Fix session's InvalidRequestError because of nested rollback

This patch addresses the issue by removing the extra nesting
(which is effectively redundant).

The longer story about this patch is the following:

Race conditions may cause DBDuplicateEntry exceptions that
require a transaction to be rollbacked back, and yet make
the whole operation succeed. A classic example is what has
been solved in commit fbc6b99. If the rollback is done in a
nested transaction, the above mentioned exception is raised.

To address the problem, we could use savepoints by means of
sqlalchemy's begin_nested(); Even though this approach is
preferable, it causes quite a bit of changes in the unit
tests (because of sqlite); it may also require that certain
DBMS, or certain DB backend configurations, support savepoints.

In the end, the simpler approach was chosen.

Closes-bug: #1354072

Change-Id: Ic9393319e55f71d681124bd3052e939724ebab6b

diff --git a/neutron/scheduler/l3_agent_scheduler.py b/neutron/scheduler/l3_agent_scheduler.py
index 265d2011ad86f8c93364744a29ee4d191028ea2d..8da4ed7d599c9ccc7c39b397553b9ca49dcfbb65 100644 (file)
--- a/neutron/scheduler/l3_agent_scheduler.py
+++ b/neutron/scheduler/l3_agent_scheduler.py
@@ -224,9 +224,8 @@ class ChanceScheduler(L3Scheduler):
 
     def schedule(self, plugin, context, router_id,
                  candidates=None, hints=None):
-        with context.session.begin(subtransactions=True):
-            return self._schedule_router(
-                plugin, context, router_id, candidates=candidates, hints=hints)
+        return self._schedule_router(
+            plugin, context, router_id, candidates=candidates, hints=hints)
 
     def _choose_router_agent(self, plugin, context, candidates):
         return random.choice(candidates)
@@ -237,9 +236,8 @@ class LeastRoutersScheduler(L3Scheduler):
 
     def schedule(self, plugin, context, router_id,
                  candidates=None, hints=None):
-        with context.session.begin(subtransactions=True):
-            return self._schedule_router(
-                plugin, context, router_id, candidates=candidates, hints=hints)
+        return self._schedule_router(
+            plugin, context, router_id, candidates=candidates, hints=hints)
 
     def _choose_router_agent(self, plugin, context, candidates):
         candidate_ids = [candidate['id'] for candidate in candidates]