Richard, apart from performance, what's the advantage of this approach over the fork+exec version?
Because it seems more complicated, and although I didn't have a look a this last patch, I guess that most of the fork+exec version could be factorized with the Windows version, no?
Since it's only intented to be used as a "debugging"/special-purpose replacement - it would probably be better if it could be made as simple as possible. Also, as you've noted, FD passing isn't supported by all Unices out there (and we've had some reliability issues on OS-X, too).