Then here's a hopeful fix for the Windows situation that relies on the socketpair() operation reusing FDs from the lowest value. I'm adding asserts to check that this is actually the case. (These are actual assert statements to indicate that they are verifying an assumption internal to the test, not verifying the functionality under test.)

I'll test it when I next get near a Windows box (Monday in the office) -- or someone else with Windows access can let me know.