Backporting a Fix

tl;dr: Only propose a cherry pick from a merged commit, even if you want to backport the patch to multiple stable branches. Doing them all at once doesn’t speed anything up, because the cinder-stable-maint team will not approve a backport to branch n-1 until the patch has been merged into branch n.

From time to time, you may find a bug that’s been fixed in master, and you’d like to have that fix in the release you’re currently using (for example, Wallaby). What you want to do is propose a backport of the fix.

Note

The Cinder project observes the OpenStack Stable Branch Policy. Thus, not every change in master is backportable to the stable branches. In particular, features are never backportable. A really complicated bugfix may not be backportable if what it fixes is low-occurrence and there’s a high risk that it may cause a regression elsewhere in the software.

How can you tell? Ask in the #openstack-cinder channel on IRC or during the open discussion part of the weekly Cinder team meeting.

Since we use git for source code version control, backporting is done by cherry-picking a change that has already been merged into one branch into another branch. The gerrit web interface makes it really easy to do this. In fact, maybe too easy. Here are some guidelines:

• Before you cherry-pick a change, make sure it has already merged to master. If the change hasn’t merged yet, it may require further revision, and the commit you’ve cherry-picked won’t be the correct commit to backport.

• Backports must be done in reverse chronological order. Since OpenStack releases are named alphabetically, this means reverse alphabetical order: stable/yoga, stable/xena, stable/wallaby, etc.

• The cherry-pick must have merged into the closest most recent branch before it will be considered for a branch, that is, a cherry-pick to stable/xena will not be considered until it has merged into stable/yoga first.

  • This is because sometimes a backport requires revision along the way. For example, different OpenStack releases support different versions of Python. So if a fix uses a language feature introduced in Python 3.8, it will merge just fine into current master (during zed development), but it will not pass unit tests in stable/yoga (which supports Python 3.6). Likewise, if you already cherry-picked the patch from master directly to stable/xena, it won’t pass tests there either (because xena also supports Python 3.6).

    So it’s better to follow the policy and wait until the patch is merged into stable/yoga before you propose a backport to stable/xena.

• You can propose backports directly from git instead of using the gerrit web interface, but if you do, you must include the fact that it’s a cherry-pick in the commit message. Gerrit does this automatically for you if you cherry-pick from a merged commit (which is the only kind of commit you should cherry-pick from in Gerrit); git will do it for you if you use the -x flag when you do a manual cherry-pick.

  This will keep the history of this backport intact as it goes from branch to branch. We want this information to be in the commit message and to be accurate, because if the fix causes a regression (which is always possible), it will be helpful to the poor sucker who has to fix it to know where this code came from without digging through a bunch of git history.

If you have questions about any of this, or if you have a bug to fix that is only present in one of the stable branches, ask for advice in #openstack-cinder on IRC.

Backport CI Testing

Like all code changes, backports should undergo continuous integration testing. This is done automatically by Zuul for changes that affect the main cinder code.

When a vendor driver patch backport is proposed, we would like to see a clear statement on the gerrit review that the patch has been tested in an appropriate environment.

This shouldn’t be a big deal because presumably you’ve done local testing with your backend to ensure that the code works as expected in a stable branch; we’re simply asking that this be documented on the backport.

A good example of how to document this can be found on https://review.opendev.org/c/openstack/cinder/+/821893/.