Reference on xgcc-*-libgcc package

[veehaitch]

Issue description

Since #209870, every package which depends on glibc has a dependency on xgcc-*-libgcc:

nix path-info -r $(nix build --print-out-paths --no-link github:nixos/nixpkgs/5f57c2e#hello)

/nix/store/03wnlvjlmnl7apj8af79l6l7293dpbkl-libunistring-1.1
/nix/store/d3w6v846k00pn3nilj14788i3mxbfsak-libidn2-2.3.2
/nix/store/inc9pp65rs7nxi3pv0mm35kj4arp9v67-xgcc-12.2.0-libgcc
/nix/store/qgfa76mgh1m1376npkwbmlsdwycl96k1-glibc-2.35-224
/nix/store/jfyz86p3fgpadwxc16gzvwar6sh42gvw-hello-2.12.1

As far as I can tell, there's no attribute which exposes the derivation.

For some of our systemd services running in a chroot jail, we use explicit confinement. Think systemd.services.<name>.confinement.packages but for the entire unit. Consider the following example systemd service which runs hello in a chroot jail before #209870:

{ pkgs, lib, ... }:
{
  systemd.services.hello = {
    enable = true;
    wantedBy = [ "multi-user.target" ];
  
    serviceConfig = {
      ExecStart = lib.getExe pkgs.hello;
      Type = "oneshot";
      RemainAfterExit = true;

      DynamicUser = true;
  
      RuntimeDirectory = [ "hello" ];
      RootDirectory = "%t/hello";
      BindReadOnlyPaths = with pkgs; [
        hello.out
        glibc.out
        libidn2.out
        libunistring.out
      ];
    };
  };
}

Is there any way I can get a reference on xgcc-*-libgcc to add the package to BindReadOnlyPaths=? stdenv.cc.cc.libgcc is a dependency I'd expect but, quite frankly, I don't understand the bootstrapping of GCC well enough.

cc @amjoseph-nixpkgs who authored #209870.

[ghost]

Since #209870, every package which depends on glibc has a dependency on xgcc-*-libgcc:

Correct. That is not a bug.

Glibc has required libgcc for over a decade by now; we simply weren't packaging it separately.

[ghost]

Is there any way I can get a reference on xgcc-*-libgcc to add the package to BindReadOnlyPaths=?

I don't use systemd or NixOS, so maybe this makes no sense, but can't you just use stdenv.cc.cc.libgcc or null?

[veehaitch]

Thanks for responding so quickly, @amjoseph-nixpkgs!

Correct. That is not a bug.

Glibc has required libgcc for over a decade by now; we simply weren't packaging it separately.

I guess I didn't convey what's the issue: I'm well aware that you added a dedicated package intentionally. I'm just not sure how I could get a reference to that package which is now part of almost any closure.

Is there any way I can get a reference on xgcc-*-libgcc to add the package to BindReadOnlyPaths=?

I don't use systemd or NixOS, so maybe this makes no sense, but can't you just use stdenv.cc.cc.libgcc or null?

That would be totally acceptable to me, yes! However, that's not the correct package:

$ nix why-depends github:nixos/nixpkgs/5f57c2e#hello github:nixos/nixpkgs/5f57c2e#stdenv.cc.cc.libgcc
'github:nixos/nixpkgs/5f57c2e#hello' does not depend on 'github:nixos/nixpkgs/5f57c2e#stdenv.cc.cc.libgcc'
$ nix build --print-out-paths --no-link github:nixos/nixpkgs/5f57c2e#stdenv.cc.cc.libgcc
/nix/store/5vw1jps97aj3r76rkj7jmq8wijssipji-gcc-12.2.0-libgcc

The required dependency is xgcc-12.2.0-libgcc, not gcc-12.2.0-libgcc:

$ nix path-info -r $(nix build --print-out-paths --no-link github:nixos/nixpkgs/5f57c2e#hello) | grep "libgcc"
/nix/store/inc9pp65rs7nxi3pv0mm35kj4arp9v67-xgcc-12.2.0-libgcc
$ nix why-depends github:nixos/nixpkgs/5f57c2e#hello /nix/store/inc9pp65rs7nxi3pv0mm35kj4arp9v67-xgcc-12.2.0-libgcc
/nix/store/jfyz86p3fgpadwxc16gzvwar6sh42gvw-hello-2.12.1
└───/nix/store/qgfa76mgh1m1376npkwbmlsdwycl96k1-glibc-2.35-224
    └───/nix/store/inc9pp65rs7nxi3pv0mm35kj4arp9v67-xgcc-12.2.0-libgcc

[wegank]

Are you looking for: glibc.libgcc?

[veehaitch]

Are you looking for: glibc.libgcc?

That was easy:

$ nix why-depends github:nixos/nixpkgs/5f57c2e#hello github:nixos/nixpkgs/5f57c2e#glibc.libgcc
/nix/store/jfyz86p3fgpadwxc16gzvwar6sh42gvw-hello-2.12.1
└───/nix/store/qgfa76mgh1m1376npkwbmlsdwycl96k1-glibc-2.35-224
    └───/nix/store/inc9pp65rs7nxi3pv0mm35kj4arp9v67-xgcc-12.2.0-libgcc

Thanks a lot @wegank 🙏🏻

[ghost]

The required dependency is xgcc-12.2.0-libgcc, not gcc-12.2.0-libgcc:

Ah, try stdenv.cc.cc.stdenv.cc.cc.libgcc (note the extra .stdenv.cc.cc which means "the compiler that compiled that compiler):

nix-instantiate . -A stdenv.cc.cc.stdenv.cc.cc
warning: you did not specify '--add-root'; the result might be removed by the garbage collector
/nix/store/wg247ls59dbnd2mipj2daifq33mbr5kv-xgcc-12.2.0.drv