Details on https://github.com/NixOS/nixops/issues/1063#issuecomment-453253666.
`partOf` makes that if `smokeping.service` is stopped, `thttpd.service` will
be stopped as well.
(But not that `thttpd` will be started when `smokeping` is started).
Once `thttpd.service` is stopped that way, `Restart = always` will not apply.
When the smokeping config options are changed, NixOS's `switch-configuration.pl`
will stop `smokeping` (whit shuts down thttpd due to `partOf`), and then restart
smokeping; but this does not start thttpd.
As a result, thttpd will be off after changing the config, which isn't desired.
This commit fixes it by removing the `partOf`, which makes `Restart` work
as expected.
This avoids a common problem:
Until now, port forwarding to multiple hosts running smokeping did not work;
they all show the data of the first smokeping instance.
That ws because the image URLs generated by smokeping are absolute
(`imgurl` setting).
Consequently, if you ran
ssh node-1 -L 8081:localhost:8081
ssh node-2 -L 8081:localhost:8082
ssh node-3 -L 8081:localhost:8083
and try to open http://localhost:8081, http://localhost:8082 and
http://localhost:8083, they all would show the images of node-1!
Using a relative `imgurl` fixes that.
As per smokeping docs on `imgurl`:
> Either an absolute URL to the `imgcache` directory or one relative to the
> directory where you keep the SmokePing cgi.
qemu_kvm is only built for one architecture, so it's smaller and takes
MUCH less time to build if it has to be built from source. And this
module doesn't support running a VM for one architecture from another
architecture, so the one architecture is all we'll need.
By using the new extendModules function to produce the specialisations,
we avoid reimplementing the eval-config.nix logic in reverse and fix
cross compilation support for specialisations in the process.
Some specialisations (such as those which affect various boot-time
attributes) cannot be switched to at runtime. This allows picking the
specialisation at boot time.
