The same socketpair Matthew Green pointed out is also (possibly) 
   uni-directional?

no, i was pointing out the old sys_pipe() call that is a frontend
to the socketpair code.  see kern/uipc_syscalls.c:sys_pipe():


	if ((error = falloc(p, &rf, &fd)) != 0)
		goto free2;
	retval[0] = fd;
	rf->f_flag = FREAD;
	rf->f_type = DTYPE_SOCKET;
	rf->f_ops = &socketops;
	rf->f_data = (caddr_t)rso;
	if ((error = falloc(p, &wf, &fd)) != 0)
		goto free3;
	wf->f_flag = FWRITE;
	wf->f_type = DTYPE_SOCKET;
	wf->f_ops = &socketops;
	wf->f_data = (caddr_t)wso;
	retval[1] = fd;
	if ((error = unp_connect2(wso, rso)) != 0)
		goto free4;


socketpair(2) itself is correctly bi-directional!