Skip bidirectional data check for permanent fast-forward sessions

Allow permanent fast-forward sessions (SESSION_FORWARD_TYPE_PERMANENT)
to continue processing when bidirectional data flow is detected,
instead of treating it as a fatal error. This prevents unnecessary
session termination in these specific cases while maintaining the
original strict validation for all other session types.

lib/mysql_data_stream.cpp

@@ -451,10 +451,15 @@ void MySQL_Data_Stream::shut_hard() {
 
 void MySQL_Data_Stream::check_data_flow() {
 	if ( (PSarrayIN->len || queue_data(queueIN) ) && ( PSarrayOUT->len || queue_data(queueOUT) ) ){
-		// there is data at both sides of the data stream: this is considered a fatal error
-		proxy_error("Session=%p, DataStream=%p -- Data at both ends of a MySQL data stream: IN <%d bytes %d packets> , OUT <%d bytes %d packets>\n", sess, this, PSarrayIN->len , queue_data(queueIN) , PSarrayOUT->len , queue_data(queueOUT));
-		shut_soft();
-		generate_coredump();
+		if (sess && sess->status == FAST_FORWARD && sess->session_fast_forward == SESSION_FORWARD_TYPE_PERMANENT) {
+			// Permanent fast-forward sessions: log warning but continue
+			proxy_warning("Session=%p, DataStream=%p -- Data at both ends of a MySQL data stream: IN <%d bytes %d packets> , OUT <%d bytes %d packets>\n", sess, this, queue_data(queueIN), PSarrayIN->len, queue_data(queueOUT), PSarrayOUT->len);
+		} else {
+			// All other sessions: treat as fatal error
+			proxy_error("Session=%p, DataStream=%p -- Data at both ends of a MySQL data stream: IN <%d bytes %d packets> , OUT <%d bytes %d packets>\n", sess, this, queue_data(queueIN), PSarrayIN->len, queue_data(queueOUT), PSarrayOUT->len);
+			shut_soft();
+			generate_coredump();
+		}
 	}
 	if ((myds_type==MYDS_BACKEND) && myconn && (myconn->fd==0) && (revents & POLLOUT)) {
 		int rc;