[Bug 1950679] Re: [ovn] neutron_ovn_db_sync_util hangs on sync_routers_and_rports

[Corey Bryant]

** Also affects: cloud-archive
   Importance: Undecided
       Status: New

** Also affects: cloud-archive/wallaby
   Importance: Undecided
       Status: New

** Changed in: cloud-archive
       Status: New => Invalid

** Changed in: cloud-archive/wallaby
       Status: New => Triaged

** Changed in: cloud-archive/wallaby
   Importance: Undecided => High

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https://bugs.launchpad.net/bugs/1950679

Title:
  [ovn] neutron_ovn_db_sync_util hangs on sync_routers_and_rports

Status in Ubuntu Cloud Archive:
  Invalid
Status in Ubuntu Cloud Archive wallaby series:
  Triaged
Status in neutron:
  Fix Released

Bug description:
  neutron-ovn-db-sync-util hangs in certain scenarios while running
  sync_routers_and_rports.

  Specifically, it seems to be hanging on self.l3_plugin.get_routers(ctx)
  -> model_query.get_collection(...) of get_routers(...) in neutron.db.l3_db.py
  -> get_collection(...) in neutron_lib.db.model_query.py runs dict_funcs which somehow reaches to nb_ovn property accessor in neutron.plugins.ml2.drivers.ovn.mech_driver.mech_driver.py
  -> which runs self._post_fork_event.wait() 

  That mutex seems to never be "set" and blocks further execution
  because it might not be applicable to this flow.

  It looks like the neutron-ovn-db-sync-util might need to always "set"
  it since it mocks other parts of the NB/DB client in a similar fashion
  to some unit tests.

  I'm not yet sure what kind of exact circumstances lead to that access
  and that wait(), syncing via the util to an empty OVN NB/DB seems to
  work. I see the issue more frequently on subsequent runs.

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