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[Rui Chen <chenrui.momo@xxxxxxxxx>]

Public bug reported:

This applies only when the nova/neutron event reporting mechanism is
enabled.

Boot instance, like this:
nova boot --image xxx --flavor xxx --nic port-id=xxx test_vm

The booting instance is successful, but instance nw_info cache is empty.
This is a probabilistic problem, not always can be reproduced.

After analysis the booting instance and nova/neutron event mechanism workflow,
I get the reproduce timeline:

1. neutronv2.api.allocate_for_instance when booting instance
2. neutronclient.update_port trigger neutron network_change event
3. nova get the port change event, start to dispose event
4. instance.get_by_uuid in external_instance_event , at this time instance.nw_info_cache is empty, 
because nw_info cache hadn't been added into db in booting instance thread.
5. booting instance thread start to save the instance nw_info cache into db.
6. event disposing thread start to update instance nw_info cache to empty.

Face this issue in Juno.
I add some breakpoints in order to reproduce this bug in my devstack.

** Affects: nova
     Importance: Undecided
     Assignee: Rui Chen (kiwik-chenrui)
         Status: New

** Changed in: nova
     Assignee: (unassigned) => Rui Chen (kiwik-chenrui)

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https://bugs.launchpad.net/bugs/1407664

Title:
  Race: instance nw_info cache is updated to empty list because of
  nova/neutron event mechanism

Status in OpenStack Compute (Nova):
  New

Bug description:
  This applies only when the nova/neutron event reporting mechanism is
  enabled.

  Boot instance, like this:
  nova boot --image xxx --flavor xxx --nic port-id=xxx test_vm

  The booting instance is successful, but instance nw_info cache is empty.
  This is a probabilistic problem, not always can be reproduced.

  After analysis the booting instance and nova/neutron event mechanism workflow,
  I get the reproduce timeline:

  1. neutronv2.api.allocate_for_instance when booting instance
  2. neutronclient.update_port trigger neutron network_change event
  3. nova get the port change event, start to dispose event
  4. instance.get_by_uuid in external_instance_event , at this time instance.nw_info_cache is empty, 
  because nw_info cache hadn't been added into db in booting instance thread.
  5. booting instance thread start to save the instance nw_info cache into db.
  6. event disposing thread start to update instance nw_info cache to empty.

  Face this issue in Juno.
  I add some breakpoints in order to reproduce this bug in my devstack.

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