Comment 5 for bug 1815697

Reviewed: https://review.opendev.org/684405
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=7d54f91e9f5782413807b284a77b3f34f5551fa0
Submitter: Zuul
Branch: stable/stein

commit 7d54f91e9f5782413807b284a77b3f34f5551fa0
Author: Matt Riedemann <email address hidden>
Date: Mon Apr 1 17:41:15 2019 -0400

    Make nova.compute.rpcapi.ComputeAPI.router a singleton

    When starting nova-api before any nova-computes are started
    and registered in the cell DBs, and with
    [upgrade_levels]/compute=auto, the compute RPC API client
    construction will iterate all cells looking for a minimum
    nova-compute service version, not find one, and thus not
    cache the result in the LAST_VERSION global.

    There are 30+ API controller classes that construct an
    instance of nova.compute.api.API which itself constructs
    a nova.compute.rpcapi.ComputeAPI object which determines
    the version cap as described above, and that is per API
    worker. Each cell DB call goes through RequestContext.set_target_cell
    which has a lock in it, so in this scenario on start of
    nova-api there can be a lot of locking log messages for
    get_or_set_cached_cell_and_set_connections.

    The RPC API ClientRouter can be a singleton and just constructed
    on first access to avoid the redundant database queries which
    is what this change does.

    To preserve the LAST_VERSION re-calculation that was in
    ComputeManager.reset(), we have to also reset the _ROUTER global
    so ComputeManager.reset() now resets all of the compute RPC API
    globals.

    Change-Id: I48109d5e32a2e9635c240da1c77f7f6cc7e3c76d
    Related-Bug: #1807219
    Related-Bug: #1815697
    (cherry picked from commit ae659668b5679cf7223474193d3b9a584dd3f016)