given
we are just hitting library freeze. Without this we can't build
regression tests for mysql specific bugs.

Related-Bug: #1548960

Change-Id: I2c4148972e91dcf1d6df0960198dc842b633d9e7

nova/tests/fixtures.py

@@ -23,6 +23,7 @@ import warnings
 
 import fixtures
 from oslo_config import cfg
+from oslo_db.sqlalchemy import enginefacade
 from oslo_messaging import conffixture as messaging_conffixture
 import six
 
@@ -476,3 +477,39 @@ class StableObjectJsonFixture(fixtures.Fixture):
 
         self.useFixture(fixtures.MonkeyPatch(
             'nova.objects.base.NovaObject.obj_to_primitive', _doit))
+
+
+class EngineFacadeFixture(fixtures.Fixture):
+    """Fixture to isolation EngineFacade during tests.
+
+    Because many elements of EngineFacade are based on globals, once
+    an engine facade has been initialized, all future code goes
+    through it. This means that the initialization of sqlite in
+    databases in our Database fixture will drive all connections to
+    sqlite. While that's fine in a production environment, during
+    testing this means we can't test againts multiple backends in the
+    same test run.
+
+    oslo.db does not yet support a reset mechanism here. This builds a
+    custom in tree engine facade fixture to handle this. Eventually
+    this will be added to oslo.db and this can be removed. Tracked by
+    https://bugs.launchpad.net/oslo.db/+bug/1548960
+
+    """
+    def __init__(self, ctx_manager, engine, sessionmaker):
+        super(EngineFacadeFixture, self).__init__()
+        self._ctx_manager = ctx_manager
+        self._engine = engine
+        self._sessionmaker = sessionmaker
+
+    def setUp(self):
+        super(EngineFacadeFixture, self).setUp()
+
+        self._existing_factory = self._ctx_manager._root_factory
+        self._ctx_manager._root_factory = enginefacade._TestTransactionFactory(
+            self._engine, self._sessionmaker, apply_global=False,
+            synchronous_reader=True)
+        self.addCleanup(self.cleanup)
+
+    def cleanup(self):
+        self._ctx_manager._root_factory = self._existing_factory