pal inconsistency among implementations

[yamt]

consider the underlying socket is non-blocking.

1. the socket version of mqtt_pal_sendall seems to return
   MQTT_ERROR_SOCKET_ERROR on any errors, including EAGAIN.

   other implementations look similar.
   except the mbedtls version, which returns the bytes sent in case of EAGAIN-like conditions.

   IMO, the behavior of mbedtls version is better because
   __mqtt_send needs to know who many bytes has been sent successfully.
   (to update send_offset)

2. the socket version of mqtt_pal_recvall ignores EAGAIN and keeps busy looping.
   the mbedtls version of it returns the bytes read on EAGAIN-like conditions.
   IMO, mbedtls behavior is better, to give the caller a chance to perform poll()-wait.

i think the ideal behavior (both fo send and recv) is:

• on an error, if some bytes have been sent/received already, return the number of bytes
• otherwise, if the error is EAGAIN equivalent, return 0. (or something distinguishable, like MQTT_ERROR_SOCKET_AGAIN)
• otherwise, return MQTT_ERROR_SOCKET_ERROR.

[LiamBindle]

Thanks for raising this @yamt. Sorry for my delayed response. I like your suggestion. Are you able to make a PR for this?

[yamt]

i guess i can submit a patch. but the only implementation i can actually test by myself is mbedtls.

[yamt]

mbedtls and unix socket: #156