nix-rehash/systemd.nix

{ pkgs, config, ... }:
with pkgs.lib;
let
  services = config.systemd.services;

  isOneShot = cfg: hasAttr "Type" cfg.serviceConfig && cfg.serviceConfig.Type == "oneshot";

  runServices = filterAttrs (name: cfg: !(isOneShot cfg)) services;

  oneShotServices = filterAttrs (name: cfg: isOneShot cfg) services;

  filterCommand = cmd:
    let
      filtered = substring 1 (stringLength cmd -2) cmd;
      splitted = pkgs.lib.splitString " " filtered;
    in if eqStrings (substring 0 1 cmd) "@" then
        traceVal (head splitted) + concatStringsSep " " (drop 2 splitted)
       else cmd;

  configToCommand = name: cfg: ''
      #!/bin/sh -e
      ${if hasAttr "preStart" cfg then cfg.preStart else ""}
      ${if hasAttr "ExecStart" cfg.serviceConfig then
          filterCommand cfg.serviceConfig.ExecStart
        else if hasAttr "script" cfg then
          cfg.script
        else
          ""
      }
      ${if hasAttr "postStart" cfg then cfg.postStart else ""}
      '';

in {

  options = {
    systemd.services = mkOption {
      default = {};
    }; # TODO make more specific
  };

  config = {
    userNix.startScripts."1-systemd-oneshot" = concatMapStrings (name: "${configToCommand name (getAttr name oneShotServices)}\n") (attrNames oneShotServices);

    supervisord.services = listToAttrs (map (name:
      let
        cfg = getAttr name services;
      in
        {
          name = name;
          value = {
            command = pkgs.writeScript "${name}-run" (configToCommand name cfg);
            environment = (if hasAttr "environment" cfg then cfg.environment else {}) //
              (if hasAttr "path" cfg then
                { PATH = "%(ENV_PATH)s:" + concatStringsSep ":" (map (prg: "${prg}/bin") cfg.path); }
               else {
               PATH="%(ENV_PATH)s"; });
          };
        }
      ) (attrNames runServices));
  };
}