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[OpenStack Infra <1864665@xxxxxxxxxxxxxxxxxx>]

This issue was fixed in the openstack/nova queens-eol  release.

** Changed in: nova/queens
       Status: Fix Committed => Fix Released

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https://bugs.launchpad.net/bugs/1864665

Title:
  Circular reference error during re-schedule

Status in OpenStack Compute (nova):
  Invalid
Status in OpenStack Compute (nova) ocata series:
  New
Status in OpenStack Compute (nova) pike series:
  Fix Released
Status in OpenStack Compute (nova) queens series:
  Fix Released
Status in OpenStack Compute (nova) rocky series:
  Fix Released

Bug description:
  Description
  ===========
  Server cold migration fails after re-schedule.

  Steps to reproduce
  ==================
  * create a devstack with two compute hosts with libvirt driver
  * set allow_resize_to_same_host=True on both computes
  * set up cellsv2 without cell conductor and rabbit separation to allow re-schedule logic to call back to the super conductor / scheduler
  * enable NUMATopologyFilter and make sure both computes has NUMA resources
  * create a flavor with hw:cpu_policy='dedicated' extra spec
  * boot a server with the flavor. Check which compute the server is placed (let's call it host1)
  * boot enough servers on host2 so that the next scheduling request could still be fulfilled by both computes but host1 will be preferred by the weighers
  * cold migrate the pinned server

  Expected result
  ===============
  * scheduler selects host1 first but that host fails with UnableToMigrateToSelf exception as libvirt does not have the capability
  * re-schedule happens
  * scheduler selects host2 where the server spawns successfully

  Actual result
  =============
  * during the re-schedule when the conductor sends prep_resize RPC to host2 the json serialization of the request spec fails with Circural reference error.

  Environment
  ===========
  * two node devstack with libvirt driver
  * stable/pike nova. But expected to be reproduced in newer branches but not since stein. See triage part

  Triage
  ======
  The json serialization blows up in the migrate conductor task. [1] After debugging I see that the infinit loop happens when jsonutils.to_primitive tries to serialize a VirtCPUTopology instance.

  The problematic piece of code has been removed by
  I4244f7dd8fe74565180f73684678027067b4506e in Stein.

  [1]
  https://github.com/openstack/nova/blob/4224a61b4f3a8b910dcaa498f9663479d61a6060/nova/conductor/tasks/migrate.py#L87

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