docs: Add comprehensive Doxygen documentation for session variable tracking

Add detailed architectural documentation for PR 5166 session variable tracking:

- High-level architecture overview in session_track_variables struct explaining the 3-phase workflow
- Detailed documentation for handler_rc0_Process_Variables explaining the core processing workflow
- Technical implementation details for MySQL_Connection::get_variables protocol interface
- Configuration logic documentation for handler_again___verify_backend_session_track_variables
- Added inline comments explaining why session tracking is needed and performance considerations

This documentation provides a complete understanding of how MySQL session variable tracking integrates with ProxySQL's existing state machine and leverages MySQL's native session tracking capabilities.

include/MySQL_Session.h

@@ -225,6 +225,35 @@ 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();
+	/**
+	 * @brief Verify and configure session variable tracking on backend connections.
+	 *
+	 * === PR 5166: Backend Configuration Entry Point ===
+	 *
+	 * This function is the main orchestrator for setting up session tracking on backend
+	 * connections. It's called during connection initialization to ensure tracking is
+	 * properly configured before query processing begins.
+	 *
+	 * CONFIGURATION LOGIC:
+	 * 1. Check if global mysql-session_track_variables is enabled
+	 * 2. For each tracking flag that's not yet set on the connection:
+	 *    - Mark the flag as sent (prevents duplicate configuration)
+	 *    - Transition session to appropriate configuration state
+	 *    - Return true to indicate state machine needs re-processing
+	 *
+	 * STATE MACHINE INTEGRATION:
+	 * - SETTING_SESSION_TRACK_VARIABLES: Sets session_track_system_variables="*"
+	 * - SETTING_SESSION_TRACK_STATE: Sets session_track_state_change=ON
+	 * - Returns true to continue state machine processing until both are configured
+	 *
+	 * WHY THIS APPROACH:
+	 * - Ensures tracking is configured exactly once per backend connection
+	 * - Integrates cleanly with existing ProxySQL session state machine
+	 * - Handles both tracking capabilities independently for flexibility
+	 * - Prevents redundant SET commands on already configured connections
+	 *
+	 * @return true if session state needs to be re-processed (configuration pending), false otherwise
+	 */
 	bool handler_again___verify_backend_session_track_variables();
 	bool handler_again___verify_backend_multi_statement();
 	bool handler_again___verify_backend_user_schema();
@@ -233,7 +262,27 @@ class MySQL_Session: public Base_Session<MySQL_Session, MySQL_Data_Stream, MySQL
 	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 *);
+	/**
+	 * @brief Handle the SETTING_SESSION_TRACK_VARIABLES state.
+	 *
+	 * This method executes the SET command to configure session_track_system_variables="*"
+	 * on the backend connection, enabling the server to track changes to all system
+	 * variables and report them back to ProxySQL.
+	 *
+	 * @param _rc Pointer to return code that will be set with the operation result
+	 * @return true if session state needs to be re-processed, false otherwise
+	 */
 	bool handler_again___status_SETTING_SESSION_TRACK_VARIABLES(int *);
+	/**
+	 * @brief Handle the SETTING_SESSION_TRACK_STATE state.
+	 *
+	 * This method executes the SET command to configure session_track_state_change=ON
+	 * on the backend connection, enabling the server to report when session state
+	 * changes occur (including system variable changes).
+	 *
+	 * @param _rc Pointer to return code that will be set with the operation result
+	 * @return true if session state needs to be re-processed, false otherwise
+	 */
 	bool handler_again___status_SETTING_SESSION_TRACK_STATE(int *);
 	bool handler_again___status_CHANGING_CHARSET(int *_rc);
 	bool handler_again___status_CHANGING_SCHEMA(int *);
@@ -283,6 +332,34 @@ 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);
+	/**
+	 * @brief Process session variable changes from backend connection response.
+	 *
+	 * === PR 5166: Variable Processing Workflow ===
+	 *
+	 * This function is the core of the variable tracking system and is called after
+	 * every successful query execution when SERVER_SESSION_STATE_CHANGED flag is set.
+	 *
+	 * DETAILED WORKFLOW:
+	 * 1. Extract variable changes from MySQL's session tracking data via get_variables()
+	 * 2. Iterate through all tracked variables in mysql_tracked_variables array
+	 * 3. For each variable that changed in the backend:
+	 *    - Update both client and server variable maps for state consistency
+	 *    - Handle character set variables specially (convert names to internal IDs)
+	 * 4. This ensures ProxySQL's internal state matches the actual backend state
+	 *
+	 * WHY THIS IS NEEDED:
+	 * - SQL statement parsing cannot detect all variable changes (e.g., stored procedures)
+	 * - Some variables are changed implicitly by MySQL server operations
+	 * - Without this tracking, client and backend states can diverge
+	 *
+	 * PERFORMANCE CONSIDERATIONS:
+	 * - Only called when SERVER_SESSION_STATE_CHANGED flag is set
+	 * - Processes all tracked variables but only updates those that actually changed
+	 * - Character set conversions are done only for relevant variables
+	 *
+	 * @param myconn Pointer to the MySQL connection from which to extract variable changes
+	 */
 	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);

include/MySQL_Thread.h

@@ -336,10 +336,41 @@ struct th_metrics_map_idx {
 	};
 };
 
