{ lib, config, pkgs, ... }:
let
  inherit (lib) mkEnableOption mkOption types mkIf mapAttrsToList mkPackageOption concatStringsSep;
  cfg = config.bagel.nixpkgs.channel-scripts;
  toml = pkgs.formats.toml { };
  configFile = toml.generate "forkos.toml" cfg.settings;
  orderLib = import ./service-order.nix { inherit lib; };
  makeUpdateJob = channelName: mainJob: {
    name = "update-${channelName}";
    value = {
      description = "Update channel ${channelName}";
      path = with pkgs; [ git ];
      serviceConfig = {
        Type = "oneshot";
        RemainAfterExit = false;
        User = "channel-scripts";
        DynamicUser = true;
        StateDirectory = "channel-scripts";
        MemoryHigh = "80%";
        EnvironmentFile = [
          cfg.releaseBucketCredentialsFile
        ];
        Environment = cfg.extraEnvironment;
        # TODO: we should have our own secret for this.
        LoadCredential = [ "password:${config.age.secrets.alloy-push-password.path}" ];
      };
      unitConfig.After = [ "networking.target" ];
      script =
      ''
        # A stateful copy of nixpkgs
        dir=/var/lib/channel-scripts/nixpkgs
        if ! [[ -e $dir ]]; then
          git clone --bare ${cfg.nixpkgsUrl} $dir
        fi
        GIT_DIR=$dir git config remote.origin.fetch '+refs/heads/*:refs/remotes/origin/*'

        CREDENTIAL=$(echo -en "promtail:$(cat $CREDENTIALS_DIRECTORY/password)" | base64)
        export OTEL_EXPORTER_OTLP_HEADERS="Authorization=Basic $CREDENTIAL"
        # TODO: use escapeShellArgs
        exec ${cfg.package}/bin/mirror-forkos -c ${configFile} ${concatStringsSep " " cfg.extraArgs} apply ${channelName} ${mainJob}
      '';
    };
  };
  updateJobs = orderLib.mkOrderedChain (mapAttrsToList (n: { job, ... }: makeUpdateJob n job) cfg.channels);
  channelOpts = { ... }: {
    options = {
      job = mkOption {
        type = types.str;
        example = "nixos/trunk-combined/tested";
      };

      variant = mkOption {
        type = types.enum [ "primary" "small" "darwin" "aarch64" ];
        example = "primary";
      };

      status = mkOption {
        type = types.enum [ "beta" "stable" "deprecated" "unmaintained" "rolling" ];
        example = "rolling";
      };
    };
  };
in
{
  options.bagel.nixpkgs.channel-scripts = {
    enable = mkEnableOption ''the channel scripts.
      Fast forwarding channel branches which are read-only except for this privileged bot
      based on our Hydra acceptance tests.
    '';

    otlp.enable = mkEnableOption "the OTLP export process";

    s3 = {
      release = mkOption {
        type = types.str;
      };

      channel = mkOption {
        type = types.str;
      };
    };

    package = mkPackageOption pkgs "mirror-forkos" { };

    settings = mkOption {
      type = types.attrsOf types.anything;
    };

    nixpkgsUrl = mkOption {
      type = types.str;
      default = "https://cl.forkos.org/nixpkgs.git";
      description = "URL to the nixpkgs repository to clone and to push to";
    };

    binaryCacheUrl = mkOption {
      type = types.str;
      default = "https://cache.forkos.org";
      description = "URL to the binary cache";
    };

    baseUriForGitRevisions = mkOption {
      type = types.str;
      description = "Base URI to generate link to a certain revision";
    };

    extraArgs = mkOption {
      type = types.listOf types.str;
      default = [ ];
      description = "Extra arguments passed to the mirroring program";
    };

    releaseBucketCredentialsFile = mkOption {
      type = types.path;
      description = ''Path to the release bucket credentials file exporting S3-style environment variables.
        For example, `AWS_ACCESS_KEY_ID`, `AWS_SECRET_ACCESS_KEY` for the S3 operations to work.
      '';
    };

    deployKeyFile = mkOption {
      type = types.path;
      description = ''Path to the private SSH key which is allowed to deploy things to the protected channel references on the Git repository.
      '';
    };

    hydraUrl = mkOption {
      type = types.str;
      default = "https://hydra.forkos.org";
      description = "URL to the Hydra instance";
    };

    channels = mkOption {
      type = types.attrsOf (types.submodule channelOpts);
      description = "List of channels to mirror";
    };

    extraEnvironment = mkOption {
      type = types.listOf types.str;
      default = [
        "RUST_LOG=info"
      ];
    };
  };

  config = mkIf cfg.enable {
    bagel.nixpkgs.channel-scripts.extraEnvironment = mkIf cfg.otlp.enable [
      ''OTEL_EXPORTER_OTLP_TRACES_ENDPOINT="https://tempo.forkos.org/v1/traces"''
    ];
    bagel.nixpkgs.channel-scripts.settings = {
      hydra_uri = cfg.hydraUrl;
      binary_cache_uri = cfg.binaryCacheUrl;
      base_git_uri_for_revision = cfg.baseUriForGitRevisions;
      nixpkgs_dir = "/var/lib/channel-scripts/nixpkgs";
      s3_release_bucket_name = cfg.s3.release;
      s3_channel_bucket_name = cfg.s3.channel;
    };

    users.users.channel-scripts = {
      description = "Channel scripts user";
      isSystemUser = true;
      group = "channel-scripts";
    };
    users.groups.channel-scripts = {};

    systemd.services = (lib.listToAttrs updateJobs) // {
      "update-all-channels" = {
        description = "Start all channel updates.";
        unitConfig = {
          After = map
            (service: "${service.name}.service")
            updateJobs;
          Wants = map
            (service: "${service.name}.service")
            updateJobs;
        };
        script = "true";
      };
    };

    systemd.timers."update-all-channels" = {
      description = "Start all channel updates.";
      wantedBy = [ "timers.target" ];
      timerConfig = {
        OnUnitInactiveSec = 600;
        OnBootSec = 900;
        AccuracySec = 300;
      };
    };
  };
}