MySQL: Trace session-specific system variables changes

wazir-ahmed · wazir-ahmed · commit 934709ee175f · 2025-10-13T13:39:23.000+05:30
- Configure `session_track_system_variables` and `session_track_state_change`
  in all backend connections.
- Utilize notifications from backend servers to capture system variable changes
  that cannot be handled by `MySQL_Set_Stmt_Parser`
- Update both client and server variable maps based on backend responses.

Signed-off-by: Wazir Ahmed &lt;wazir@proxysql.com&gt;
diff --git a/include/MySQL_Session.h b/include/MySQL_Session.h
@@ -224,13 +224,15 @@ class MySQL_Session: public Base_Session<MySQL_Session, MySQL_Data_Stream, MySQL
 	bool handler_again___verify_ldap_user_variable();
 	bool handler_again___verify_backend_autocommit();
 	bool handler_again___verify_backend_session_track_gtids();
+	bool handler_again___verify_backend_session_track_variables();
 	bool handler_again___verify_backend_multi_statement();
 	bool handler_again___verify_backend_user_schema();
 	bool handler_again___verify_multiple_variables(MySQL_Connection *);
 	bool handler_again___status_SETTING_INIT_CONNECT(int *);
 	bool handler_again___status_SETTING_LDAP_USER_VARIABLE(int *);
 	bool handler_again___status_SETTING_SQL_MODE(int *);
 	bool handler_again___status_SETTING_SESSION_TRACK_GTIDS(int *);
+	bool handler_again___status_SETTING_SESSION_TRACK_VARIABLES(int *);
 	bool handler_again___status_CHANGING_CHARSET(int *_rc);
 	bool handler_again___status_CHANGING_SCHEMA(int *);
 	bool handler_again___status_CONNECTING_SERVER(int *);
@@ -279,6 +281,7 @@ class MySQL_Session: public Base_Session<MySQL_Session, MySQL_Data_Stream, MySQL
 	int RunQuery(MySQL_Data_Stream *myds, MySQL_Connection *myconn);
 	void handler___status_WAITING_CLIENT_DATA();
 	void handler_rc0_Process_GTID(MySQL_Connection *myconn);
+	void handler_rc0_Process_Variables(MySQL_Connection *myconn);
 	void handler_rc0_RefreshActiveTransactions(MySQL_Connection* myconn);
 	void handler___status_WAITING_CLIENT_DATA___STATE_SLEEP___MYSQL_COM_INIT_DB_replace_CLICKHOUSE(PtrSize_t& pkt);
 	void handler___status_WAITING_CLIENT_DATA___STATE_SLEEP___MYSQL_COM_QUERY___not_mysql(PtrSize_t& pkt);
diff --git a/include/mysql_connection.h b/include/mysql_connection.h
@@ -89,6 +89,7 @@ class MySQL_Connection {
 		char *ldap_user_variable;
 		char *ldap_user_variable_value;
 		bool session_track_gtids_sent;
+		bool session_track_variables_sent;
 		bool ldap_user_variable_sent;
 		uint8_t protocol_version;
 		int8_t last_set_autocommit;
@@ -262,6 +263,7 @@ class MySQL_Connection {
 	void reset();
 
 	bool get_gtid(char *buff, uint64_t *trx_id);
+	bool get_variables(std::unordered_map<std::string, std::string>&);
 	void reduce_auto_increment_delay_token() { if (auto_increment_delay_token) auto_increment_delay_token--; };
 
 	bool match_ff_req_options(const MySQL_Connection *c);
diff --git a/include/proxysql_structs.h b/include/proxysql_structs.h
@@ -310,6 +310,7 @@ enum session_status {
 	SETTING_NEXT_TRANSACTION_READ,
 	PROCESSING_EXTENDED_QUERY_SYNC,
 	RESYNCHRONIZING_CONNECTION,
+	SETTING_SESSION_TRACK_VARIABLES,
 	session_status___NONE // special marker
 };
 
diff --git a/lib/MySQL_Session.cpp b/lib/MySQL_Session.cpp
@@ -1923,6 +1923,16 @@ bool MySQL_Session::handler_again___verify_backend_session_track_gtids() {
 	return ret;
 }
 
+bool MySQL_Session::handler_again___verify_backend_session_track_variables() {
+	if (mybe->server_myds->myconn->options.session_track_variables_sent == false) {
+		mybe->server_myds->myconn->options.session_track_variables_sent = true;
+		set_previous_status_mode3();
+		NEXT_IMMEDIATE_NEW(SETTING_SESSION_TRACK_VARIABLES);
+	}
+
+	return false;
+}
+
 
 bool MySQL_Session::handler_again___verify_multiple_variables(MySQL_Connection* myconn) {
 	for (auto i = 0; i < SQL_NAME_LAST_LOW_WM; i++) {
@@ -2535,6 +2545,12 @@ bool MySQL_Session::handler_again___status_SETTING_GENERIC_VARIABLE(int *_rc, co
 		free(query);
 		query=NULL;
 	}
+
+	if (rc == 0 && strncasecmp(var_name, "session_track_system_variables", strlen("session_track_system_variables")) == 0) {
+		char *q = (char *)"SET session_track_state_change=ON";
+		rc = myconn->async_send_simple_command(myds->revents, q, strlen(q));
+	}
+
 	if (rc==0) {
 		myds->revents|=POLLOUT;	// we also set again POLLOUT to send a query immediately!
 		myds->DSS = STATE_MARIADB_GENERIC;
@@ -2723,6 +2739,13 @@ bool MySQL_Session::handler_again___status_SETTING_SESSION_TRACK_GTIDS(int *_rc)
 	return ret;
 }
 
