hydrafy-gitlab

Allows a gitlab-runner to share the /nix store between builds and thus improve compilation times and leverages the full potential of nix. Only works for docker-runners.

Use Case

• You don't want to rebuild large dependencies on every run (I am looking at you, CUDA)
• You want to build large, multi staged derivations which depend each other
• You want to share build results across multiple repositories
• You already have a bunch of gitlab runner and can't be bothered to host a real hydra
• You cannot host your gitlab runner on a nixos machine

HowTo

The gitlab runners still require the nixos/nix base image. To share the nix-store you'll have to copy its initial contents to the runner- host and mount the resulting dir in all docker runners.

To do so, you'll have to follow these steps

1. "Pin" the nixos/nix-Image hash: Check the docker repo and copy the sha-hash. This enables you to reference the image f.e. by nixos/nix@sha256:2da921898aa6c89e2e60b1fb72d74525b8464b47412482c7f1cf77b8e707a099
2. Execute copy-nix-store.sh from this repository on your host machine. This script takes the image name as its first and only parameter (the nixos/nix@hash combination) from which it should copy the nix store. The result is stored in $CWD/nix-backup.
3. Locate your gitlab-runner config.toml (see docs) and find the correct runner.docker section (take care: there may be multiple, one for each of your registered repositories).
4. Update your volumes entry to contain two new mount points:

volumes=["/path/to/nix-backup/nix/store:/nix/store:rw","/path/to/nix-backup/nix/var/nix/db:/nix/var/nix/db:rw"]

5. Restart the runner ( by using gitlab-runner restart) or wait for it to pickup the changes.
6. Update your .gitlab-ci.yml to only use the image you have pinned above.

Upgrading

To upgrade the nixos/nix base image, simply delete your nix-backup dir and execute copy-nix-store.sh again with your new base image. You may need to stop your runner before you delete the shared folders and restart it afterwards.

Pitfalls

• The /nix dir will be available in every docker-image you use in your pipeline, even non-nixos images if you use any.

See also

• Original idea by Arian van Putten over at https://gitlab.com/arianvp/nixos-gitlab-runner