Add "is_started" flag to enginefacade

Some module reloading scenarios such as that which occurs
within mod_wsgi mean that an existing module-level enginefacade
is already in the "started" state, however initialization
routines from the calling application may still attempt to
call the ``.configure`` method.  Add a new flag is_started
to both _TransactionContextManager and _TransactionFactory so
that calling code can check for this state ahead of time;
additionally, promote the TypeError raised to a specific subclass
enginefacade.AlreadyStartedError to allow for better optimistic
schemes.

Change-Id: I2f5a9e35c2fae0c28b78beef3dcd2c4794362766
References: I704196711d30c1124e713ac31111a8ea6fa2f1ba

oslo_db/sqlalchemy/enginefacade.py

@@ -120,6 +120,14 @@ class _Default(object):
             hasattr(conf_namespace, key)
 
 
+class AlreadyStartedError(TypeError):
+    """Raises when a factory is being asked to initialize a second time.
+
+    Subclasses :class:`.TypeError` for legacy support.
+
+    """
+
+
 class _TransactionFactory(object):
     """A factory for :class:`._TransactionContext` objects.
 
@@ -314,7 +322,8 @@ class _TransactionFactory(object):
     def _configure(self, as_defaults, kw):
 
         if self._started:
-            raise TypeError("this TransactionFactory is already started")
+            raise AlreadyStartedError(
+                "this TransactionFactory is already started")
         not_supported = []
         for k, v in kw.items():
             for dict_ in (
@@ -464,6 +473,11 @@ class _TransactionFactory(object):
             if self._reader_engine is not self._writer_engine:
                 self._reader_engine.pool.dispose()
 
+    @property
+    def is_started(self):
+        """True if this :class:`._TransactionFactory` is already started."""
+        return self._started
+
     def _start(self, conf=False, connection=None, slave_connection=None):
         with self._start_lock:
             # self._started has been checked on the outside
@@ -777,6 +791,11 @@ class _TransactionContextManager(object):
         """The :class:`._TransactionFactory` associated with this context."""
         return self._root._root_factory
 
+    @property
+    def is_started(self):
+        """True if this manager is already started."""
+        return self._factory.is_started
+
     def configure(self, **kw):
         """Apply configurational options to the factory.
 

oslo_db/tests/sqlalchemy/test_enginefacade.py

@@ -479,6 +479,32 @@ class MockFacadeTest(oslo_test_base.BaseTestCase):
             [mock.call.dispose()]
         )
 
+    def test_started_flag(self):
+        facade = enginefacade.transaction_context()
+
+        self.assertFalse(facade.is_started)
+        facade.configure(connection=self.engine_uri)
+        facade.writer.get_engine()
+
+        self.assertTrue(facade.is_started)
+
+    def test_started_exception(self):
+        facade = enginefacade.transaction_context()
+
+        self.assertFalse(facade.is_started)
+        facade.configure(connection=self.engine_uri)
+        facade.writer.get_engine()
+
+        exc = self.assertRaises(
+            enginefacade.AlreadyStartedError,
+            facade.configure,
+            connection=self.engine_uri
+        )
+        self.assertEqual(
+            "this TransactionFactory is already started",
+            exc.args[0]
+        )
+
     def test_session_reader_decorator(self):
         context = oslo_context.RequestContext()
 

releasenotes/notes/add_facade_started-14f9bc34fac89371.yaml

@@ -0,0 +1,8 @@
+---
+features:
+  - |
+    Added new ``.is_started`` boolean flag to enginefacade context manager
+    and factory objects, so that double-configure scenarios can be prevented
+    by calling code.   Additionally, the ``TypeError`` raised when configure
+    is called after the factory is started is now a specific subclass
+    ``enginefacade.AlreadyStartedError``.