Re: [PATCH 1/3] gitlab: always build container images

["Daniel P. Berrangé" <berrange@redhat.com>]

On Tue, Feb 09, 2021 at 07:37:51AM +0100, Thomas Huth wrote:
> On 08/02/2021 17.33, Daniel P. Berrangé wrote:
> [...]
> > For example, consider pushing 5 commits, one of which contains a
> > dockerfile change. This will trigger a CI pipeline for the
> > containers. Now consider you do some more work on the branch and push 3
> > further commits, so you now have a branch of 8 commits. For the second
> > push GitLab will only look at the 3 most recent commits, the other 5
> > were already present. Thus GitLab will not realize that the branch has
> > dockerfile changes that need to trigger the container build.
> > 
> > This can cause real world problems:
> > 
> >   - Push 5 commits to branch "foo", including a dockerfile change
> > 
> >      => rebuilds the container images with content from "foo"
> >      => build jobs runs against containers from "foo"
> > 
> >   - Refresh your master branch with latest upstream master
> > 
> >      => rebuilds the container images with content from "master"
> >      => build jobs runs against containers from "master"
> > 
> >   - Push 3 more commits to branch "foo", with no dockerfile change
> > 
> >      => no container rebuild triggers
> >      => build jobs runs against containers from "master"
> > 
> > The "changes" conditional in gitlab is OK, *provided* your build
> > jobs are not relying on any external state from previous builds.
> > 
> > This is NOT the case in QEMU, because we are building container
> > images and these are cached. This is a scenario in which the
> > "changes" conditional is not usuable.
> > 
> > The only other way to avoid this problem would be to use the git
> > branch name as the container image tag, instead of always using
> > "latest".
> I'm basically fine with your patch, but let me ask one more thing: Won't we
> still have the problem if the user pushes to different branches
> simultaneously? E.g. the user pushes to "foo" with changes to dockerfiles,
> containers start to get rebuild, then pushes to master without waiting for
> the previous CI to finish, then the containers get rebuild from the "master"
> job without the local changes to the dockerfiles. Then in the "foo" CI
> pipelines the following jobs might run with the containers that have been
> built by the "master" job...

Yes,  this is the issue I describe in the cover letter.

> So if we really want to get it bulletproof, do we have to use the git branch
> name as the container image tag?

That is possible, but I'm somewhat loathe to do that, as it means the
container registry in developers forks will accumulate a growing list
of image tags. I know gitlab will force expire once it gets beyond a
certain number of tags, but it still felt pretty wasteful of space
to create so many tags.

Having said that, maybe this is not actually wasteful if we always
use the "master" as a cache for docker, then the "new" images we
build on each branch will just re-use existing docker layers and
thus not add to disk usage. We'd only see extra usage if the branch
contained changes to dockerfiles.

Regards,
Daniel
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