We were closing before calling del_fd, which also closed.

The shutdown() logic applies when a child and parent are using the
*same* socket fd to communicate to each other.  That's really unusual
(who would you connect to?), and should probably be done by the user.

Generally, you'd use socketpair() for this child-parent case.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>