Hello,
it looks to me like this comment is wrong:

gnulib/lib/passfd.c:
101 /* recvfd receives a file descriptor through the socket.
102    The flags are a bitmask, possibly including O_CLOEXEC (defined in <fcntl.h>).
103 
104    Return 0 on success, or -1 with errno set in case of error.
105 */
106 int
107 recvfd (int sock, int flags)

It returns the fd, not 0, on success, right?

Jann