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

From: Daniel P . Berrangé
Subject: Re: [PATCH 1/3] gitlab: always build container images
Date: Wed, 10 Feb 2021 11:17:00 +0000
User-agent: Mutt/1.14.6 (2020-07-11)

On Tue, Feb 09, 2021 at 09:58:29AM +0000, Daniel P. Berrangé wrote:
> 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.

The challenge here is that I need the docker tag name to be in an env
variable in the gitlab-ci.yml file.

I can directly use $CI_COMMIT_REF_NAME  to get the branch name but
the list of valid characters for a git branch is way more permissive
than valid characters for a docker tag.

So we need to filter the git branch name to form a valid docker tag,
and AFAICT, there's no way todo that when setting a global env variable
in the gitlab-ci.yml.  I can only do filtering once in the before_script:
stage, and that's too late to use it in the image name for the job.

We could ignore the problem and hope people always have sane branch
names ? 


  "A tag name must be valid ASCII and may contain lowercase and 
   uppercase letters, digits, underscores, periods and dashes. 
   A tag name may not start with a period or a dash and may 
   contain a maximum of 128 characters."

that rule would cover all my git branch names, but then ASCII covers
most common english needs.  I worry that we might have contributors
who genuinely use non-ASCII chars in their git branch names, especially
those speakers of non-english/european languages eg persian, chinese,
japanese languages for example. Git is very permissive, allowing
everything except a short list


  "A branch name can not:
        - Have a path component that begins with "."
        - Have a double dot ".."
        - Have an ASCII control character, "~", "^", ":" or SP, anywhere
        - End with a "/"
        - End with ".lock"
        - Contain a "\" (backslash"

The result will be if someone names their git branch "🏂", then all
the CI jobs will fail in gitlab.

 $ git branch 🏂


 $ docker  tag 470671670cac foo:🏂
  Error: invalid reference format


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