oslo.db: transition from legacy facade

WIP

Direct use enginefacade in api module for creation of engines
and sessions for use in watcher.

Change-Id: I36daf1bb11191583b83264b9ed269a08cbf069ba
Closes-Bug: 2056181

watcher/db/sqlalchemy/api.py

@@ -22,7 +22,7 @@ import operator
 
 from oslo_config import cfg
 from oslo_db import exception as db_exc
-from oslo_db.sqlalchemy import session as db_session
+from oslo_db.sqlalchemy import enginefacade
 from oslo_db.sqlalchemy import utils as db_utils
 from oslo_utils import timeutils
 from sqlalchemy.inspection import inspect
@@ -44,22 +44,32 @@ _FACADE = None
 def _create_facade_lazily():
     global _FACADE
     if _FACADE is None:
-        # FIXME(amoralej): Remove autocommit=True (and ideally use of
-        # LegacyEngineFacade) asap since it's not compatible with SQLAlchemy
-        # 2.0.
-        _FACADE = db_session.EngineFacade.from_config(CONF,
-                                                      autocommit=True)
-    return _FACADE
+        _FACADE = True
+        enginefacade.configure(expire_on_commit=True)
+
+
+def _get_transaction_context(reader=False):
+    _create_facade_lazily()
+    # TODO(gthiemonge) Create and use new functions to get read-only sessions
+    if reader:
+        context = enginefacade.reader
+    else:
+        context = enginefacade.writer
+    return context
+
+
+def _get_sessionmaker(reader=False):
+    context = _get_transaction_context(reader)
+    return context.get_sessionmaker()
 
 
 def get_engine():
-    facade = _create_facade_lazily()
-    return facade.get_engine()
+    context = _get_transaction_context()
+    return context.get_engine()
 
 
 def get_session(**kwargs):
-    facade = _create_facade_lazily()
-    return facade.get_session(**kwargs)
+    return _get_sessionmaker()(**kwargs)
 
 
 def get_backend():