feat(ci): add dedicated Nix cache prebuild runner

.github/workflows/build_systems.yml

@@ -8,8 +8,23 @@ on:
     - cron: "0 22 * * *"
 
 jobs:
+  prebuild-common:
+    name: prebuild-common-x86-64-v3
+    runs-on: nix-cache-builder
+    steps:
+      - uses: actions/checkout@v4
+      # portal-1 is the smallest system closure: 95% of its derivations are
+      # shared by all five systems, so it is a maintainable common cache seed.
+      # Keep going so one failing package does not stop unrelated cache entries
+      # from being built.
+      - name: Build common packages
+        run: nixos-rebuild build --keep-going --accept-flake-config --flake ./#portal-1
+      - name: Copy common packages to nix-cache
+        run: nix copy --accept-flake-config --to unix:///host-nix/var/nix/daemon-socket/socket .#nixosConfigurations.portal-1.config.system.build.toplevel
+
   build:
     name: build-${{ matrix.system }}
+    needs: prebuild-common
     runs-on: self-hosted
     strategy:
       matrix:

systems/jeeves/runners/default.nix

@@ -21,5 +21,16 @@
     nix-builder-12.enable = true;
     nix-builder-13.enable = true;
     nix-builder-14.enable = true;
+
+    # Warm the shared x86-64-v3 cache before the smaller per-system runners
+    # start. Eight jobs with eight cores each can use Jeeves' 64 logical CPUs,
+    # while the 6000% quota leaves some capacity for its normal services.
+    nix-cache-builder = {
+      enable = true;
+      labels = [ "nix-cache-builder:host" ];
+      cores = 8;
+      maxJobs = 8;
+      cpuQuota = "6000%";
+    };
   };
 }

systems/jeeves/runners/nix_builder.nix

@@ -11,11 +11,8 @@ let
   cfg = config.services.nix_builder;
   runnerUsername = "gitea-runner";
   runnerUserid = 601;
-  runnerLabels = [
-    "self-hosted:host"
-    "nixos:host"
-  ];
   containerConfig =
+    containerCfg:
     {
       config,
       pkgs,
@@ -50,8 +47,8 @@ let
         useHostResolvConf = false;
       };
       nix.settings = {
-        cores = 8;
-        max-jobs = 2;
+        inherit (containerCfg) cores;
+        max-jobs = containerCfg.maxJobs;
         system-features = lib.mkAfter [
           "gccarch-x86-64-v2"
           "gccarch-x86-64-v3"
@@ -94,7 +91,7 @@ let
         enable = true;
         name = "jeeves-nix-builder";
         url = "http://192.168.99.14:6443/";
-        labels = runnerLabels;
+        labels = containerCfg.labels;
         tokenFile = "/run/secrets/gitea-runners/registration-token";
         settings.runner.timeout = "12h";
         hostPackages = with pkgs; [
@@ -120,20 +117,21 @@ let
           User = mkForce runnerUsername;
           Group = mkForce runnerUsername;
           ExecStartPre = mkForce [
-            "${getExe registerRunner} builder http://192.168.99.14:6443/ ${runnerConfigFile} ${escapeShellArgs runnerLabels}"
+            "${getExe registerRunner} builder http://192.168.99.14:6443/ ${runnerConfigFile} ${escapeShellArgs containerCfg.labels}"
           ];
         };
       };
       system.stateVersion = "24.05";
     };
-  sharedContainerPath =
+  mkContainerPath =
+    containerCfg:
     (import "${pkgs.path}/nixos/lib/eval-config.nix" {
       modules = [
         {
           boot.isNspawnContainer = true;
           nixpkgs.pkgs = pkgs;
         }
-        containerConfig
+        (containerConfig containerCfg)
       ];
       system = null;
     }).config.system.build.toplevel;
@@ -151,7 +149,36 @@ in
         types.submodule (
           { name, ... }:
           {
-            options.enable = mkEnableOption "Gitea runner container";
+            options = {
+              enable = mkEnableOption "Gitea runner container";
+
+              labels = mkOption {
+                type = types.listOf types.str;
+                default = [
+                  "self-hosted:host"
+                  "nixos:host"
+                ];
+                description = "Gitea Actions labels advertised by this runner.";
+              };
+
+              cores = mkOption {
+                type = types.ints.positive;
+                default = 8;
+                description = "Number of cores made available to each Nix build job.";
+              };
+
+              maxJobs = mkOption {
+                type = types.ints.positive;
+                default = 2;
+                description = "Maximum number of Nix build jobs run in parallel.";
+              };
+
+              cpuQuota = mkOption {
+                type = types.str;
+                default = "800%";
+                description = "systemd CPU quota for the runner container.";
+              };
+            };
           }
         )
       );
@@ -173,7 +200,7 @@ in
     containers = mapAttrs (
       name: containerCfg:
       mkIf containerCfg.enable {
-        path = sharedContainerPath;
+        path = mkContainerPath containerCfg;
         autoStart = true;
         privateNetwork = true;
         hostBridge = cfg.bridgeName;
@@ -199,16 +226,14 @@ in
     ) cfg.containers;
 
     systemd = {
-      services = builtins.listToAttrs (
-        map (name: {
-          name = "container@${name}";
-          value = {
-            requires = [ "gitea.service" ];
-            after = [ "gitea.service" ];
-            serviceConfig.CPUQuota = "800%";
-          };
-        }) (builtins.attrNames (filterAttrs (_: c: c.enable) cfg.containers))
-      );
+      services = mapAttrs' (
+        name: containerCfg:
+        nameValuePair "container@${name}" {
+          requires = [ "gitea.service" ];
+          after = [ "gitea.service" ];
+          serviceConfig.CPUQuota = containerCfg.cpuQuota;
+        }
+      ) (filterAttrs (_: c: c.enable) cfg.containers);
 
       tmpfiles.rules = [
         "d ${vars.uv_cache} 0755 ${runnerUsername} ${runnerUsername} - -"