[OVN] Add support for WSGI mod

[Lucas Alvares Gomes]

Currently the OVN driver initialization does not work with uWSGI because the initialization is different than eventlet and the post_fork_initialization() method [0] does not get called in the process.

The post_fork_initialization() method is the one responsible for a few things:

* Starting the OVN database connections
* Starting the maintenance task
* Overwriting the agent and availability zone methods for OVN use [1]
* Running the database synchronization script (in log or repair mode) [2]

These steps would need to be factored out to adhere to the uWSGI initialization process.

[0] https://github.com/openstack/neutron/blob/19de0598227e2156b26f6d5eca874eaba09106a7/neutron/plugins/ml2/drivers/ovn/mech_driver/mech_driver.py#L181-L186

[1] https://github.com/openstack/neutron/blob/19de0598227e2156b26f6d5eca874eaba09106a7/neutron/plugins/ml2/drivers/ovn/mech_driver/mech_driver.py#L284-L292

[2] https://github.com/openstack/neutron/blob/19de0598227e2156b26f6d5eca874eaba09106a7/neutron/plugins/ml2/drivers/ovn/mech_driver/mech_driver.py#L300-L315

How to reproduce:

Deploy Devstack with OVN (https://docs.openstack.org/neutron/latest/contributor/testing/ovn_devstack.html) with NEUTRON_DEPLOY_MOD_WSGI=True set in local.conf.

[Slawek Kaplonski (slaweq)]

I don't think this is an rfe which we need to discuss in the drivers team and approve. For me it is just yet another feature parity gap which we should close ASAP. So I will remove "rfe" tag from that and lets keep it as a bug.

[Rabi Mishra (rabi)]

Any plans for fixing this soon? Looks like this would need some major refactoring?

[OpenStack Infra (hudson-openstack)]

Related fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/neutron/+/922074

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/neutron/+/922470

[OpenStack Infra (hudson-openstack)]

Related fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/neutron/+/922613

[OpenStack Infra (hudson-openstack)]

Related fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/neutron/+/923198

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/c/openstack/neutron/+/922074
Committed: https://opendev.org/openstack/neutron/commit/980f9bdab2f8a0895bb0373fc73788cec93ee649
Submitter: "Zuul (22348)"
Branch: master

commit 980f9bdab2f8a0895bb0373fc73788cec93ee649
Author: Rodolfo Alonso Hernandez <email address hidden>
Date: Mon Jun 17 07:10:03 2024 +0000

    [OVN] Add a new process to spawn ML2/OVN maintenance worker

    When using the Neutron WSGI module, the ML2/OVN maintenance worker needs
    to be spawned in a separate service. This patch adds the service
    ``neutron-ovn-maintenance-worker``, that is a single process service tha
    runs the ``MaintenanceWorker`` instance. This process is in charge of
    performing periodic routines related to the ML2/OVN driver.

    This new service should be included in any deployment project that
    allows to spawn Neutron ML2/OVN with WSGI. Along with this patch, a new
    one for devstack will be proposed.

    Related-Bug: #1912359
    Change-Id: Iea2995adb3343aae74a1b617fbccfce5c62c6b87

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/c/openstack/neutron/+/922470
Committed: https://opendev.org/openstack/neutron/commit/cfab008eef3d11055ad16b8dd0d49aaa1801bc95
Submitter: "Zuul (22348)"
Branch: master

commit cfab008eef3d11055ad16b8dd0d49aaa1801bc95
Author: Rodolfo Alonso Hernandez <email address hidden>
Date: Fri Jun 21 12:39:24 2024 +0000

    [OVN] Enable the WSGI module for the OVN mechanism driver

    This patch enables the use of the WSGI module with the ML2/OVN
    mechanism driver. The ML2/OVN requires two events that are called
    during the Neutron eventlet server initialization:
    * BEFORE_SPAWN: called once before the API workers have been created
      and after the ML2 plugin code has been initalizated.
    * AFTER_INIT: called when the API worker is started; at this point
      the different worker processes have been spawned.

    The WSGI module didn't make these event calls. Now these events are
    called during the API server initialization, after the ML2 plugin
    has been initalizated but before the server is running and attending
    any request.

    This approach differs from the Neutron eventlet server event calls
    because the BEFORE_SPAWN event is called for all API workers; that
    means the method ``OVNMechanismDriver.pre_fork_initialize`` is called
    as many times as workers are configured.

    Closes-Bug: #1912359
    Change-Id: I684c6cea620308a6617b665400ce608650a2adfd

[OpenStack Infra (hudson-openstack)]

Change abandoned by "Rodolfo Alonso <email address hidden>" on branch: master
Review: https://review.opendev.org/c/openstack/neutron/+/922613
Reason: neutron patch merged, devstack one under review

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/c/openstack/neutron/+/923198
Committed: https://opendev.org/openstack/neutron/commit/308db2e048754e6b35eca625dee6bad1971b1055
Submitter: "Zuul (22348)"
Branch: master

commit 308db2e048754e6b35eca625dee6bad1971b1055
Author: Rodolfo Alonso Hernandez <email address hidden>
Date: Sat Jun 29 16:58:18 2024 +0000

    [OVN] Release note for ``neutron-ovn-maintenance-worker`` process

    Related-Bug: #1912359
    Change-Id: Ic7f7edb22c2824a841a7b978b5b3c8eff68fb281