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.
to do this we must replace derivations with attrsets in make-options-doc, since
xml can represent derivations differently from attrset but json cannot. this
also given asciidoc and mddoc the ability to handle derivation differently,
which they previously didn't have.
use the json file derivation we already have to also generate the asciidoc and
md options docs instead of formatting the options in nix. docbook docs are
already produced in derivations.
the new script produce the exact same output as the old in-nix generation.
we need the file itself as a dependency for the docbook build, but we don't need
it to be properly sorted at the nix level. push the sort out to a python script
instead to save eval time. on the machine used to write this `nix-instantiate
<nixos/nixos> -A system` went down from 7.1s to 5.4s and GC heap size decreased
by 50MB (or 70MB max RSS).
When using `documentation.nixos.includeAllModules = true;` with external
modules, the string context might contain dependencies to derivations
and so `toFile` refuses to evaluate;
```
error: in 'toFile': the file 'options.xml' cannot refer to derivation outputs, at
[...]/nixpkgs/nixos/lib/make-options-doc/default.nix:89:16
```
This is not an issue when using `writeText` (instead of manually
stripping the context).
Motivation is to support other repositories containing nixos
modules that would like to generate options documentation:
- nix-darwin
- private repos
- arion
- ??
