athens/modules/postgresql/default.nix

{ pkgs, config, ... }: {
  services = {
    postgresql = {
      enable = true;
      package = pkgs.postgresql_12;
    };

    postgresqlBackup = {
      enable = true;
    };
  };

  # Postgres upgrades: https://nixos.org/manual/nixos/stable/index.html#module-services-postgres-upgrading
  environment.systemPackages = [
    (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/12"

      # XXX specify the postgresql package you'd like to upgrade to
      export NEWBIN="${pkgs.postgresql_12}/bin"

      export OLDDATA="${config.services.postgresql.dataDir}"
      export OLDBIN="${config.services.postgresql.package}/bin"

      install -d -m 0700 -o postgres -g postgres "$NEWDATA"
      cd "$NEWDATA"
      sudo -u postgres $NEWBIN/initdb -D "$NEWDATA"

      sudo -u postgres $NEWBIN/pg_upgrade \
        --old-datadir "$OLDDATA" --new-datadir "$NEWDATA" \
        --old-bindir $OLDBIN --new-bindir $NEWBIN \
        "$@"
    '')
  ];
}