ml2 "while True" assumes fresh data on transactions

[Kevin Benton]

both delete_network and delete_subnet in ML2 use the following pattern:

while True:
    with session.begin():
        record = get_thing_from_db()
        ... Logic to determine if out of transaction cleaning should be done
        if no_cleaning:
            context.session.delete(record)

    do_cleaning_up()

The problem here is that it assumes it will get fresh data on each iteration. However, due to to the identity map pattern followed by sqlalchemy[1], new data will not be reflected in the 'record' var above on subsequent iterations if the primary key still exists in the DB.

This can lead to infinite loops on delete_subnet if a concurrent delete request (or network update) bumps the network revision number. This is because the network that is in the session will always have the stale revision number even though the lookup methods are called on each iteration.

This can be reproduced with the following steps:
1. Place a pdb breakpoint here: https://github.com/openstack/neutron/blob/27928c0ddfb8d62843aa72ecf943d1f46ef30099/neutron/plugins/ml2/plugin.py#L1062
2. create a network with two subnets (ensure the dhcp agent is running so it gets an IP on each)
3. issue an API subnet delete call
4. first time breakpoint hits, nothing is looked up yet, so just continue
5. second time breakpoint hits, issue the following query in mysql: 'update standardattributes set revision_number=500000;'. this will alter the revision number in the DB but the loop won't ever get the new one.
6. continue
7. continue
8. continue
9. do this as long as you want :)

1. http://docs.sqlalchemy.org/en/latest/orm/session_basics.html#is-the-session-a-cache

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/370920

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/370920
Committed: https://git.openstack.org/cgit/openstack/neutron/commit/?id=1279640da845d4f6bc6274e6706875610e179326
Submitter: Jenkins
Branch: master

commit 1279640da845d4f6bc6274e6706875610e179326
Author: Kevin Benton <email address hidden>
Date: Wed Sep 14 22:35:02 2016 -0700

    Expire DB objects in ML2 infinity loops

    We must expire all DB objects in the session to ensure
    that we are getting the latest object state from the DB
    on each iteration of these 'while True:' loops to ensure
    we don't continue forever on stale data.

    Closes-Bug: #1623990
    Change-Id: I2f4ea6cce5d83c13fd37650d28a7089c5aa9a4c0

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/neutron 9.0.0.0rc1 release candidate.