setup-systemd-units.nix

# | Build a script to install and start a set of systemd units on any
# systemd-based system.
#
# Creates a symlink at /etc/systemd-static/${namespace} for slightly
# improved atomicity.
{ writeScriptBin
, bash
, coreutils
, systemd
, runCommand
, lib
}:
  { units     # : AttrSet String (Either Path { path : Path, wanted-by : [ String ] })
              # ^ A set whose names are unit names and values are
              # either paths to the corresponding unit files or a set
              # containing the path and the list of units this unit
              # should be wanted-by (none by default).
              #
              # The names should include the unit suffix
              # (e.g. ".service")
  , namespace # : String
              # The namespace for the unit files, to allow for
              # multiple independent unit sets managed by
              # `setupSystemdUnits`.
  }:
    let static = runCommand "systemd-static" {}
          ''
            mkdir -p $out
            ${lib.concatStringsSep "\n" (lib.mapAttrsToList (nm: file:
                "ln -sv ${file.path or file} $out/${nm}"
             ) units)}
          '';
        add-unit-snippet = name: file:
          ''
            oldUnit=$(readlink -f "$unitDir/${name}" || echo "$unitDir/${name}")
            if [ -f "$oldUnit" -a "$oldUnit" != "${file.path or file}" ]; then
              unitsToStop+=("${name}")
            fi
            ln -sf "/etc/systemd-static/${namespace}/${name}" \
              "$unitDir/.${name}.tmp"
            mv -T "$unitDir/.${name}.tmp" "$unitDir/${name}"
            ${lib.concatStringsSep "\n" (map (unit:
                ''
                  mkdir -p "$unitDir/${unit}.wants"
                  ln -sf "../${name}" \
                    "$unitDir/${unit}.wants/.${name}.tmp"
                  mv -T "$unitDir/${unit}.wants/.${name}.tmp" \
                    "$unitDir/${unit}.wants/${name}"
                ''
              ) file.wanted-by or [])}
            unitsToStart+=("${name}")
          '';
    in
      writeScriptBin "setup-systemd-units"
        ''
          #!${bash}/bin/bash -e
          export PATH=${coreutils}/bin:${systemd}/bin

          unitDir=/etc/systemd/system
          if [ ! -w "$unitDir" ]; then
            unitDir=/nix/var/nix/profiles/default/lib/systemd/system
            mkdir -p "$unitDir"
          fi
          declare -a unitsToStop unitsToStart

          oldStatic=$(readlink -f /etc/systemd-static/${namespace} || true)
          if [ "$oldStatic" != "${static}" ]; then
            ${lib.concatStringsSep "\n"
                (lib.mapAttrsToList add-unit-snippet units)}
            if [ ''${#unitsToStop[@]} -ne 0 ]; then
              echo "Stopping unit(s) ''${unitsToStop[@]}" >&2
              systemctl stop "''${unitsToStop[@]}"
            fi
            mkdir -p /etc/systemd-static
            ln -sfT ${static} /etc/systemd-static/.${namespace}.tmp
            mv -T /etc/systemd-static/.${namespace}.tmp /etc/systemd-static/${namespace}
            systemctl daemon-reload
            echo "Starting unit(s) ''${unitsToStart[@]}" >&2
            systemctl start "''${unitsToStart[@]}"
          else
            echo "Units unchanged, doing nothing" >&2
          fi
        ''