Verify it works correctly on Apache/Linux
Making this an issue so I have a concrete place to say "Yes, this happened."
It would appear that the check performed by Server.isCGI is returning the wrong value under Apache on Ubuntu. When invoked under cgi-bin with only mod_cgi enabled, it still returns success (with an AF_UNIX address of 0.0.0.0:0), instead of ENOTSOCK.
I've verified that it is not actually running in FastCGI mode because attempts to accept a connection on stdin fail with EINVAL.
Rather, I should say that it is returning the right thing (because Apache is communicating with the CGI process via a local socket; double-check whether I'm using mod_cgi or mod_cgid), but this breaks the test.