Infinite loop in mqtt

[cmeister2]

I did this

In one tab:

nc -l 127.0.0.1 1883 -c 'printf "\x91\x02"'

In another tab:

docker run --rm --network host curlimages/curl:8.18.0 -v mqtt://127.0.0.1/H

Observe that netcat returns immediately, but curl enters an infinite loop until forcefully terminated:

* mqtt_doing: state [2]
* mqtt_doing: state [2]
* mqtt_doing: state [2]
* mqtt_doing: state [2]
* mqtt_doing: state [2]
* mqtt_doing: state [2]
* mqtt_doing: state [2]
* mqtt_doing: state [2]
* mqtt_doing: state [2]
* mqtt_doing: state [2]
* mqtt_doing: state [2]
* mqtt_doing: state [2]
* mqtt_doing: state [2]
^C
got 3 SIGTERM/SIGINTs, forcefully exiting

I expected the following

curl to not enter an infinite loop

curl/libcurl version

curl 8.18.0 (and DEV)

operating system

WSL / ubi9

[cmeister2]

Codex proposes:

In mqtt.c inside mqtt_recv_atleast(), treat nread == 0 (EOF / no more peer data) as an error (CURLE_RECV_ERROR) instead of falling through to CURLE_AGAIN.

Right now, that path returns CURLE_AGAIN, which later gets normalized to OK (mqtt.c:946) so mqtt_doing keeps re-entering and spins in state 2 (MQTT_CONNACK).

Net effect: malformed/incomplete handshake exits fast instead of timing out in a loop.

Optional hardening on top: also reject unexpected first packet type when waiting for CONNACK, so 0x91 fails immediately as protocol error.

[icing]

Propose fix in #20830