set up distributed builds

services/distributed-builds.nix

@@ -0,0 +1,45 @@
+{ config, lib, pkgs, ... }:
+
+let
+  inherit (builtins) elem;
+  inherit (lib) mkIf mkEnableOption mkOption types optional;
+  cfg = config.qenya.services.distributed-builds;
+in
+{
+  options.qenya.services.distributed-builds = {
+    enable = mkEnableOption "distributed builds";
+    keyFile = mkOption {
+      type = types.path;
+      description = ''
+        Path to the OpenSSH private key to be used for distributed builds.
+      '';
+    };
+    builders = mkOption {
+      type = types.listOf types.str;
+      default = [ ];
+      description = ''
+        List of builders to attempt to use for distributed builds.
+      '';
+      example = [ "kalessin" ];
+    };
+  };
+
+  config = mkIf cfg.enable {
+    assertions = [{
+      assertion = cfg ? keyFile;
+      message = "must specify a private key to be used for distributed builds";
+    }];
+
+    nix.distributedBuilds = true;
+    nix.settings.builders-use-substitutes = true;
+
+    nix.buildMachines =
+      (optional (elem "kalessin" cfg.builders) {
+        hostName = config.birdsong.hosts."kalessin".ipv4;
+        sshUser = "remotebuild";
+        sshKey = cfg.keyFile;
+        systems = [ "aarch64-linux" "x86_64-linux" ];
+        supportedFeatures = [ ];
+      });
+  };
+}