If I understand this correctly, spin should be written as:

spin = do
    block $ do
        (t, _) <- accept s
        unblock (forkIO $ doStuff t) `finally` sClose t
    spin

I think the `finally` portion should be done in the forked process context. Otherwise once the process is forked, the socket gets closed by the parent process. Something more along the lines of:

unblock (forkIO $ doStuff t `finally` sClose t)