refactor(ops/nixos): Pass `depot` as a special argument r/2626

This changes the evaluation order for the `depot` argument and ensures
it is partially evaluated before the module system starts resolving
imports.

This way we can import modules from `depot.path` without `depot`
having to come from readTree.

Fixes b/129.

Change-Id: Icf4dd2be15011055dac8b27e991a4ff6a12bf827
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3156
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

2 files changed, 36 insertions, 7 deletions

diff --git a/ops/nixos.nix b/ops/nixos.nix
index 0fdb50a99b43..19ba0168c07e 100644
--- a/ops/nixos.nix
+++ b/ops/nixos.nix
@@ -5,10 +5,6 @@ let inherit (lib) findFirst isAttrs;
 in rec {
   # This provides our standard set of arguments to all NixOS modules.
   baseModule = { ... }: {
-    _module.args = {
-      inherit (args) depot;
-    };
-
     # Ensure that pkgs == third_party.nix
     nixpkgs.pkgs = depot.third_party.nixpkgs;
     nix.nixPath = [
@@ -24,6 +20,10 @@ in rec {
         configuration
       ];
     };
+
+    specialArgs = {
+      inherit (args) depot;
+    };
   });
 
   findSystem = hostname:
diff --git a/third_party/default.nix b/third_party/default.nix
index 0b1e94c7f86c..5a244ed22039 100644
--- a/third_party/default.nix
+++ b/third_party/default.nix
@@ -17,7 +17,36 @@
 {
   # Expose a partially applied NixOS, expecting an attribute set with
   # a `configuration` key. Exposing it like this makes it possible to
-  # modify some of the base configuration used by NixOS. passed to
-  # this.
-  nixos = import "${pkgs.path}/nixos";
+  # modify some of the base configuration used by NixOS.
+  #
+  # This partially reimplements the code in
+  # <nixpkgs/nixos/default.nix> as we need to modify its internals to
+  # be able to pass `specialArgs`. We depend on this because `depot`
+  # needs to be partially evaluated in NixOS configuration before
+  # module imports are resolved.
+  nixos = {
+    configuration,
+    specialArgs ? {},
+    system ? builtins.currentSystem,
+    ...
+  }:
+  let
+    eval = import "${pkgs.path}/nixos/lib/eval-config.nix" {
+      inherit specialArgs system;
+      modules = [ configuration ];
+    };
+
+    # This is for `nixos-rebuild build-vm'.
+    vmConfig = (import "${pkgs.path}/nixos/lib/eval-config.nix" {
+      inherit specialArgs system;
+      modules = [
+        configuration
+        "${pkgs.path}/nixos/modules/virtualisation/qemu-vm.nix"
+      ];
+    }).config;
+  in {
+    inherit (eval) pkgs config options;
+    system = eval.config.system.build.toplevel;
+    vm = vmConfig.system.build.vm;
+  };
 }