Add flush_on_subtransaction=True to engine config

By default, the new enginefacade doesn't emulate the same
autoflush behavior we had on exits of session context managers
with the old style. As of olso.db 4.18, we have an option to
get this same behavior. This patch implements it in our db.api
to help with the adoption of the new enginefacade by providing
the same flush semantics.

Related-Bug: #1664643
Partially-Implements: blueprint enginefacade-switch
Change-Id: I71e274d0f0199896f28daae6db71241d9b43a2f3

neutron_lib/db/api.py

@@ -22,7 +22,7 @@ def _create_context_manager():
     global _CTX_MANAGER
     if _CTX_MANAGER is None:
         _CTX_MANAGER = enginefacade.transaction_context()
-        _CTX_MANAGER.configure(sqlite_fk=True)
+        _CTX_MANAGER.configure(sqlite_fk=True, flush_on_subtransaction=True)
 
     return _CTX_MANAGER
 

releasenotes/notes/flush_on_subtransaction-99ef11dfb56b706d.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - The oslo.db context manager returned by ``db.api.get_context_manager()``
+    will be configured with flush_on_subtransaction=True to more closely
+    match the behavior of the previous session.begin(subtransactions=True)
+    pattern we used everywhere before. See bug 1664643 for more details.