Samba CI on gitlab/Under the hood
How GitLab CI works in Samba
Running remote scripts, displaying the output
Like the Samba build farm of old, GitLab CI is a system for running scripts on remote hosts against a git checkout.
Samba uses a feature called GitLab Pipelines to orchestrate our CI.
In Samba's case, the remote script is script/autobuild.py plus some housekeeping before and after. The details is recorded in the .gitlab-ci*.yml files in the Samba tree (so it is maintained with the code).
.gitlab-ci-private.yml vs .gitlab-ci.yml
We have two different CI configurations, one using the default name .gitlab-ci.yml (so picked up by default by forks of our repo) and one that we specify in the Common development repo (.gitlab-ci-private.yml)
The .gitlab-ci-private.yml file includes .gitlab-ci.yml to as to avoid duplication.
The motivation here is to use the shared runners where possible as these are provided by gitlab.com at no cost to Samba Team.
To get a consistent build environment container images are used, so the scripts described above all run inside a container.
The image used is defined in the .gitlab-ci.yml file.
GitLab CI is best thought of as a fancy way to run commands in containers and report their results.
GitLab CI uses Docker as the container runtime.
While the container image format can be consumed by and the containers started using other tools, to closely replicate the environment on the runners, use Docker.
A bit like running in a chroot
The way containers are used by GitLab CI is very much akin to downloading a tarball (the image), unpacking it and calling chroot into it (entering the container). Modern container concepts like namespaces etc are used to make it more seamless, but this conceptualization may assist those struggling with the concepts.
On a private VM
To allow us to accept and test code from a broader range of contributors, and to enable scaling at times of peak load, the docker container is started in a private VM using Docker Machine. This applies for both the private and shared (provided by gitlab.com) runners.
Multiple VMs in parallel
Each section in the .gitlab-ci*.yml file is a job, and each job is distributed to an independent VM, allowing execution in parallel.
Providing the private VMs
- The Samba team provides the private VMs in the Rackspace cloud paid for by the team using donations.
Ansible management scripts
- The scripts used to configure and operate this service are available.
- These scripts allow a new bastion host to be fully built by just running single script invocation:
Future CI services
As all the complex parts of Samba's build and test system are still below autobuild, migration to a different CI service in the future or in parallel is quite practical.
Not tied to gitlab.com
If needed, private GitLab hosts running the Open Source GitLab CE can interpret the same configuration and operate against the same runners (just without the free shared runners, naturally).
This gives the Samba Team options if gitlab.com hosting becomes a problem for any reason.