emqx / esockd / 599

67%

master: 67%

Pull #228

github

feat: refund connection-rate token for connections that die before engaging

A connection whose peer FIN/RSTs before any application data was exchanged
consumes a connection-rate token at accept but never engages: under a storm
of dead sockets the limiter bucket can be driven into debt, pausing every
acceptor even though no real connection ever established.

Refund such tokens:
- esockd_limiter:refund/1 returns one token only while the bucket is in debt
  (tokens < 0), so no free tokens are ever created; missing buckets are no-op.
- esockd_transport:upgrade/2 exits the connection process with the
  {shutdown, {pre_establishment, Reason}} marker when an upgrade fun (e.g.
  the TLS handshake) fails with a peer-gone reason (closed/einval/enotconn),
  so TLS dead connections are detected by esockd itself, without any upper
  layer cooperation.
- esockd_connection_sup unwraps the marker: the inner reason is counted in
  shutdown_count as before, and the token is refunded. set_conn_limiter/2
  hands the limiter bucket to the sup (process dictionary, hot-upgrade safe).
- esockd_listener_sup wires the bucket to the connection sup at listener
  start and on set_max_conn_rate reconfiguration.

The emqx side (emqx_connection) produces the same marker for plain-TCP
connections that die before their first bytes (see the paired PR).

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