+/**
+ * @brief Configuration structure for session variable tracking functionality.
+ *
+ * === PR 5166: Session Variable Tracking Architecture ===
+ *
+ * This PR introduces comprehensive tracking of session-specific system variables
+ * by leveraging MySQL's native session tracking capabilities. The overall workflow
+ * consists of three main phases:
+ *
+ * 1. CONFIGURATION PHASE:
+ *    - Global enable/disable via mysql-session_track_variables variable
+ *    - Per-connection tracking state managed via flags in MySQL_Connection
+ *    - New session states handle configuration of tracking on backends
+ *
+ * 2. BACKEND SETUP PHASE:
+ *    - When enabled, configure session_track_system_variables="*"
+ *    - Configure session_track_state_change=ON
+ *    - MySQL server then automatically tracks all system variable changes
+ *
+ * 3. PROCESSING PHASE:
+ *    - After each query, check SERVER_SESSION_STATE_CHANGED flag
+ *    - Extract variable changes via MySQL's session tracking protocol
+ *    - Update both client and server variable maps for state consistency
+ *    - Special handling for character set variables (name → ID conversion)
+ *
+ * BENEFITS:
+ * - Captures changes that SQL parsing alone cannot detect (stored procedures, etc.)
+ * - Redundant tracking ensures accurate client/backend state synchronization
+ * - Leverages MySQL server's native capabilities for reliability
+ * - Performance optimized: only processes when session state actually changes
+ */
 struct session_track_variables {
 	enum mode {
-		DISABLED = 0,
-		ENABLED
+		DISABLED = 0,	///< Session variable tracking is disabled (default)
+		ENABLED			///< Session variable tracking is enabled
 	};
 };
 
@@ -597,6 +628,15 @@ class MySQL_Threads_Handler
 		int processlist_max_query_length;
 
 		bool ignore_min_gtid_annotations;
+		/**
+		 * @brief Configuration flag to enable/disable session variable tracking.
+		 *
+		 * When set to session_track_variables::ENABLED (1), ProxySQL will configure
+		 * backend connections to track system variable changes using MySQL's
+		 * session_track_system_variables and session_track_state_change capabilities.
+		 *
+		 * Default: session_track_variables::DISABLED (0)
+		 */
 		int session_track_variables;
 	} variables;
 	struct {

include/mysql_connection.h

@@ -88,9 +88,9 @@ class MySQL_Connection {
 		char * session_track_gtids;
 		char *ldap_user_variable;
 		char *ldap_user_variable_value;
-		bool session_track_gtids_sent;
-		bool session_track_variables_sent;
-		bool session_track_state_sent;
+		bool session_track_gtids_sent;		///< Flag indicating if GTID session tracking has been configured on this connection
+		bool session_track_variables_sent;	///< Flag indicating if session_track_system_variables has been configured on this connection
+		bool session_track_state_sent;		///< Flag indicating if session_track_state_change has been configured on this connection
 		bool ldap_user_variable_sent;
 		uint8_t protocol_version;
 		int8_t last_set_autocommit;
@@ -264,6 +264,35 @@ class MySQL_Connection {
 	void reset();
 
 	bool get_gtid(char *buff, uint64_t *trx_id);
+	/**
+	 * @brief Extract session variable changes from MySQL's session tracking system.
+	 *
+	 * === PR 5166: Backend Variable Extraction ===
+	 *
+	 * This method is the interface to MySQL's native session tracking protocol and
+	 * extracts system variable changes that occurred during the last query execution.
+	 *
+	 * TECHNICAL DETAILS:
+	 * - Uses mysql_session_track_get_first/next() to iterate through SESSION_TRACK_SYSTEM_VARIABLES
+	 * - Only processes when SERVER_SESSION_STATE_CHANGED flag is set and no errors occurred
+	 * - Handles the variable/value pairing protocol where variables and values are returned alternately
+	 * - Populates a map with variable names as keys and their new values as values
+	 *
+	 * PROTOCOL FLOW:
+	 * 1. Check if session state changed and no errors occurred
+	 * 2. Get first tracked system variable (name)
+	 * 3. Iterate to get corresponding value, next variable name, etc.
+	 * 4. Build variable_name → value mappings in the provided map
+	 *
+	 * INTEGRATION POINT:
+	 * This is called by handler_rc0_Process_Variables() which then processes the
+	 * extracted changes to update ProxySQL's internal state. The separation allows
+	 * for testing and potential future extensions of the tracking protocol.
+	 *
+	 * @param variables Reference to unordered_map that will be populated with
+	 *                 variable names as keys and their new values as values
+	 * @return true if session variable changes were found and extracted, false otherwise
+	 */
 	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--; };