* Added an option `nixpkgs.config' that allows the Nixpkgs options to

be set from the NixOS configuration.  For instance, you can say

    nixpkgs.config.firefox.enableGeckoMediaPlayer = true;

    environment.systemPackages = [ pkgs.firefox ];

  but the more interesting application is to apply global overrides to
  Nixpkgs throughout NixOS, e.g.

    nixpkgs.config.packageOverrides = pkgs:
      { glibc = pkgs.glibc27;
        gcc = pkgs.gcc42;
      };

  would build the whole system with Glibc 2.7 and GCC 4.2.  (There are
  some issues with "useFromStdenv" in all-packages.nix that need to be
  fixed for packages in the stdenv bootstrap though.)

  The implementation of this option is kind of evil though due to the
  need to prevent a circularity between the evaluation of
  nixpkgs.config and the "pkgs" module argument.

svn path=/nixos/trunk/; revision=16866

lib/eval-config.nix

@@ -5,41 +5,69 @@
 { configuration
 , system ? builtins.currentSystem
 , nixpkgs ? import ./from-env.nix "NIXPKGS" /etc/nixos/nixpkgs
-, pkgs ? import nixpkgs {inherit system;}
+, pkgs ? null
+, baseModules ? import ../modules/module-list.nix
 , extraArgs ? {}
 , extraModules ? []
 }:
 
-let extraArgs_ = extraArgs; in
+let extraArgs_ = extraArgs; pkgs_ = pkgs; in
 
 rec {
-  inherit nixpkgs pkgs;
 
+  # These are the NixOS modules that constitute the system configuration.
   configComponents =
-    [ configuration
-      ./check-config.nix
-    ]
+    [ configuration ]
     ++ extraModules
-    ++ (import ../modules/module-list.nix);
+    ++ baseModules;
 
+  # Merge the option definitions in all modules, forming the full
+  # system configuration.  This is called "configFast" because it's
+  # not checked for undeclared options.
+  configFast =
+    pkgs.lib.definitionsOf configComponents extraArgs;
+
+  # These are the extra arguments passed to every module.  In
+  # particular, Nixpkgs is passed through the "pkgs" argument.
   extraArgs = extraArgs_ // {
     inherit pkgs optionDeclarations;
     modulesPath = ../modules;
   };
 
-  config_ =
-    pkgs.lib.definitionsOf configComponents extraArgs;
+  # Import Nixpkgs, allowing the NixOS option nixpkgs.config to
+  # specify the Nixpkgs configuration (e.g., to set package options
+  # such as firefox.enableGeckoMediaPlayer, or to apply global
+  # overrides such as changing GCC throughout the system).  This is
+  # tricky, because we have to prevent an infinite recursion: "pkgs"
+  # is passed as an argument to NixOS modules, but the value of "pkgs"
+  # depends on config.nixpkgs.config, which we get from the modules.
+  # So we call ourselves here with "pkgs" explicitly set to an
+  # instance that doesn't depend on nixpkgs.config.
+  pkgs =
+    if pkgs_ != null
+    then pkgs_
+    else import nixpkgs {
+      inherit system;
+      config =
+        (import ./eval-config.nix {
+          inherit configuration system nixpkgs extraArgs extraModules;
+          # For efficiency, leave out most NixOS modules; they don't
+          # define nixpkgs.config, so it's pointless to evaluate them.
+          baseModules = [ ../modules/misc/nixpkgs.nix ];
+          pkgs = import nixpkgs { inherit system; config = {}; };
+        }).configFast.nixpkgs.config;
+    };
 
   # "fixableDeclarationsOf" is used instead of "declarationsOf" because some
   # option default values may depends on the definition of other options.
   # !!! This seems inefficent.  Didn't definitionsOf already compute
   # the option declarations?
   optionDeclarations =
-    pkgs.lib.fixableDeclarationsOf configComponents extraArgs config_;
+    pkgs.lib.fixableDeclarationsOf configComponents extraArgs configFast;
 
   # Optionally check wether all config values have corresponding
   # option declarations.
-  config = pkgs.checker config_
-    config_.environment.checkConfigurationOptions
-    optionDeclarations config_;
+  config = pkgs.checker configFast
+    configFast.environment.checkConfigurationOptions
+    optionDeclarations configFast;
 }

modules/misc/nixpkgs.nix

@@ -0,0 +1,17 @@
+{ config, pkgs, ... }:
+
+{
+  options = {
+
+    nixpkgs.config = pkgs.lib.mkOption {
+      default = {};
+      example = {
+        firefox.enableGeckoMediaPlayer = true;
+      };
+      description = ''
+        The configuration of the Nix Packages collection.
+      '';
+    };
+
+  };
+}

modules/module-list.nix

@@ -15,8 +15,10 @@
   ./installer/tools/nixos-checkout.nix
   ./installer/tools/tools.nix
   ./misc/assertions.nix
+  ./misc/check-config.nix
   ./misc/ids.nix
   ./misc/locate.nix
+  ./misc/nixpkgs.nix
   ./misc/passthru.nix
   ./programs/bash/bash.nix
   ./programs/info.nix