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.
Dnscrypt-proxy needs some options to be set before it can do anything useful.
Currently, we only apply what the user configured which, by default, is nothing.
This leads to the dnscrypt-proxy2 service failing to start when you only set
`enable = true;` which is not a great user experience.
This patch makes the module take the example config from the upstream repo as a
base on top of which the user-specified settings are applied (it contains sane
defaults).
An option has been added to restore the old behaviour.
it does happen that `dnscrypt-proxy` exit when it is unable to
synchronise its resolvers metadata on startup. this can happen due
to network connectivity issues for example. not restarting it automatically
means no dns resolution will work until a manual restart is performed.
