Comment 14 for bug 1841967

Reviewed: https://review.opendev.org/681663
Committed: https://git.openstack.org/cgit/openstack/neutron/commit/?id=f913394d70dede8d40d6e96485bd5333dfb6f43f
Submitter: Zuul
Branch: stable/rocky

commit f913394d70dede8d40d6e96485bd5333dfb6f43f
Author: Mark Goddard <email address hidden>
Date: Fri Aug 30 16:58:34 2019 +0100

    Create _mech_context before delete to avoid race

    When a network is deleted, precommit handlers are notified prior to the
    deletion of the network from the database. One handler exists in the ML2
    plugin - _network_delete_precommit_handler. This handler queries the
    database for the current state of the network and uses it to create a
    NetworkContext which it saves under context._mech_context. When the
    postcommit handler _network_delete_after_delete_handler is triggered
    later, it passess the saved context._mech_context to mechanism drivers.

    A problem can occur with provider networks since the segments service
    also registers a precommit handler - _delete_segments_for_network. Both
    precommit handlers use the default priority, so the order in which they
    are called is random, and determined by dict ordering. If the segment
    precommit handler executes first, it will delete the segments associated
    with the network. When the ML2 plugin precommit handler runs it then
    sees no segments for the network and sets the provider attributes of the
    network in the NetworkContext to None.

    A mechanism driver that is passed a NetworkContext without provider
    attributes in its delete_network_postcommit method will not have the
    information to perform the necessary actions. In the case of the
    networking-generic-switch mechanism driver where this was observed, this
    resulted in the driver ignoring the event, because the network did not
    look like a VLAN.

    This change uses a priority of zero for ML2 network delete precommit
    handler, to ensure they query the network and store the NetworkContext
    before the segments service has a chance to delete segments.

    A similar change has been made for subnets, both to keep the pattern
    consistent and avoid any similar issues.

    Change-Id: I6482223ed2a479de4f5ef4cef056c311c0281408
    Closes-Bug: #1841967
    Depends-On: https://review.opendev.org/680001
    (cherry picked from commit fea2d9091f71a2ec88318121ed9a22180e1ae96f)