OpenSSL: fix spurious SSL connection aborts

Description

Was seeing spurious SSL connection aborts using libmosquitto and OpenSSL. I tracked it down to uncleared error state on the OpenSSL error stack - patch attached deals with that.

Rough idea of problem:

Code that uses libmosquitto calls some library that uses OpenSSL but don't clear the OpenSSL error stack after an error. lib/net_mosq.c calls SSL_read which eventually gets an EWOULDBLOCK from the OS. Returns -1 to indicate an error lib/net_mosq.c calls SSL_get_error. First thing, SSL_get_error calls ERR_get_error to check the OpenSSL error stack, finds an old error and returns SSL_ERROR_SSL instead of SSL_ERROR_WANT_READ or SSL_ERROR_WANT_WRITE.

lib/net_mosq.c returns an error and aborts the connection

Solution:

Clear the openssl error stack before calling SSL_* operation if we're going to call SSL_get_error afterwards.

Notes:

This is much more likely to happen with multi because it's easier to intersperse other calls to the OpenSSL library in the same thread.

Thank you for contributing your time to the Mosquitto project!

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The problem appears after #commit.

eclipse / mosquitto