+bool MySQL_Session::handler_again___status_SETTING_SESSION_TRACK_VARIABLES(int *_rc) {
+	bool ret=false;
+	assert(mybe->server_myds->myconn);
+	ret = handler_again___status_SETTING_GENERIC_VARIABLE(_rc, (char *)"session_track_system_variables", "*", false);
+	return ret;
+}
+
 bool MySQL_Session::handler_again___status_CHANGING_SCHEMA(int *_rc) {
 	bool ret=false;
 	//fprintf(stderr,"CHANGING_SCHEMA\n");
@@ -4829,6 +4852,28 @@ void MySQL_Session::handler_rc0_Process_GTID(MySQL_Connection *myconn) {
 	}
 }
 
+void MySQL_Session::handler_rc0_Process_Variables(MySQL_Connection *myconn) {
+	std::unordered_map<string, string> var_map;
+
+	if(myconn->get_variables(var_map)) {
+		const char *variable = nullptr;
+		const char *value = nullptr;
+
+		for (int idx = 0 ; idx < SQL_NAME_LAST_HIGH_WM ; idx++) {
+			variable = mysql_tracked_variables[idx].set_variable_name;
+
+			auto itr = var_map.find(variable);
+			if(itr != var_map.end()) {
+				value = itr->second.c_str();
+				proxy_debug(PROXY_DEBUG_MYSQL_CONNECTION, 7, "Session=%p, backend=%p. Notification for session_track_system_variables: variable=%s, value=%s\n", this, this->mybe, variable, value);
+
+				mysql_variables.client_set_value(this, idx, value);
+				mysql_variables.server_set_value(this, idx, value);
+			}
+		}
+	}
+}
+
 void MySQL_Session::handler_KillConnectionIfNeeded() {
 	if ( // two conditions
 		// If the server connection is in a non-idle state (ASYNC_IDLE), and the current time is greater than or equal to mybe->server_myds->wait_until
@@ -5026,6 +5071,10 @@ int MySQL_Session::handler() {
 									goto handler_again;
 								}
 
+								if (handler_again___verify_backend_session_track_variables()) {
+									goto handler_again;
+								}
+
 								// Optimize network traffic when we can use 'SET NAMES'
 								if (verify_set_names(this)) {
 									goto handler_again;
@@ -5106,6 +5155,8 @@ int MySQL_Session::handler() {
 
 					handler_rc0_Process_GTID(myconn);
 
+					handler_rc0_Process_Variables(myconn);
+
 					// if we are locked on hostgroup, the value of autocommit is copied from the backend connection
 					// see bug #3549
 					if (locked_on_hostgroup >= 0) {
@@ -5402,6 +5453,9 @@ bool MySQL_Session::handler_again___multiple_statuses(int *rc) {
 		case SETTING_SESSION_TRACK_GTIDS:
 			ret = handler_again___status_SETTING_SESSION_TRACK_GTIDS(rc);
 			break;
+		case SETTING_SESSION_TRACK_VARIABLES:
+			ret = handler_again___status_SETTING_SESSION_TRACK_VARIABLES(rc);
+			break;
 		case SETTING_SET_NAMES:
 			ret = handler_again___status_CHANGING_CHARSET(rc);
 			break;
diff --git a/lib/mysql_connection.cpp b/lib/mysql_connection.cpp
@@ -452,6 +452,7 @@ MySQL_Connection::MySQL_Connection() {
 	options.init_connect_sent=false;
 	options.session_track_gtids = NULL;
 	options.session_track_gtids_sent = false;
+	options.session_track_variables_sent = false;
 	options.ldap_user_variable=NULL;
 	options.ldap_user_variable_value=NULL;
 	options.ldap_user_variable_sent=false;
@@ -3082,6 +3083,7 @@ void MySQL_Connection::reset() {
 		options.session_track_gtids = NULL;
 		options.session_track_gtids_sent = false;
 	}
+	options.session_track_variables_sent = false;
 }
 
 bool MySQL_Connection::get_gtid(char *buff, uint64_t *trx_id) {
@@ -3116,6 +3118,48 @@ bool MySQL_Connection::get_gtid(char *buff, uint64_t *trx_id) {
 	return ret;
 }
 
+bool MySQL_Connection::get_variables(std::unordered_map<string, string>& variables) {
+	bool ret = false;
+
+	if ((mysql != nullptr)
+		&& (mysql->net.last_errno == 0)
+		&& (mysql->server_status & SERVER_SESSION_STATE_CHANGED)) {
+		// when there is no error and status changed
+		const char *data;
+		size_t length;
+
+		if (mysql_session_track_get_first(mysql, SESSION_TRACK_SYSTEM_VARIABLES, &data, &length) == 0) {
+			string var_name(data, length);
+			string val;
+
+			// get_first() returns a variable_name
+			// get_next() will return the value
+			bool expect_value = true;
+
+			while (mysql_session_track_get_next(mysql, SESSION_TRACK_SYSTEM_VARIABLES, &data, &length) == 0) {
+				if (expect_value) {
+					val = string(data, length);
+					variables[var_name] = val;
+					// got a value in this iteration
+					// in the next iteration, we have to expect a variable_name
+					expect_value = false;
+				} else {
+					var_name = string(data, length);
+					// got a variable_name in this iteration
+					// in the next iteration, we have to expect the value of this variable
+					expect_value = true;
+				}
+			}
+
+			// update counters
+			// __sync_fetch_and_add(&myds->sess->thread->status_variables.stvar[st_var_gtid_session_collected],1);
+			ret = true;
+		}
+	}
+
+	return ret;
+}
+
 void MySQL_Connection::set_ssl_params(MYSQL *mysql, MySQLServers_SslParams *ssl_params) {
 	if (ssl_params == NULL) {
 		mysql_ssl_set(mysql,
