Hi all,

as per: https://github.com/openstack/neutron-specs/blob/master/specs/kilo/core-vendor-decomposition.rst, neutron is going to spin off vendor plugins into separate trees outside of neutron core team control. This raises several questions on how we are going to handle stable branches that will still include plugin code for several cycles.

1) If a plugin is already spinned off and a patch is applicable for stable branches, there are two cases:
- patch is not merged into vendor repo;
- patch is merged into the vendor repo.

My take is:
- if it's merged in the vendor repo, then we just cherry-pick from there (it should just work if vendor repo was created with the whole master history saved). - if it's not merged into the repo, I would recommend the author to propose and merge it there first. If there are any justifiable issues with proposing it for vendor repo inclusion, then we can consider stable-only merge.

2) If a plugin is in the middle of spinning off and a patch is applicable for stable branch, then there are two options: - require plugin to spin off first and then apply the patch to vendor repo, or - allow some types of patches to be merged into master while vendors are working on spinning off the code.

Examples of those patches are:
- https://review.openstack.org/#/c/147976/
- https://review.openstack.org/#/c/148369/

Currently the patches above are blocked for master inclusion assuming the spin off must take place first, and then bugs should be fixed in vendor repo. At the same time, we usually avoid backports unless the code is not in master anymore, but that's not the case here. So the current approach effectively blocks any bug fixes for plugins in stable branches.

If we would be sure that a plugin is out of the tree till Kilo, then it would indeed be a waste of time to review the code for neutron/master since it would be guaranteed to be released as a separate packagr e anyway. In that case, it would be ok to forbid any patches for the plugin code till its spin off from master, and the patch would go directly to stable branches.

That said, it would potentially introduce regressions if we consider upgrades from Juno to Kilo + vendor repo. We may say that since the regression would be on vendor plugin side, and neutron team does not have anything to do with spinned off plugins, that would be fine for us.

But: we cannot guarantee that a plugin wil leave the neutron tree this cycle. The spec explicitly gives permission to stay in the tree till L-cycle, and in that case it will be our responsibility to handle regressions in Kilo that we may introduce by blocking master fixes.

I think we should try to set procedure that would avoid potential regressions even if they will come from vendor repos.

We allow fixes that are not applicable for final releases for master if it's to be backported in stable branches. F.e. see https://review.openstack.org/#/c/127633/ that was merged into master while pecan migration should make it useless for Kilo.

It's my belief plugin code bug fixes in stable branches should be treated the same way.

That said, we should expect vendors to run third party CI for stable branches if they want to see backports merged in.

I think the correct approach here is:
- once a plugin is spinned off, consider it is a 'master' for the code, and backport to stable branches directly from there; - before a plugin is spinned off, assume that it's not going to be spinned off in Kilo, and hence allow bug fixes in neutron/master (but not new features); - once we get to L release that requires all vendor plugin to go out, forbid any fixes for the code, assuming they will either spin off or will be dropped anyway.

The approach is pretty similar to how oslo project handles new library spin-offs from oslo-incubator. Yes, there is a difference here: in neutron, we loose any control on spinned off repos. Though I don't feel it justifies stable-only fixes while we can easily add value to vendor code by asking people to consider fixing the bug there first. More importantly, nothing should justify blocking bug fixing for stable branches.



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