hyperhive/nix/host-modules/swarm-controller.nix

# The swarm-level controller daemon. Per-host opt-in: a swarm has one
# controller, so most hives leave this off and point at the hive that
# runs it. Distinct from hive-c0re, which every hive runs — c0re owns
# the agents on one host, this owns what is true across hives.
#
# Serves HTTP over a unix socket rather than a TCP port: the gateway's
# nginx is the only intended client, it runs on this same host and so
# reaches the socket by path, and a socket that is never bound to an
# address cannot be reached from off-host by mistake.
{
  pkgs,
  lib,
  config,
  ...
}:
let
  cfg = config.services.hyperhive.swarm.controller;
  autheliaCfg = config.services.hyperhive.swarm.authelia;

  # What `swarmctl` needs in order to act on authelia from the host.
  #
  # Only set when authelia actually runs **here**: the controller can be
  # enabled on a host that is not the swarm's SSO host, and in that case
  # the right behaviour is for `swarmctl user add` to fail saying the
  # value is unset. A guessed path would resolve cleanly and write a file
  # nothing reads, which is the failure mode that costs an afternoon.
  autheliaEnv = lib.optionalAttrs autheliaCfg.enable {
    # The CONFIGURED authelia, not whatever is on PATH: the argon2
    # parameters baked into a hash have to match the verifier's.
    SWARMCTL_AUTHELIA_BIN = "${autheliaCfg.package}/bin/authelia";
    SWARMCTL_AUTHELIA_USERS_FILE = autheliaCfg.hostUsersFile;
    SWARMCTL_AUTHELIA_MACHINE = autheliaCfg.machine;
    SWARMCTL_AUTHELIA_UNIT = autheliaCfg.unit;
  };

  # Wrapped rather than documented: every one of these values is derived
  # from an option this deployment already set, so making the operator
  # re-supply them on the command line would be asking them to repeat the
  # config back to it — and to get it wrong the day one of them changes.
  swarmctlConfigured = pkgs.symlinkJoin {
    name = "swarmctl-configured";
    paths = [ cfg.swarmctlPackage ];
    nativeBuildInputs = [ pkgs.makeWrapper ];
    postBuild = ''
      wrapProgram $out/bin/swarmctl ${
        lib.concatStringsSep " " (
          lib.mapAttrsToList (name: value: "--set ${name} ${lib.escapeShellArg value}") autheliaEnv
        )
      }
    '';
  };
in
{
  options.services.hyperhive.swarm.controller = {
    enable = lib.mkOption {
      type = lib.types.bool;
      default = false;
      description = ''
        Run the swarm-controller daemon on this host. Off by default and
        deliberately not derived from `services.hyperhive.enable`: a swarm
        has one controller, so enabling it per hive is a decision about
        swarm topology, not about whether hyperhive is installed.

        `services.hyperhive.enableAllLocalDefaults` does assert it, and
        that is not an exception to the rule above — it is the rule
        applied. That mode says "this box is the whole deployment", which
        answers the topology question outright, where
        `services.hyperhive.enable` alone never can.
      '';
    };

    package = lib.mkOption {
      type = lib.types.package;
      defaultText = lib.literalExpression "hyperhive.packages.\${system}.swarm-controller";
      description = ''
        swarm-controller package. Wired by default from this flake's own
        package set (see `flake.nix`); override to run a different build.
      '';
    };

    swarmctlPackage = lib.mkOption {
      type = lib.types.package;
      defaultText = lib.literalExpression "hyperhive.packages.\${system}.swarmctl";
      description = ''
        `swarmctl` package — the swarm operator's CLI, installed on this
        host alongside the daemon and wrapped with the paths it needs.

        A separate option from `package` rather than a second binary in
        the same derivation: the CLI runs as root and acts directly,
        the daemon runs unprivileged and serves a socket, and pinning one
        without the other is a thing an operator may legitimately want.
      '';
    };

    socketPath = lib.mkOption {
      type = lib.types.str;
      default = "/run/swarm-controller/controller.sock";
      description = ''
        Unix socket the daemon serves on, and the path the gateway's nginx
        proxies to.

        The **directory** is the access control here, not the socket mode:
        the socket itself is `0666` (nginx runs as another user, and
        `connect(2)` needs write), exactly as hive-c0re publishes the
        per-agent sockets. What keeps that safe is that the directory holds
        exactly one socket and is traverse-only (`0751`) for everyone else.
        Moving this path under a directory that carries anything else —
        `/run/hyperhive`, which holds the host admin socket, above all —
        exposes whatever else lives there to the same reachability.

        Changing this therefore means re-checking what else lives in the
        new directory, not just the daemon.
      '';
    };
  };

  config = lib.mkIf (config.services.hyperhive.enable && cfg.enable) {
    users.users.swarm-controller = {
      isSystemUser = true;
      group = "swarm-controller";
      description = "hyperhive swarm-controller daemon";
    };
    users.groups.swarm-controller = { };

    # Installed host-wide, not into the daemon's unit: `swarmctl` is run
    # by a human on this box and acts as root, so the daemon's sandbox is
    # exactly what it must not inherit.
    environment.systemPackages = [ swarmctlConfigured ];

    systemd.services.swarm-controller = {
      description = "hyperhive swarm-level controller daemon";
      wantedBy = [ "multi-user.target" ];
      after = [ "network.target" ];

      serviceConfig = {
        ExecStart = "${cfg.package}/bin/swarm-controller";
        User = "swarm-controller";
        Group = "swarm-controller";
        Restart = "on-failure";
        RestartSec = "5s";

        # `/run/swarm-controller` — its own directory, holding only the
        # socket. See `socketPath`'s description for why that is a security
        # property and not tidiness.
        RuntimeDirectory = "swarm-controller";
        # 0751: traverse-only for others, so the gateway's nginx can reach
        # the socket path without being able to list the directory. Same
        # shape (and same reason) as hive-c0re's runtime dir.
        RuntimeDirectoryMode = "0751";
        # Preserved across restarts so the path never vanishes from under a
        # running nginx. The daemon unlinks a stale socket on start, which
        # is what makes preservation safe.
        RuntimeDirectoryPreserve = "yes";

        StateDirectory = "swarm-controller";
        StateDirectoryMode = "0750";

        # Nothing here needs a writable filesystem, real privileges, or a
        # view of the rest of the machine; the daemon reads its socket path
        # from config and serves.
        PrivateTmp = true;
        ProtectSystem = "strict";
        ProtectHome = true;
        NoNewPrivileges = true;
        PrivateDevices = true;
        ProtectKernelTunables = true;
        ProtectKernelModules = true;
        ProtectControlGroups = true;
        RestrictAddressFamilies = [
          "AF_UNIX"
        ];
      };

      environment.SWARM_CONTROLLER_SOCKET = cfg.socketPath;
    };
  };
}