doc/nixos/postgress: Streamline upgrade script

{ config, pkgs, ... }:

{

  <xref linkend="opt-environment.systemPackages" /> = [

    (pkgs.writeScriptBin "upgrade-pg-cluster" ''

    (let

      # XXX specify the postgresql package you'd like to upgrade to.

      newPostgres = pkgs.postgresql_13;

    in pkgs.writeScriptBin "upgrade-pg-cluster" ''

      set -eux

      # XXX it's perhaps advisable to stop all services that depend on postgresql

      systemctl stop postgresql

      # XXX replace `<new version>` with the psqlSchema here

      export NEWDATA="/var/lib/postgresql/<new version>"

      export NEWDATA="/var/lib/postgresql/${newPostgres.psqlSchema}"

      # XXX specify the postgresql package you'd like to upgrade to

      export NEWBIN="${pkgs.postgresql_13}/bin"

      export NEWBIN="${newPostgres}/bin"

      export OLDDATA="${config.<xref linkend="opt-services.postgresql.dataDir"/>}"

      export OLDBIN="${config.<xref linkend="opt-services.postgresql.package"/>}/bin"