server.nix

{
  network.description = "hc-web continuous integration server";

  jenkinsMaster =
    { config, pkgs, ... }:
    {
      # server config ...
    };
}

test.nix

import <nixpkgs/nixos/tests/make-test.nix> ({ pkgs, ... }: {
  name = "boot";
  machine = (import server.nix).jenkinsMaster;
  testScript = ''
    $machine->start;
    # ... lotsa tests ...
  '';
 }

So test requires some more memory otherwise jenkins explodes.

I cannot add virtualisation.memorySize to the server.nix because nixops will choke on that since it's only valid for the tests..

building all machine configurations... 
error: The option `virtualisation.memorySize' defined in `/home/gilligan/Development/hc-web-ci-server/infrastructure/hc-web-ci-server.nix' does not exist.     
(use ‘--show-trace’ to show detailed location information)                     
error: unable to build all machine configurations               

How can I add the virtualisation attribute in test.nix ? The only thing I could think of now is to move the attribute set that the jenkinsMaster function returns to a separate .nix file and then just do something like ...

{
  machine = { config, pkgs, ... }: import config.nix // { virtualisation.memorySize = 4096; };
}

Actually just thought of that while typing this up. Is this what you would do? I am thinking there is a different way ...

To be honest, I don’t know how nixops evaluates the module configs you hand it, e.g. in the example from the manual:

$ nixops create ./trivial.nix ./trivial-vbox.nix -d trivial

There are two attribute sets of modules given, only one of them contains nixops-specific options (stuff from https://nixos.org/nixops/manual/#ch-options). I’d think nixops might do something like merge every attribute with the same name in all files you give it on the command line, as follows:

listOfAttributeModules:
import <nixpkgs/nixos> { 
  configuration = {
    imports = [ nixopsOptionDeclarations ] ++ listOfAttributeModules;
  };
}

The part where all nixops-specific options are imported is important.
That’s just my guess, to be sure you’d have to look at the nixops source.