Only build attributes not found in any cache.

[Mic92]

This would speed up CI builds significantly and save bandwidth.
Currently the cachix-action will pull built packages:
https://github.com/Mic92/nur-packages/runs/589727774
In the example above there were 500 MB pulled without a single package built.

This is how it can be implemented:

1. Get all packages: nix-env -f ./non-broken.nix -qaP \* --out-path --xml --meta > packages.xml
2. Get all build hashes: grep -oP '(?<=/nix/store/)[^-]*' < packages.xml (obviously this should be done by proper xml parsing)
3. Only build those attributes where curl -I https://<binary-cache>/<hash>.narinfo does not return a 404

[domenkozar]

Similar idea for upstream NixOS/nix#3428

[Mic92]

Once there would be a wrapper around nix-build, error logging could be improved as well. Right now it nix-build is quite verbose and it is hard to search for the error that broke a particular package because the build log is quite mix of all builds. It would be better if there was a nix build output that would just iterate over full logs of each failed package individually at the end of execution.

[domenkozar]

This should probably be a wrapper or (I prefer) part of nix-build. Either way I'll leave it open here since it's relevant.

[Mic92]

This is wrapper I came up with: https://github.com/Mic92/ci-nix-build
It's a single static binary (2mb). Still look for a better name.

[domenkozar]

Nice! :)

I think it's going to be tricky to get argument passing to nix-build to work, but it's quite nice for 90% of the cases where nix-build does the job.

[Mic92]

I will add support for some build arguments later.

[zimbatm]

nice! I have a name suggestion: nix-build-uncached. I started adding a nix-build --uncached attribute to Nix itself but this is still at a very early stage.

[Mic92]

https://github.com/Mic92/nix-build-uncached it is. Even if your flag lands in Nix it is still going to take a while until it ends in a released version. Until than nix-build-cached will pay the builds.