Comment 14 for bug 1731668

Reviewed: https://review.opendev.org/756636
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=7d2a491695078742c3e11cd22bbeae0a1e4e1591
Submitter: Zuul
Branch: stable/queens

commit 7d2a491695078742c3e11cd22bbeae0a1e4e1591
Author: melanie witt <email address hidden>
Date: Thu Oct 8 02:33:14 2020 +0000

    [stable-only] Add functional test for bug 1731668

    This reproduces the bug to demonstrate the problem in a real MySQL
    database (the bug cannot be reproduced using sqlite) and verify that
    the fix works properly.

    The bug has to do with parallel transactions and isolation between
    transactions and the sqlite database does not have any isolation
    between operations on the same database connection:

      https://www.sqlite.org/isolation.html

    and we can't use separate in-memory database connections to sqlite for
    each transaction because each separate connection creates its own
    copy of the database. What we want is one copy but isolated
    transactions writing and reading from that one copy.

    Note: this is a stable-only change because the placement code
    containing the bug was temporary data-transitioning code that no longer
    exists in placement after Queens.

    Related-Bug: #1731668

    Change-Id: I93d34c68bf89511dfeb53a4776c478b88463f24f