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> Would it be possible to modify FreeBSD to enable it by default? Or is there a reason to not enable it by default?
That's very unlikely to happen. I believe it was added as an opt-in feature for some linux compatibility situations. The correct solution is to use closefrom(2), as it is the optimal current solution, and in our case, safe to use.
> Which patch? https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=221700#c3 or something else?
Yep, that one.
> It seems like closefrom() is available on:
Per #8052 there was some concerns about closefrom(2) not being async-signal safe. [1] 
I can't speak to other implementations, but this is what I requested clarity from our FreeBSD developers on, and confirmed that our implementation is indeed safe. That is the reason why I thought scoping closefrom(2) to __FreeBSD__ may be warranted, just like like the linux specific bits in https://hg.python.org/cpython/rev/61aa484a3e54 from #8052
But I'll leave the decision as to how its implemented (configure checkls, ifdef'd or not) in your capable hands.
Summary: FreeBSD's closefrom(2) is safe to anywhere in Python where it needs to close a range of fd's, including the subprocess module.
[1] https://bugs.python.org/issue8052#msg132307