{
  writeShellApplication,
  postgresql_15,
  postgresql_16,
  ...
}:

let
  newPostgres = postgresql_16;
  oldPostgres = postgresql_15;
in
writeShellApplication {
  name = "upgrade-pg-cluster";
  # Only the new one should be in scope, because that is the
  # one that we will be referencing directly with the sudo command
  # down farther.
  runtimeInputs = [ newPostgres ];
  text = ''
    systemctl stop postgresql

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

    export OLDDATA="/var/lib/postgresql/${oldPostgres.psqlSchema}"
    export OLDBIN="${oldPostgres}/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 "$NEWBIN" \
        "$@"
  '';
}