New interface hashing breaks upgrades for LinuxBridge

[Sean M. Collins]

Code introduced in https://review.openstack.org/#/c/224064/ touches code that did the interface name hashing. Let's ensure that it doesn't break upgrades.

IRC conversation:

http://eavesdrop.openstack.org/irclogs/%23openstack-neutron/%23openstack-neutron.2015-10-09.log.html#t2015-10-09T16:26:18

This bug is to fix upgrade, not fix the naming scheme.

[Cedric Brandily (cbrandily)]

The old algorithm doesn't hash interface names so it fails to generate valid interface names when:

  # len('%s.%s' % (physical_interface + vlan)) > 15
  len(physical_interface) > 14 - len(vlan)

The new algorithm hashs interface names (and generate different names) when:

  # large_vlan is a 4 digit vlan
  # len('%s.%s' % (physical_interface + large_vlan)) > 15
  # it's sadly not highlighted in https://review.openstack.org/#/c/224064/8/neutron/tests/unit/plugins/common/test_utils.py
  len(physical_interface) > 10

It means that old/new algorithms generate valid but different interface names when:

  10 < len(physical_interface) <= 14 - len(vlan) # implies vlan < 1000

So they behave differently when old algorithm generate valid names only for a subset of all vlans

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/233715

[Andreas Scheuring (andreas-scheuring)]

uff. Good point. I haven't spent much thoughts about upgrade compatibility - sorry for that! And thanks for finding this!!
Let's roll it back and discuss how to solve the issue!

[Andreas Scheuring (andreas-scheuring)]

looks like we have to live with the fact that when you have a long device name like "abcdefghijklm", vlan devices will be named

abcdefghijklm.1
abcde<hash>.111

This is the only way to stay backwards compatible. It's not very nice for debugging - but I don't see any other feasible solution.

Any ideas?

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/233715
Committed: https://git.openstack.org/cgit/openstack/neutron/commit/?id=e7488570e70d0a92cc3d8fa3b5853df7fbb33564
Submitter: Jenkins
Branch: master

commit e7488570e70d0a92cc3d8fa3b5853df7fbb33564
Author: Sean M. Collins <email address hidden>
Date: Mon Oct 12 12:43:02 2015 -0400

    Revert "Make OVS interface name hashing algorithm common and extend it"

    Based on discussion with Ihar and Cedric, this may break upgrades. Let's
    revert it for the time being and reassess.

    This reverts commit 3d0db13370eee09aaee5923c54d0e3188509c4e5.

    Change-Id: I69d74befb08e074c1bccd823cca0899de6ed57d6
    Closes-Bug: 1504647

[Thierry Carrez (ttx)]

This issue was fixed in the openstack/neutron 8.0.0.0b1 development milestone.