{
  nodes,
  config,
  lib,
  pkgs,
  ...
}:
let
  cfg = config.bagel.services.buildbot;
  ssh-keys = import ../../common/ssh-keys.nix;
  freeGbDiskSpace = 20;
  extraTenantSpecificBuilders = {
    lix = import ./lix.nix {
      inherit config nodes;
    };
    floral = [ ];
  }.${cfg.tenant or (throw "${cfg.tenant} is not a known tenant")};
  clientId = {
    lix = "buildbot";
    floral = "forkos-buildbot";
  }.${cfg.tenant or (throw "${cfg.tenant} is not a known tenant")};
  inherit (lib) mkEnableOption mkOption mkIf types;
in
{
  options.bagel.services.buildbot = {
    enable = mkEnableOption "Buildbot";

    tenant = mkOption {
      type = types.enum [ "lix" "floral" ];
      description = "Which buildbot tenant to enable";
    };

    domain = mkOption {
      type = types.str;
      description = "Domain name for this Buildbot";
    };

    gerrit = {
      domain = mkOption {
        type = types.str;
        description = "Canonical domain of the Gerrit associated to this Buildbot";
        example = [ "cl.forkos.org" ];
      };

      port = mkOption {
        type = types.port;
        description = "Gerrit SSH port for this Buildbot";
      };

      username = mkOption {
        type = types.str;
        description = "Gerrit service username for this Buildbot";
      };
    };

    cors.allowedOrigin = mkOption {
      type = types.str;
      example = "*.forkos.org";
      description = "Allowed origin for Buildbot and NGINX for CORS without the protocol";
    };

    buildSystems = mkOption {
      type = types.listOf (types.enum [ "x86_64-linux" "aarch64-linux" "x86_64-darwin" "aarch64-darwin" ]);
      default = [ "x86_64-linux" ];
      example = [ "x86_64-linux" "aarch64-linux" ];
      description = "Supported build systems for this buildbot instance.";
    };

    projects = mkOption {
      type = types.listOf types.str;
      example = [ "nixpkgs" ];
      description = "Static list of projects enabled for Buildbot CI";
    };

    builders = mkOption {
      type = types.listOf types.str;
      description = "List of builders to configure for Buildbot";
      example = [ "builder-2" "builder-3" ];
    };
  };

  config = mkIf cfg.enable {
    networking.firewall.allowedTCPPorts = [ 80 443 ];
    bagel.secrets.files = [
      "buildbot-worker-password"
      "buildbot-oauth-secret"
      "buildbot-workers"
      "buildbot-service-key"
      "buildbot-signing-key"
      "buildbot-remote-builder-key"
    ];
    age.secrets.buildbot-signing-key = {
      owner = "buildbot-worker";
      group = "buildbot-worker";
    };
    age.secrets.buildbot-remote-builder-key = {
      file = ../../secrets/${cfg.tenant}/buildbot-remote-builder-key.age;
      owner = "buildbot-worker";
      group = "buildbot-worker";
    };

    services.nginx = {
      appendHttpConfig = ''
        # Our session stuff is too big with the TWISTED_COOKIE in addition.
        # Default is usually 4k or 8k.
        large_client_header_buffers 4 16k;
      '';
      virtualHosts.${cfg.domain} = {
        forceSSL = true;
        enableACME = true;
        extraConfig = ''
          add_header Access-Control-Allow-Credentials 'true' always;
          add_header Access-Control-Allow-Origin 'https://${cfg.cors.allowedOrigin}' always;
        '';
      };
    };

    services.buildbot-nix.worker = {
      enable = true;
      workerPasswordFile = config.age.secrets.buildbot-worker-password.path;
      # All credits to eldritch horrors for this beauty.
      workerArchitectures =
      {
        # nix-eval-jobs runs under a lock, error reports do not (but are cheap)
        other = 8;
      } // (
        lib.filterAttrs
          (n: v: lib.elem n config.services.buildbot-nix.coordinator.buildSystems)
          (lib.zipAttrsWith
            (_: lib.foldl' lib.add 0)
            (lib.concatMap
              (m: map (s: { ${s} = m.maxJobs; }) m.systems)
              config.services.buildbot-nix.coordinator.buildMachines))
        );
    };

    services.buildbot-nix.coordinator = {
      enable = true;

      inherit (cfg) domain;
      # TODO(raito): is that really necessary when we can just collect buildMachines' systems?
      inherit (cfg) buildSystems;

      oauth2 = {
        name = "Lix";
        inherit clientId;
        clientSecretFile = config.age.secrets.buildbot-oauth-secret.path;
        resourceEndpoint = "https://identity.lix.systems";
        authUri = "https://identity.lix.systems/realms/lix-project/protocol/openid-connect/auth";
        tokenUri = "https://identity.lix.systems/realms/lix-project/protocol/openid-connect/token";
        userinfoUri = "https://identity.lix.systems/realms/lix-project/protocol/openid-connect/userinfo";
      };

      # TODO(raito): this is not really necessary, we never have remote buildbot workers.
      # we can replace all of this with automatic localworker generation on buildbot-nix side.
      workersFile = config.age.secrets.buildbot-workers.path;

      allowedOrigins = [
        cfg.cors.allowedOrigin
      ];

      buildMachines = map (n: {
          hostName = nodes.${n}.config.networking.fqdn;
          protocol = "ssh-ng";
          # Follows Hydra.
          maxJobs = 8;
          sshKey = config.age.secrets.buildbot-remote-builder-key.path;
          sshUser = "buildbot";
          systems = [ "x86_64-linux" ];
          supportedFeatures = nodes.${n}.config.nix.settings.system-features;
          # Contrary to how Nix works, here we can specify non-base64 public host keys.
          publicHostKey = ssh-keys.machines.${n};
        }
      ) cfg.builders ++ extraTenantSpecificBuilders;

      gerrit = {
        # Manually managed account…
        # TODO: https://git.lix.systems/the-distro/infra/issues/69
        inherit (cfg.gerrit) domain port username;
        privateKeyFile = config.age.secrets.buildbot-service-key.path;
        inherit (cfg) projects;
      };

      evalWorkerCount = 6;
      evalMaxMemorySize = "4096";

      signingKeyFile = config.age.secrets.buildbot-signing-key.path;
    };

    # Make PostgreSQL restart smoother.
    systemd.services.postgresql.serviceConfig = {
      Restart = "always";
      RestartMaxDelaySec = "5m";
      RestartSteps = 10;
    };


    nix.settings.keep-derivations = true;
    nix.gc = {
      automatic = true;
      dates = "hourly";
      options = ''
        --max-freed "$((${toString freeGbDiskSpace} * 1024**3 - 1024 * $(df -P -k /nix/store | tail -n 1 | ${pkgs.gawk}/bin/awk '{ print $4 }')))"
      '';
    };
  };
}