refactor(ops): Split //ops/nixos into different locations r/2482

Splits //ops/nixos into:

* //ops/nixos.nix - utility functions for building systems
* //ops/machines - shared machine definitions (read by readTree)
* //ops/modules - shared NixOS modules (skipped by readTree)

This simplifies working with the configuration fixpoint in whitby, and
is overall a bit more in line with how NixOS systems in user folders
currently work.

Change-Id: I1322ec5cc76c0207c099c05d44828a3df0b3ffc1
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2931
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>
Reviewed-by: glittershark <grfn@gws.fyi>

1 files changed, 45 insertions, 0 deletions

diff --git a/ops/nixos.nix b/ops/nixos.nix
new file mode 100644
index 000000000000..465a2eed3198
--- /dev/null
+++ b/ops/nixos.nix
@@ -0,0 +1,45 @@
+# Helper functions for instantiating depot-compatible NixOS machines.
+{ depot, lib, pkgs, ... }@args:
+
+let inherit (lib) findFirst isAttrs;
+in rec {
+  # This provides our standard set of arguments to all NixOS modules.
+  baseModule = { ... }: {
+    _module.args = {
+      inherit (args) depot;
+    };
+  };
+
+  nixosFor = configuration: (depot.third_party.nixos {
+    configuration = { ... }: {
+      imports = [
+        baseModule
+        configuration
+      ];
+    };
+  });
+
+  findSystem = hostname:
+    (findFirst
+      (system: system.config.networking.hostName == hostname)
+      (throw "${hostname} is not a known NixOS host")
+      (map nixosFor depot.ops.machines.all-systems));
+
+  rebuild-system = pkgs.writeShellScriptBin "rebuild-system" ''
+    set -ue
+    if [[ $EUID -ne 0 ]]; then
+      echo "Oh no! Only root is allowed to rebuild the system!" >&2
+      exit 1
+    fi
+
+    echo "Rebuilding NixOS for $HOSTNAME"
+    system=$(nix-build -E "((import ${toString depot.depotPath} {}).ops.nixos.findSystem \"$HOSTNAME\").system" --no-out-link --show-trace)
+
+    nix-env -p /nix/var/nix/profiles/system --set $system
+    $system/bin/switch-to-configuration switch
+  '';
+
+  # Systems that should be built in CI
+  whitbySystem = (nixosFor depot.ops.machines.whitby).system;
+  meta.targets = [ "whitbySystem" ];
+}