feat(users/flokli/nixos/nixos-tvix-cache): init

This is a fetch-through mirror of cache.nixos.org, hosted by NumTide.

The current machine is a SX65 Hetzner dedicated server with 4x22TB SATA disks,
and 2x1TB NVMe disks.

The goals of this machine:

 - Exercise tvix-store and nar-bridge code
 - Collect usage metrics (see https://nixos.tvix.store/grafana)
 - Identify bottlenecks
 - Replace cache.nixos.org?

Be however aware that there's zero availability guarantees. Since Tvix doesn't
support garbage collection yet, we either will delete data or order a bigger
box.

Change-Id: Id24baa18cae1629a06caaa059c0c75d4a01659d5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12811
Tested-by: BuildkiteCI
Reviewed-by: Jonas Chevalier <zimbatm@zimbatm.com>
Reviewed-by: flokli <flokli@flokli.de>

users/flokli/nixos/nixos-tvix-cache/nar-bridge-module.nix

@@ -0,0 +1,75 @@
+{ config
+, lib
+, pkgs
+, depot
+, ...
+}:
+let
+  cfg = config.services.nar-bridge;
+
+  package = depot.tvix.nar-bridge.override (old: {
+    features = old.features or [ "default" ] ++ [ "xp-store-composition-cli" ];
+    runTests = true;
+  });
+
+  storeCompositionFormat = pkgs.formats.toml { };
+
+  storeCompositionFile = storeCompositionFormat.generate "store-composition.toml" cfg.settings;
+
+  args = [
+    "--listen-address"
+    "sd-listen"
+    "--experimental-store-composition"
+    storeCompositionFile
+  ];
+in
+{
+  options = {
+    services.nar-bridge = {
+      enable = lib.mkEnableOption "nar-bridge service";
+
+      settings = lib.mkOption {
+        type = storeCompositionFormat.type;
+        default = { };
+      };
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    users.users.nar-bridge = {
+      isSystemUser = true;
+      group = "nar-bridge";
+    };
+
+    users.groups.nar-bridge = { };
+
+    systemd.sockets.nar-bridge = {
+      description = "nar-bridge socket";
+      wantedBy = [ "sockets.target" ];
+
+      socketConfig = {
+        LimitNOFILE = 65535;
+        ListenStream = "/run/nar-bridge.sock";
+        SocketMode = "0666";
+        SocketUser = "root";
+      };
+    };
+
+    systemd.services.nar-bridge = {
+      description = "NAR Bridge";
+      requires = [ "nar-bridge.socket" ];
+      after = [ "nar-bridge.socket" ];
+      wantedBy = [ "multi-user.target" ];
+      serviceConfig = {
+        ExecStart = "${package}/bin/nar-bridge ${lib.escapeShellArgs args}";
+
+        Restart = "always";
+        RestartSec = "10";
+
+        User = "nar-bridge";
+        Group = "nar-bridge";
+        StateDirectory = "nar-bridge";
+      };
+    };
+  };
+}