What does this package do?

[arximboldi]

Hi!

This package has very little documentation, so I am unsure whether it's fit for my purpose...

I am looking for a way to use ccache inside Nix builds, to save CI time when rebuilding big packages of which only small bits change. I guess to do this really, one would need disable the sandbox somehow, since the ccache cache needs to be persisted across builds. Or maybe there is another way that a Nix mastermind like @edolstra has in mind :)

Is this doing this? What is this package achieving instead?

[milahu]

same question ^^

relevant code is nix-ccache in flake.nix and cc-wrapper.sh

but i dont see how cc-wrapper.sh is using ccache. probably this is just an early draft

compare this to ccacheStdenv.mkDerivation with cc = ccache.links

# nixpkgs/pkgs/top-level/all-packages.nix
{
  # Wrapper that works as gcc or g++
  # It can be used by setting in nixpkgs config like this, for example:
  #    replaceStdenv = { pkgs }: pkgs.ccacheStdenv;
  # But if you build in chroot, you should have that path in chroot
  # If instantiated directly, it will use $HOME/.ccache as the cache directory,
  # i.e. /homeless-shelter/.ccache using the Nix daemon.
  # You should specify a different directory using an override in
  # packageOverrides to set extraConfig.
  #
  # Example using Nix daemon (i.e. multiuser Nix install or on NixOS):
  #    packageOverrides = pkgs: {
  #     ccacheWrapper = pkgs.ccacheWrapper.override {
  #       extraConfig = ''
  #         export CCACHE_COMPRESS=1
  #         export CCACHE_DIR=/var/cache/ccache
  #         export CCACHE_UMASK=007
  #       '';
  #     };
  # You can use a different directory, but whichever directory you choose
  # should be owned by user root, group nixbld with permissions 0770.
  ccacheWrapper = makeOverridable ({ extraConfig, cc }:
    cc.override {
      cc = ccache.links {
        inherit extraConfig;
        unwrappedCC = cc.cc;
      };
    }) {
      extraConfig = "";
      inherit (stdenv) cc;
    };

  ccacheStdenv = lowPrio (makeOverridable ({ stdenv, ... } @ extraArgs:
    overrideCC stdenv (buildPackages.ccacheWrapper.override ({
      inherit (stdenv) cc;
    } // lib.optionalAttrs (builtins.hasAttr "extraConfig" extraArgs) {
      extraConfig = extraArgs.extraConfig;
    }))) {
      inherit stdenv;
    });

related: my failed experiment with static build graph

see also my experiment, to implement ccache/distcc in nix

https://discourse.nixos.org/t/distributed-nix-build-split-large-package-into-many-derivations/15979

• dry-run the buildPhase to get all compile objects
• create one derivation per compile object
• build the derivations. can be distributed across multiple machines -> distcc
• link the compile objects

no need for recursive nix, all derivations are generated in nix (not in bash)

[edolstra]

It doesn't use ccache, it's just named after it because it provides similar functionality (caching and distributed builds). It should probably be renamed.

[milahu]

aah, of course

to make this work, i had to add recursive-nix to both experimental-features and system-features

full config example

# /etc/nixos/configuration.nix
{
  nix.extraOptions = ''
    experimental-features = nix-command flakes recursive-nix
    system-features = nixos-test benchmark big-parallel kvm recursive-nix
    keep-outputs = true
    keep-derivations = true
  '';
  nix.package = pkgs.nixUnstable;

rename ... nixrecc, the recursive nix compiler? nix-objcache?

[arximboldi]

Ohhh, this is very interesting:
NixOS/nix#13
And also this repository. I will keep following, thank you all for the details!