nix-build-uncached

Deprecation nix-eval-jobs exposes the binary cache information now also in --check-cache-status, which allows to build a similar feature. nix-fast-build uses nix-eval-jobs and exposes the --skip-cached flag to replace nix-build-uncached. Otherwise NixOS/nix#3946 might track information about possible features in nix itself.

nix-build by default will download already built packages, resulting in unnecessary downloads even if no package has been changed. nix-build-uncached will only build packages not yet in binary caches.

USAGE

nix-build-uncached is available in nixpkgs.

For nix versions bigger than 2.3.x you need to enable at least experimental-features = nix-command in your /etc/nix/nix.conf or $HOME/.config/nix/nix.conf or pass -build-flags "--experimental-features nix-command" to nix-build-uncached.

In the following example ci.nix contains all expressions that should be built. Since only hello-nur is not yet in the binary cache, all other packages are skipped.

[joerg@turingmachine] nix-build-uncached ci.nix
$ nix-build --dry-run ci.nix --keep-going
these derivations will be built:
  /nix/store/s5alllpjx9fmdj26mf9cmxzs3xyxjn7f-hello-2.00.tar.gz.drv
  /nix/store/03m2lwg4zia58zqm7hqlb3r0cgfq53cn-hello-2.00.drv
these paths will be fetched (198.28 MiB download, 681.37 MiB unpacked):
  /nix/store/0mijq2b50xmgk6akxdrg3x8x0k7784jb-python3.8-kiwisolver-1.2.0
  /nix/store/0q1g21q160w2cj2745r24pfn2yb8pmda-python3.8-jupyter_client-6.1.5
  /nix/store/0qxi1gzv1xgpxy58nfk9pxlfqybv1198-cntr-1.2.0
 # ...
$ nix build --keep-going /nix/store/s5alllpjx9fmdj26mf9cmxzs3xyxjn7f-hello-2.00.tar.gz.drv /nix/store/03m2lwg4zia58zqm7hqlb3r0cgfq53cn-hello-2.00.drv
[1/2 built, 1 copied (0.7 MiB)] connecting to 'ssh://Mic92@prism.r'

We can pass all arguments that also nix-build accept:

[joerg@turingmachine] nix-build-uncached -E 'with import <nixpkgs> {}; hello'
$ nix-build --dry-run -E with import <nixpkgs> {}; hello --keep-going
these paths will be fetched (0.04 MiB download, 0.20 MiB unpacked):
  /nix/store/aldyr0pjzqydf1vn9lzz7p5gvc141fhn-hello-2.10

However this only affects the evaluation during the dry build, if you want to pass arguments to the final nix build instead, use -build-flags:

[joerg@turingmachine] nix-build-uncached -build-flags '--builders ""' ci.nix
$ nix-build --dry-run ci.nix --builders
these derivations will be built:
  /nix/store/s5alllpjx9fmdj26mf9cmxzs3xyxjn7f-hello-2.00.tar.gz.drv
  /nix/store/03m2lwg4zia58zqm7hqlb3r0cgfq53cn-hello-2.00.drv
these paths will be fetched (198.28 MiB download, 681.37 MiB unpacked):
  /nix/store/0mijq2b50xmgk6akxdrg3x8x0k7784jb-python3.8-kiwisolver-1.2.0
  /nix/store/0q1g21q160w2cj2745r24pfn2yb8pmda-python3.8-jupyter_client-6.1.5
  /nix/store/0qxi1gzv1xgpxy58nfk9pxlfqybv1198-cntr-1.2.0
 # ...
$ nix build --builders   /nix/store/s5alllpjx9fmdj26mf9cmxzs3xyxjn7f-hello-2.00.tar.gz.drv /nix/store/03m2lwg4zia58zqm7hqlb3r0cgfq53cn-hello-2.00.drv
[1/2 built, 1 copied (0.7 MiB)] connecting to 'ssh://Mic92@prism.r'

Flakes

We cannot support flakes directly at the time because nix-build does not accept those and nix build's --no-dry-run is broken. However it is possible to add a wrapper nix expression that imports a flake. The following example imports hydra jobs from a nix flake the same directory. It assumes that the flake also has nixpkgs in its inputs.

let
  outputs = builtins.getFlake (toString ./.);
  pkgs = outputs.inputs.nixpkgs;
  drvs = pkgs.lib.collect pkgs.lib.isDerivation outputs.hydraJobs;
in drvs

$ nix-build-uncached ./ci.nix

Packages with allowSubstitutes = false

If your package set you are building has packages at the top level scope that have the attribute allowSubstitutes = false; set, than nix-build-uncached will build/download them everytime. This attribute is set for some builders such as writeText or writeScriptBin. A workaround is to use the following nix library and save it as force_cached.nix. Than wrap your attribute set like this:

let
  pkgs = import <nixpkgs> {};
in (pkgs.callPackage ./force_cached.nix {}) {
  hello = pkgs.writeScriptBin "hello" ''
    #!/bin/sh
    exec ${pkgs.hello}/bin/hello
  '';
}

Real-world examples

• Using nix-build-uncached in github actions