nixos/documentation: split options doc build

most modules can be evaluated for their documentation in a very
restricted environment that doesn't include all of nixpkgs. this
evaluation can then be cached and reused for subsequent builds, merging
only documentation that has changed into the cached set. since nixos
ships with a large number of modules of which only a few are used in any
given config this can save evaluation a huge percentage of nixos
options available in any given config.

in tests of this caching, despite having to copy most of nixos/, saves
about 80% of the time needed to build the system manual, or about two
second on the machine used for testing. build time for a full system
config shrank from 9.4s to 7.4s, while turning documentation off
entirely shortened the build to 7.1s.

1 files changed, 27 insertions, 1 deletions

diff --git a/nixos/doc/manual/development/meta-attributes.section.md b/nixos/doc/manual/development/meta-attributes.section.md
index ca4ba007f7dc0..946c08efd0a35 100644
--- a/nixos/doc/manual/development/meta-attributes.section.md
+++ b/nixos/doc/manual/development/meta-attributes.section.md
@@ -5,7 +5,7 @@ extra information. Module meta attributes are defined in the `meta.nix`
 special module.
 
 `meta` is a top level attribute like `options` and `config`. Available
-meta-attributes are `maintainers` and `doc`.
+meta-attributes are `maintainers`, `doc`, and `buildDocsInSandbox`.
 
 Each of the meta-attributes must be defined at most once per module
 file.
@@ -24,6 +24,7 @@ file.
   meta = {
     maintainers = with lib.maintainers; [ ericsagnes ];
     doc = ./default.xml;
+    buildDocsInSandbox = true;
   };
 }
 ```
@@ -38,3 +39,28 @@ file.
     ```ShellSession
     $ nix-build nixos/release.nix -A manual.x86_64-linux
     ```
+
+-  `buildDocsInSandbox` indicates whether the option documentation for the
+   module can be built in a derivation sandbox. This option is currently only
+   honored for modules shipped by nixpkgs. User modules and modules taken from
+   `NIXOS_EXTRA_MODULE_PATH` are always built outside of the sandbox, as has
+   been the case in previous releases.
+
+   Building NixOS option documentation in a sandbox allows caching of the built
+   documentation, which greatly decreases the amount of time needed to evaluate
+   a system configuration that has NixOS documentation enabled. The sandbox also
+   restricts which attributes may be referenced by documentation attributes
+   (such as option descriptions) to the `options` and `lib` module arguments and
+   the `pkgs.formats` attribute of the `pkgs` argument, `config` and the rest of
+   `pkgs` are disallowed and will cause doc build failures when used. This
+   restriction is necessary because we cannot reproduce the full nixpkgs
+   instantiation with configuration and overlays from a system configuration
+   inside the sandbox. The `options` argument only includes options of modules
+   that are also built inside the sandbox, referencing an option of a module
+   that isn't built in the sandbox is also forbidden.
+
+   The default is `true` and should usually not be changed; set it to `false`
+   only if the module requires access to `pkgs` in its documentation (e.g.
+   because it loads information from a linked package to build an option type)
+   or if its documentation depends on other modules that also aren't sandboxed
+   (e.g. by using types defined in the other module).