I have this dataframe:

import polars as pl
dff = pl.DataFrame({'file':['a','a','a','a','b','b'],
                    'ru':['fe','fe','ev','ev','ba','br'],
                    'rt':[0,0,1,1,1,0],
                    })
dff

With the values:

file    ru  rt
"a" "fe"    0
"a" "fe"    0
"a" "ev"    1
"a" "ev"    1
"b" "ba"    1
"b" "br"    0

I’d like to update all ru and rt fields within the same file field, with the values of the first record, if the rt value of the first record rt == 0.
So the desired output would look like:

file    ru  rt
"a" "fe"    0
"a" "fe"    0
"a" "fe"    0
"a" "fe"    0
"b" "ba"    1
"b" "br"    0

How can I achieve that?