Duplicate file is closed when original file is closed

[slide]

From @slozier on October 30, 2016 23:58

This works with CPython. IronPython fails with ValueError: I/O operation on closed file.

import os

with open("temp.txt", "wb+") as f:
    f.write("test")
    f.seek(os.SEEK_SET, 0)
    x = os.dup(f.fileno())
    g = os.fdopen(x, "rb")
assert g.read() == "test"

Copied from original issue: IronLanguages/main#1512