infra: steal from the best

(Mic92's cluster TUM's infra.)

modules/nix-daemon.nix

@@ -0,0 +1,60 @@
+{ lib
+, config
+, pkgs
+, ...
+}:
+
+let
+  gcc-system-features = arch: lib.optionals (arch != null) ([ "gccarch-${arch}" ]
+    ++ map (x: "gccarch-${x}") lib.systems.architectures.inferiors.${arch});
+in
+{
+  options = {
+    simd.arch = lib.mkOption {
+      type = with lib.types; nullOr str;
+      default = null;
+      description = ''
+        Microarchitecture string for nixpkgs.hostPlatform.gcc.march and to generate system-features.
+        Can be determined with: gcc -march=native -Q --help=target | grep march
+      '';
+    };
+  };
+
+  imports = [ ./builder.nix ];
+
+  config = {
+    warnings = lib.optionals (config.simd.arch == null) [ "Please set simd.arch for ${config.networking.hostName}" ];
+
+    nix = {
+      gc.automatic = true;
+      gc.dates = "03:15";
+      gc.options = "--delete-older-than 30d";
+
+      # 2.11, 2.12 suffers from a bug with remote builders…
+      package = pkgs.nixVersions.nix_2_13;
+
+      # should be enough?
+      nrBuildUsers = lib.mkDefault 32;
+
+      # https://github.com/NixOS/nix/issues/719
+
+      settings = {
+        keep-outputs = true;
+        keep-derivations = true;
+        # in zfs we trust
+        fsync-metadata = lib.boolToString (!config.boot.isContainer or config.fileSystems."/".fsType != "zfs");
+        substituters = [
+          "https://nix-community.cachix.org"
+          "https://tum-dse.cachix.org"
+        ];
+        system-features = [ "benchmark" "big-parallel" "kvm" "nixos-test" ] ++ gcc-system-features config.simd.arch;
+        trusted-public-keys = [
+          "nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs="
+          "tum-dse.cachix.org-1:v67rK18oLwgO0Z4b69l30SrV1yRtqxKpiHodG4YxhNM="
+        ];
+      };
+    };
+
+    nixpkgs.config.allowUnfree = true;
+  };
+}