Etienne: since this is about solving a 3.x specific problem, it will not get backported.  Issue 1573931 looks unrelated to me at a quick glance.  FYI, you will find that you *do* have detach in 2.7 if you open a file using the io subsystem (import io).  Of course, that isn't used for the std files in 2.7.

Glen: the new IO subsystem is a complete C layer on top of only the most basic of the C runtime stuff.  It does handle cross platform issues.  Given that, and given that the input to CGI *should* be bytes, I think letting an error raise if the stream is text and detatch isn't available is fine, though we might find we want to catch it to improve the error message with extra context.

Pierre: yes, that diff is what I was looking for.  I hope to have time to look it over later today.