Comment 21 for bug 1824435

Reviewed: https://review.opendev.org/691403
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=416290f1938631a95aaaf92a41f455ebf75b0732
Submitter: Zuul
Branch: stable/train

commit 416290f1938631a95aaaf92a41f455ebf75b0732
Author: melanie witt <email address hidden>
Date: Fri Oct 11 21:08:33 2019 +0000

    Remove redundant call to get/create default security group

    In the instance_create DB API method, it ensures the (legacy) default
    security group gets created for the specified project_id if it does
    not already exist. If the security group does not exist, it is created
    in a separate transaction.

    Later in the instance_create method, it reads the default security group
    back that it wrote earlier (via the same ensure default security group
    code). But since it was written in a separate transaction, the current
    transaction will not be able to see it and will get back 0 rows. So, it
    creates a duplicate default security group record if project_id=NULL
    (which it will be, if running nova-manage db online_data_migrations,
    which uses an anonymous RequestContext with project_id=NULL). This
    succeeds despite the unique constraint on project_id because in MySQL,
    unique constraints are only enforced on non-NULL values [1].

    To avoid creation of a duplicate default security group for
    project_id=NULL, we can use the default security group object that was
    returned from the first security_group_ensure_default call earlier in
    instance_create method and remove the second, redundant call.

    This also breaks out the security groups setup code from a nested
    method as it was causing confusion during code review and is not being
    used for any particular purpose. Inspection of the original commit
    where it was added in 2012 [2] did not contain any comments about the
    nested method and it appeared to either be a way to organize the code
    or a way to reuse the 'models' module name as a local variable name.

    Closes-Bug: #1824435

    [1] https://dev.mysql.com/doc/refman/8.0/en/create-index.html#create-index-unique
    [2] https://review.opendev.org/#/c/8973/2/nova/db/sqlalchemy/api.py@1339

    Change-Id: Idb205ab5b16bbf96965418cd544016fa9cc92de9
    (cherry picked from commit 6ea945e3b126879a86fe78d9c0537f5d258cf91a)