`:gen_tcp.recv/3` requires a passive socket and returns `:einval` if
the socket is in any active mode. We were opening with
`active: :once`, sending CONNECT, then calling `:gen_tcp.recv` to
read CONNACK — which kernel-rejected every time.
The visible symptom was "Pskr.Client connect failed: :einval" every
30s on the elected leader, while a manual `:gen_tcp.connect` from
`bin/microwaveprop rpc` worked because it never tried to recv. Each
failed attempt also leaked a port (connect succeeded; recv failed
later), and we'd accumulated ~27 dangling sockets on the leader
process.
Open the socket in `active: false`, finish the synchronous
CONNECT/CONNACK/SUBSCRIBE handshake with `:gen_tcp.recv`, then
`:inet.setopts(active: :once)` so PUBLISHes arrive as
`{:tcp, socket, data}` messages going forward. Also close the
socket on any handshake failure so retries stop leaking ports.
Graham McIntire
5521b0b153