Add a way to pin a NixOS version within the module system.

This modification add a way to re-evaluate the module system with a
different version of NixOS, or with a different set of arguments.

nixos/modules/misc/nixos.nix

@@ -0,0 +1,82 @@
+{ config, options, lib, ... }:
+
+# This modules is used to inject a different NixOS version as well as its
+# argument such that one can pin a specific version with the versionning
+# system of the configuration.
+let
+  nixosReentry = import config.nixos.path {
+    inherit (config.nixos) configuration extraModules;
+    inherit (config.nixpkgs) system;
+    reEnter = true;
+  };
+in
+
+with lib;
+
+{
+  options = {
+    nixos.path = mkOption {
+      default = null;
+      example = literalExample "./nixpkgs-15.09/nixos";
+      type = types.nullOr types.path;
+      description = ''
+        This option give the ability to evaluate the current set of modules
+        with a different version of NixOS. This option can be used version
+        the version of NixOS with the configuration without relying on the
+        <literal>NIX_PATH</literal> environment variable.
+      '';
+    };
+
+    nixos.system = mkOption {
+      example = "i686-linux";
+      type = types.uniq types.str;
+      description = ''
+        Name of the system used to compile NixOS.
+      '';
+    };
+
+    nixos.extraModules = mkOption {
+      default = [];
+      example = literalExample "mkIf config.services.openssh.enable [ ./sshd-config.nix ]";
+      type = types.listOf types.unspecified;
+      description = ''
+        Define additional modules which would be loaded to evaluate the
+        configuration.
+      '';
+    };
+
+    nixos.configuration = mkOption {
+      type = types.unspecified;
+      internal = true;
+      description = ''
+        Option used by <filename>nixos/default.nix</filename> to re-inject
+        the same configuration module as the one used for the current
+        execution.
+      '';
+    };
+
+    nixos.reflect = mkOption {
+      default = { inherit config options; };
+      type = types.unspecified;
+      internal = true;
+      description = ''
+        Provides <literal>config</literal> and <literal>options</literal>
+        computed by the module system and given as argument to all
+        modules. These are used for introspection of options and
+        configuration by tools such as <literal>nixos-option</literal>.
+      '';
+    };
+  };
+
+  config = mkMerge [
+    (mkIf (config.nixos.path != null) (mkForce {
+      system.build.toplevel = nixosReentry.system;
+      system.build.vm = nixosReentry.vm;
+      nixos.reflect = { inherit (nixosReentry) config options; };
+    }))
+
+    { meta.maintainers = singleton lib.maintainers.pierron;
+      meta.doc = ./nixos.xml;
+    }
+  ];
+}

nixos/modules/misc/nixos.xml

@@ -0,0 +1,84 @@
+<chapter xmlns="http://docbook.org/ns/docbook"
+         xmlns:xlink="http://www.w3.org/1999/xlink"
+         xmlns:xi="http://www.w3.org/2001/XInclude"
+         version="5.0"
+         xml:id="module-misc-nixos">
+
+<title>NixOS Reentry</title>
+
+<!-- FIXME: render nicely -->
+
+<!-- FIXME: source can be added automatically -->
+<para><emphasis>Source:</emphasis> <filename>modules/misc/nixos.nix</filename></para>
+
+<!-- FIXME: more stuff, like maintainer? -->
+
+<para>NixOS reentry can be used for both pinning the evaluation to a
+specific version of NixOS, and to dynamically add additional modules into
+the Module evaluation.</para>
+
+<section><title>NixOS Version Pinning</title>
+
+<para>To pin a specific version of NixOS, you need a version that you can
+either clone localy, or that you can fetch remotely.</para>
+
+<para>If you already have a cloned version of NixOS in the directory
+<filename>/etc/nixos/nixpkgs-16-03</filename>, then you can specify the
+<option>nixos.path</option> with either the path or the relative path of
+your NixOS clone. For example, you can add the following to your
+<filename>/etc/nixos/configuration.nix</filename> file:
+
+<programlisting>
+nixos.path = ./nixpkgs-16-03/nixos;
+</programlisting>
+</para>
+
+<para>Another option is to fetch a specific version of NixOS, with either
+the <literal>fetchTarball</literal> builtin, or the
+<literal>pkgs.fetchFromGithub</literal> function and use the result as an
+input.
+
+<programlisting>
+nixos.path = "${builtins.fetchTarball https://github.com/NixOS/nixpkgs/archive/1f27976e03c15183191d1b4aa1a40d1f14666cd2.tar.gz}/nixos";
+</programlisting>
+</para>
+
+</section>
+
+
+<section><title>Adding Module Dynamically</title>
+
+<para>To add additional module, the recommended way is to use statically
+known modules in the list of imported arguments as described in <xref
+linkend="sec-modularity" />.  Unfortunately, this recommended method has
+limitation, such that the list of imported files cannot be selected based on
+the content of the configuration.
+
+Fortunately, NixOS reentry system can be used as an alternative to register
+new imported modules based on the content of the configuration. To do so,
+one should define both <option>nixos.path</option> and
+<option>nixos.extraModules</option> options.
+
+<programlisting>
+nixos.path = &lt;nixos&gt;;
+nixos.extraModules =
+  if config.networking.hostName == "server" then
+    [ ./server.nix ] else [ ./client.nix ];
+</programlisting>
+
+Also note, that the above can be reimplemented in a different way which is
+not as expensive, by using <literal>mkIf</literal> at the top each
+configuration if both modules are present on the file system (see <xref
+linkend="sec-option-definitions" />) and by always inmporting both
+modules.</para>
+
+</section>
+
+<section><title>Options</title>
+
+<para>FIXME: auto-generated list of module options.</para>
+
+</section>
+
+
+</chapter>

nixos/modules/module-list.nix

@@ -52,6 +52,7 @@
   ./misc/lib.nix
   ./misc/locate.nix
   ./misc/meta.nix
+  ./misc/nixos.nix
   ./misc/nixpkgs.nix
   ./misc/passthru.nix
   ./misc/version.nix