MQTTClient_publishMessage() seems to get stucked on MQTTClient_yield()

[svermigo]

It seems we have an issue similar to #774

MQTTClient_publishMessage() seems to get stucked on MQTTClient_yield().

Tested on versions: 1.3.0 and 1.3.2

To Reproduce we just need to exchange a bunch of data between client/server in a short time. It seems we will get an SSLSocket error during sending and that causes the hang of the publish method.

Product name: Eclipse Paho Synchronous MQTT C Client Library
Version: 1.3.2
Build level: 2020-04-30T06:31:15Z
OpenSSL version: OpenSSL 1.0.2p 14 Aug 2018
OpenSSL flags: compiler: arm-poky-linux-gnueabi-gcc -march=armv7-a -mfpu=neon -mfloat-abi=hard -mcpu=cortex-a8 -DL_ENDIAN -DTERMIO -O2 -pipe -g -feliminate-unused-debug-types -Wall -Wa,--noexecstack
OpenSSL build timestamp: built on: reproducible build, date unspecified
OpenSSL platform: platform: linux-armv4
OpenSSL directory: OPENSSLDIR: "/usr/lib/ssl"
/proc/version: Linux version 4.9.218-noah (oe-user@oe-host) (gcc version 7.3.0 (GCC) ) #1 PREEMPT Thu Apr 9 14:50:52 UTC 2020

The full logs of connection attached down below:
084821.log
092735.log

[icraggs]

It looks like from the first log that the last entry from the background thread is:

20200504 085023.511 Calling messageArrived for client hmir6-b12345-c123456, queue depth 1

and that the application doesn't return from the messageArrived callback. If this is the case, then it will cause the library to block.

Same for the second log at:

20200504 092830.664 Calling messageArrived for client hmir6-b12345-c123456, queue depth 1

The SSL error want_read/want_write is normal socket buffering.

[svermigo]

I think I got that .. so that means, that a message, which arrives while publishing another message must be handled first and then the publishing will finish?

[icraggs]

The application must return from any callback promptly, otherwise other background processing, like the sending of messages, may be blocked.

[svermigo]

Great :) .. Thank you mr. Craggs. I believe it will solve my issue

[ssuraci]

I've the same issue (tested on v1.3.1): got error -22 (TCPSOCKET_INTERRUPTED) on MQTTClient_publishMessage and subsequent call to MQTTClient_publishMessage got stuck in MQTTClient_yield(). Application is send-only so no callback for incoming messages was called. Now trying to force a disconnect in case of MQTTClient_publishMessage error.