Share a single connection with all systems during migrations

The purpose of the Alembic env.py file is so that an application
may define the optimal approach to database connectivity
for an application as it runs migrations.  Here, we ensure
that the connectivity comes from the same system as that of
the api.py module itself, e.g. enginefacade, and we also
ensure that the same transaction/connection used for migrations
is shared with the ORM session, so that ORM operations inline
with schema operations share the same transactional context.

Change-Id: Ie2d28d2b04e452c13d16e561b781f82cf2b75e18
Story: #2000099

subunit2sql/migrations/env.py

@@ -16,7 +16,8 @@
 from __future__ import with_statement
 from alembic import context
 from logging.config import fileConfig  # noqa
-from oslo.db.sqlalchemy import session
+
+from subunit2sql.db import api as db_api
 
 
 # this is the Alembic Config object, which provides
@@ -73,8 +74,12 @@ def run_migrations_online():
     and associate a connection with the context.
 
     """
-    engine = session.create_engine(subunit2sql_config.database.connection)
+
+    facade = db_api._create_facade_lazily()
+    engine = facade.get_engine()
     connection = engine.connect()
+    facade._session_maker.configure(bind=connection)
+
     context.configure(connection=connection, target_metadata=target_metadata)
 
     try:
@@ -82,6 +87,7 @@ def run_migrations_online():
             context.run_migrations()
     finally:
         connection.close()
+        facade._session_maker.configure(bind=engine)
 
 if context.is_offline_mode():
     run_migrations_offline()