Jeff King [off-list ref] writes:

It looks like we'd have the option of either adding F_GETFL/F_SETFL
support to compat/mingw.c's fake fcntl() function, or adding a
compat/nonblock.c that abstracts the whole thing. I'd prefer the latter,
as it makes the interface a bit narrower.

I agree that "make this non-blocking" is the preferred interface
over "emuilate fcntl".

But I'm not sure what should go on the Windows side of that #ifdef.
Unlike some other spots, I don't think we can just make it a noop, or
Windows will be subject to the same deadlock (unless for some reason its
write() does behave differently?).

Let them deadlock so that they can fix it, and until then leave it a
noop?  That may break the CI tests for them so we could hide the
known-to-be-broken test behind prerequisite to buy them time,
perhaps?