A request handler that leaves the response code unset asks for no
response to be sent. That is honoured for a piggybacked ACK (the token
gets stripped, so an empty ACK goes out) and for a non-confirmable or
reliable response (dropped in no_response()), but not for a separate
response set up with coap_register_async().
There the response is turned into a CON, and a 0.00 CON was put on the
wire. An Empty message is a ping (RFC 7252 4.2), not a response, and the
PDU still carries the request's token, which RFC 7252 4.1 does not allow.
Being confirmable, it was then retransmitted up to MAX_RETRANSMIT times,
usually at a client port that had long gone away.
Drop it like the other two cases, delete it without logging it as a
dropped response since the application asked for this, and document in
coap_handler(3) that a separate response is not sent either.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
