Manually set 'POLLOUT' event for 'SHOW_WARNINGS' session status to ensure sending by 'MySQL_Connection' #3606

lib/MySQL_Session.cpp

@@ -4646,6 +4646,13 @@ handler_again:
 			{
 				MySQL_Data_Stream *myds=mybe->server_myds;
 				MySQL_Connection *myconn=myds->myconn;
+
+				// Setting POLLOUT is required just in case this state has been reached when 'RunQuery' from
+				// 'PROCESSING_QUERY' state has immediately return. This is because in case 'mysql_real_query_start'
+				// immediately returns with '0' the session is never processed again by 'MySQL_Thread', and 'revents' is
+				// never updated with the result of polling through the 'MySQL_Thread::mypolls'.
+				myds->revents |= POLLOUT;
+
 				int rc = myconn->async_query(
 					mybe->server_myds->revents,(char *)"SHOW WARNINGS", strlen((char *)"SHOW WARNINGS")
 				);