morph evaluates nixpkgs.overlays twice

[delroth]

Example repro:

let
  # Pin the deployment package-set to a specific version of nixpkgs
  pkgs = import (builtins.fetchTarball {
    url = "https://github.com/NixOS/nixpkgs/archive/98d9589819218971a95fd64c172fe5996a9734f5.tar.gz";
    sha256 = "0blscxj13qbcnlxkzwjsyqa80ssnx9wm0wz0bg6gkc1fa412w4f9";
  }) {};
in
{
  network =  {
    inherit pkgs;
    description = "simple hosts";
  };

  "web01.example.com" = { config, pkgs, ... }: {
    boot.loader.systemd-boot.enable = true;
    boot.loader.efi.canTouchEfiVariables = true;

    nixpkgs.overlays = [(self: super: {
      my_hello = (if super?my_hello then throw "fail" else super.hello);
    })];

    environment.systemPackages = with pkgs; [ my_hello ];

    fileSystems = {
        "/" = { label = "nixos"; fsType = "ext4"; };
        "/boot" = { label = "boot"; fsType = "vfat"; };
    };
  };
}

morph build fails to build this derivation because the throw is triggered. This means that the overlay is applied twice.

This causes real world issues when for example you use an overlay to apply a patch to a package: patches = old.patches ++ [ ./mypatch ]; will end up appending the patch twice, causing build failures.

Both nixops and the normal nixos-config handle this case correctly and apply the overlay only once.

cc @Shados (again, sorry :P I'm working through the issues trying to migrate my small network from nixops to morph)