Summary

engine::purge_error_state() calls ERR_clear_error() before every SSL read and write operation. In normal conditions, the OpenSSL error queue is empty, making the clear unnecessary.

An ERR_peek_error() guard was already present for OpenSSL <1.1, but is missing for the newer version we use.

Proposed Fix

Add an ERR_peek_error() guard for OpenSSL 1.1+

See changes in this patch. 

ERR_peek_error() is a read-only peek at the head of the thread-local error queue. If it returns 0, the queue is empty and there's nothing to clear. 

Expected Impact

We saw a 1.7% throughput improvement in our 100 read YCSB test (multipatch)