Merge pull request #11106 from nbp/nixos-reentry

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

1 files changed, 84 insertions, 0 deletions

diff --git a/nixos/modules/misc/nixos.xml b/nixos/modules/misc/nixos.xml
new file mode 100644
index 000000000000..f8d3b4bc6e33
--- /dev/null
+++ b/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>