fix(pskr): handshake on passive socket, switch to active after SUBSCRIBE

`: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.

lib/microwaveprop/pskr/client.ex

@@ -178,22 +178,40 @@ defmodule Microwaveprop.Pskr.Client do
   # ---------- Connection setup ----------
 
   defp do_connect(state) do
+    # Open in passive mode for the synchronous CONNECT/CONNACK
+    # handshake — `:gen_tcp.recv/3` only works on passive sockets
+    # and returns `:einval` if the socket is in any active mode.
+    # After CONNACK we flip to `active: :once` so PUBLISHes arrive
+    # as `{:tcp, socket, data}` messages.
+    #
     # `:inet` forces IPv4 resolution + an IPv4 socket. Without
-    # this, BEAM can pick the broker's AAAA record while our K8s
-    # pod has IPv4-only egress, and `:gen_tcp.connect/4` fails
-    # with `:einval` from the kernel `connect()` syscall.
-    tcp_opts = [:binary, :inet, active: :once, packet: :raw, keepalive: true]
+    # this, BEAM can pick a broker AAAA record while our K8s pod
+    # has IPv4-only egress.
+    tcp_opts = [:binary, :inet, active: false, packet: :raw, keepalive: true]
 
-    with {:ok, socket} <- :gen_tcp.connect(@broker_host, @broker_port, tcp_opts, 10_000),
-         :ok <- :gen_tcp.send(socket, Mqtt.connect(state.client_id, @keepalive_seconds)),
+    case :gen_tcp.connect(@broker_host, @broker_port, tcp_opts, 10_000) do
+      {:ok, socket} -> handshake(socket, state)
+      {:error, _} = err -> err
+    end
+  end
+
+  defp handshake(socket, state) do
+    with :ok <- :gen_tcp.send(socket, Mqtt.connect(state.client_id, @keepalive_seconds)),
          {:ok, :connected, rest} <- await_connack(socket, <<>>),
-         :ok <- :gen_tcp.send(socket, subscribe_packet(state.bands)) do
+         :ok <- :gen_tcp.send(socket, subscribe_packet(state.bands)),
+         :ok <- :inet.setopts(socket, active: :once) do
       Logger.info("Pskr.Client connected as #{state.client_id}, subscribed to #{length(state.bands)} bands")
       # `rest` holds anything the broker sent piggybacked after
       # CONNACK — feed it back into the buffer so we don't lose
       # the SUBACK or an early PUBLISH.
       send(self(), {:tcp, socket, rest})
       {:ok, socket, schedule_ping()}
+    else
+      {:error, _} = err ->
+        # Close the connected-but-not-handshaked socket so we
+        # don't leak ports across retries.
+        _ = :gen_tcp.close(socket)
+        err
     end
   end