nixpkgs/modules/system/etc/etc.nix

# produce a script to generate /etc
{config, pkgs, ...}:

###### interface
let
  inherit (pkgs.lib) mkOption;

  option = {
    environment.etc = mkOption {
      default = [];
      example = [
        { source = "/nix/store/.../etc/dir/file.conf.example";
          target = "dir/file.conf";
          mode = "0440";
        }
      ];
      description = ''
        List of files that have to be linked in /etc.
      '';
    };
  };
in

###### implementation
let

  copyScript = {source, target, mode ? "644", own ? "root.root"}:
    assert target != "nixos";
    ''
      source="${source}"
      target="/etc/${target}"
      mkdir -p $(dirname "$target")
      test -e "$target" && rm -f "$target"
      cp "$source" "$target"
      chown ${own} "$target"
      chmod ${mode} "$target"
    '';

  makeEtc = pkgs.stdenv.mkDerivation {
    name = "etc";

    builder = ./make-etc.sh;

    /* !!! Use toXML. */
    sources = map (x: x.source) config.environment.etc;
    targets = map (x: x.target) config.environment.etc;
    modes = map (x: if x ? mode then x.mode else "symlink") config.environment.etc;
  };

in

{
  require = [option];

  system = {
    build = {
      etc = makeEtc;
    };

    activationScripts = {
      etc = pkgs.lib.fullDepEntry ''
        # Set up the statically computed bits of /etc.
        echo "setting up /etc..."
        staticEtc=/etc/static
        rm -f $staticEtc
        ln -s ${makeEtc}/etc $staticEtc
        for i in $(cd $staticEtc && find * -type l); do
            mkdir -p /etc/$(dirname $i)
            rm -f /etc/$i
            if test -e "$staticEtc/$i.mode"; then
                # Create a regular file in /etc.
                cp $staticEtc/$i /etc/$i
                chown 0.0 /etc/$i
                chmod "$(cat "$staticEtc/$i.mode")" /etc/$i
            else
                # Create a symlink in /etc.
                ln -s $staticEtc/$i /etc/$i
            fi
        done

        # Remove dangling symlinks that point to /etc/static.  These are
        # configuration files that existed in a previous configuration but not
        # in the current one.  For efficiency, don't look under /etc/nixos
        # (where all the NixOS sources live).
        for i in $(find /etc/ \( -path /etc/nixos -prune \) -o -type l); do
            target=$(readlink "$i")
            if test "''${target:0:''${#staticEtc}}" = "$staticEtc" -a ! -e "$i"; then
                rm -f "$i"
            fi
        done
      '' [
        "systemConfig"
        "defaultPath" # path to cp, chmod, chown
        "stdio"
      ];
    };
  };
}