nixpkgs/modules/tasks/swap.nix

{pkgs, config, ...}:

let

###### interface

  options = {

    swapDevices = pkgs.lib.mkOption {
      default = [];
      example = [
        { device = "/dev/hda7"; }
        { device = "/var/swapfile"; }
        { label = "bigswap"; }
      ];
      description = "
        The swap devices and swap files.  These must have been
        initialised using <command>mkswap</command>.  Each element
        should be an attribute set specifying either the path of the
        swap device or file (<literal>device</literal>) or the label
        of the swap device (<literal>label</literal>, see
        <command>mkswap -L</command>).  Using a label is
        recommended.
      ";
    };

  };


###### implementation

  inherit (pkgs) utillinux lib;

  swapDevices = config.swapDevices;

  devicesByPath =
    map (x: x.device) (lib.filter (x: x ? device) swapDevices);
    
  devicesByLabel =
    map (x: x.label) (lib.filter (x: x ? label) swapDevices);

in


{
  require = [options];

  services.extraJobs = [{
    name = "swap";

    job = ''
      start on startup
      start on new-devices

      script
          for device in ${toString devicesByPath}; do
              ${utillinux}/sbin/swapon "$device" || true
          done

          for label in ${toString devicesByLabel}; do
              ${utillinux}/sbin/swapon -L "$label" || true
          done

          # Remove swap devices not listed in swapDevices.
          # !!! disabled because it doesn't work with labels
          #for used in $(cat /proc/swaps | grep '^/' | sed 's/ .*//'); do
          #    found=
          #    for device in $ {toString swapDevices}; do
          #        if test "$used" = "$device"; then found=1; fi
          #    done
          #    if test -z "$found"; then
          #        ${utillinux}/sbin/swapoff "$used" || true
          #    fi
          #done

      end script
    '';
  }];
}