feat(ops/modules): configure builderball cache setup

Configures an experimental setup for a builderball-based public cache.

This cache only includes the two build machines (whitby & nevsky), for the time
period where both of them exist simultaneously.

The idea is this:

All participating hosts run a harmonia binary cache locally (whitby already
does). They then run builderball instances pointing at each other's harmonia
caches (through dedicated public hostnames).

When a request comes in, the first matching cache address is returned and Nix
will substitute from there.

Change-Id: Ia7d5357fd5e04f77b460205544fa24e82b100230
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12975
Autosubmit: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

ops/modules/builderball.nix

@@ -0,0 +1,51 @@
+# Configuration for builderball, the Nix cache proxy for substituting between
+# builders.
+#
+# This is in experimental state, not yet supporting any dynamic private builders.
+{ depot, config, lib, ... }:
+
+let
+  cfg = config.services.depot.builderball;
+  description = "Nix cache proxy for distribution between builders";
+  hostname = config.networing.hostName;
+in
+{
+  options.services.depot.builderball = {
+    enable = lib.mkEnableOption description;
+
+    caches = lib.mkOption {
+      type = with lib.types; listOf string;
+      description = "Public addresses of caches to use";
+
+      default = [
+        "whitby.cache.tvl.fyi"
+        "nevsky.cache.tvl.fyi"
+      ];
+    };
+
+    port = lib.mkOption {
+      type = lib.types.int;
+      description = "port on which to listen locally";
+      default = 26862; # bounc
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    systemd.services.builderball =
+      let
+        caches = lib.concatStringsSep " " (map (c: "-cache https://${c}") cfg.caches);
+      in
+      {
+        inherit description;
+        wantedBy = [ "multi-user.target" ];
+        wants = [ "network-online.target" ];
+        after = [ "network-online.target" ];
+
+        serviceConfig = {
+          ExecStart = "${depot.ops.builderball}/bin/builderball ${caches} -port ${toString cfg.port} -debug";
+          DynamicUser = true;
+          Restart = "always";
+        };
+      };
+  };
+}