Comment 3 for bug 1974070

Reviewed: https://review.opendev.org/c/openstack/nova/+/864773
Committed: https://opendev.org/openstack/nova/commit/3c022e968375c1b2eadf3c2dd7190b9434c6d4c1
Submitter: "Zuul (22348)"
Branch: master

commit 3c022e968375c1b2eadf3c2dd7190b9434c6d4c1
Author: John Garbutt <email address hidden>
Date: Wed Nov 16 17:12:40 2022 +0000

    Ironic nodes with instance reserved in placement

    Currently, when you delete an ironic instance, we trigger
    and undeploy in ironic and we release our allocation in placement.
    We do this well before the ironic node is actually available.

    We have attempted to fix this my marking unavailable nodes
    as reserved in placement. This works great until you try
    and re-image lots of nodes.

    It turns out, ironic nodes that are waiting for their automatic
    clean to finish, are returned as a valid allocation candidates
    for quite some time. Eventually we mark then as reserved.

    This patch takes a strange approach, if we mark all nodes as
    reserved as soon as the instance lands, we close the race.
    That is, when the allocation is removed the node is still
    unavailable until the next update of placement is done and
    notices that the node has become available. That may or may
    not have been after automatic cleaning. The trade off is
    that when you don't have automatic cleaning, we wait a bit
    longer to notice the node is available again.

    Note, this is also useful when a broken Ironic node is
    marked as in-maintainance while it is in-use by a nova
    instance. In a similar way, we mark the Nova as reserved
    immmeidately, rather than first waiting for the instance to be
    deleted before reserving the resources in Placement.

    Closes-Bug: #1974070
    Change-Id: Iab92124b5776a799c7f90d07281d28fcf191c8fe