Extract hostgroup routing and locking logic from MySQL and PgSQL sessions

This commit completes Milestone 3 Phase 3.5 by moving hostgroup routing
and locking decisions out of the MySQL_Session and PgSQL_Session classes.
The logic is now encapsulated in standalone, pure functions designed for
easier testing and better maintainability.

Key changes:
- Created include/MySQL_HostGroup_Routing.h and lib/MySQL_HostGroup_Routing.cpp:
  - Implements resolve_hostgroup_routing(), a pure function that handles:
    - Mirroring logic (highest priority).
    - Intercepting SHOW WARNINGS and routing to the previous HG.
    - Routing SELECT LAST_INSERT_ID() / @@IDENTITY queries to the HG
      where the last affected rows occurred.
    - Applying destination_hostgroup from Query Processor Output (QPO).
    - Enforcing hostgroup locking (mysql-set_query_lock_on_hostgroup).
- Created include/PgSQL_HostGroup_Routing.h and lib/PgSQL_HostGroup_Routing.cpp:
  - Implements resolve_pgsql_hostgroup_routing() with similar logic adapted
    for PostgreSQL sessions.
- Refactored lib/MySQL_Session.cpp and lib/PgSQL_Session.cpp:
  - Replaced scattered inline logic with calls to the new routing functions.
  - Integration points include STATE_SLEEP, STMT_PREPARE, STMT_EXECUTE,
    and the main query execution path.
- Enhanced Unit Testing:
  - Added test/tap/tests/unit/MySQL_HostGroup_Routing-t.cpp (15 test cases).
  - Added test/tap/tests/unit/PgSQL_HostGroup_Routing-t.cpp (10 test cases).
  - Verified all 25 test cases pass successfully.
- Updated Build System:
  - Modified lib/Makefile to include the new object files.
  - Updated test/tap/tests/unit/Makefile to include unit test targets.

This refactoring significantly reduces coupling between the session state
and routing logic, providing a foundation for more robust session
orchestration and simplified unit testing of critical ProxySQL features.

Issue: #5493

Author: renecannao

include/MySQL_HostGroup_Routing.h

@@ -0,0 +1,62 @@
+#ifndef __MYSQL_HOSTGROUP_ROUTING_H
+#define __MYSQL_HOSTGROUP_ROUTING_H
+
+#include <string>
+
+/**
+ * @struct MySQL_Routing_Session_State
+ * @brief Represents the session state relevant for hostgroup routing decisions.
+ */
+struct MySQL_Routing_Session_State {
+    int current_hostgroup;
+    int default_hostgroup;
+    int locked_on_hostgroup;
+    int transaction_persistent_hostgroup;
+    int last_HG_affected_rows;
+    int warning_in_hg;
+    bool autocommit;
+    int autocommit_on_hostgroup;
+    bool mirror;
+};
+
+/**
+ * @struct MySQL_Routing_QPO_State
+ * @brief Represents the Query Processor Output relevant for hostgroup routing decisions.
+ */
+struct MySQL_Routing_QPO_State {
+    int destination_hostgroup;
+    bool is_set_statement; // Derived from query parsing
+    bool is_show_warnings; // Derived from query parsing
+    bool is_last_insert_id; // Derived from query parsing
+    bool is_version_query; // Derived from query parsing
+};
+
+/**
+ * @struct MySQL_Routing_Result
+ * @brief Represents the output of the hostgroup routing decision.
+ */
+struct MySQL_Routing_Result {
+    int new_current_hostgroup;
+    int new_locked_on_hostgroup;
+    bool lock_hostgroup;
+    bool error;
+    std::string error_msg;
+};
+
+/**
+ * @brief Resolves the target hostgroup and locking decisions based on session and QPO state.
+ * 
+ * This is a pure function designed to be easily testable.
+ * 
+ * @param sess_state Current session state.
+ * @param qpo_state Query Processor Output state.
+ * @param set_query_lock_on_hostgroup Global configuration (mysql-set_query_lock_on_hostgroup).
+ * @return MySQL_Routing_Result The routing decision.
+ */
+MySQL_Routing_Result resolve_hostgroup_routing(
+    const MySQL_Routing_Session_State& sess_state,
+    const MySQL_Routing_QPO_State& qpo_state,
+    int set_query_lock_on_hostgroup
+);
+
+#endif // __MYSQL_HOSTGROUP_ROUTING_H

include/PgSQL_HostGroup_Routing.h

@@ -0,0 +1,54 @@
+#ifndef __PGSQL_HOSTGROUP_ROUTING_H
+#define __PGSQL_HOSTGROUP_ROUTING_H
+
+#include <string>
+
+/**
+ * @struct PgSQL_Routing_Session_State
+ * @brief Represents the session state relevant for hostgroup routing decisions in PostgreSQL.
+ */
+struct PgSQL_Routing_Session_State {
+    int current_hostgroup;
+    int default_hostgroup;
+    int locked_on_hostgroup;
+    int transaction_persistent_hostgroup;
+};
+
+/**
+ * @struct PgSQL_Routing_QPO_State
+ * @brief Represents the Query Processor Output relevant for hostgroup routing decisions in PostgreSQL.
+ */
+struct PgSQL_Routing_QPO_State {
+    int destination_hostgroup;
+    bool lock_hostgroup; // Derived from query parsing
+};
+
+/**
+ * @struct PgSQL_Routing_Result
+ * @brief Represents the output of the hostgroup routing decision for PostgreSQL.
+ */
+struct PgSQL_Routing_Result {
+    int new_current_hostgroup;
+    int new_locked_on_hostgroup;
+    bool lock_hostgroup;
+    bool error;
+    std::string error_msg;
+};
+
+/**
+ * @brief Resolves the target hostgroup and locking decisions based on session and QPO state.
+ * 
+ * This is a pure function designed to be easily testable.
+ * 
+ * @param sess_state Current session state.
+ * @param qpo_state Query Processor Output state.
+ * @param set_query_lock_on_hostgroup Global configuration (pgsql-set_query_lock_on_hostgroup).
+ * @return PgSQL_Routing_Result The routing decision.
+ */
+PgSQL_Routing_Result resolve_pgsql_hostgroup_routing(
+    const PgSQL_Routing_Session_State& sess_state,
+    const PgSQL_Routing_QPO_State& qpo_state,
+    int set_query_lock_on_hostgroup
+);
+
+#endif // __PGSQL_HOSTGROUP_ROUTING_H

lib/Makefile

@@ -88,7 +88,7 @@ MYCXXFLAGS := $(STDCPP) $(MYCFLAGS) $(PSQLCH) $(PSQLGA) $(PSQL31) $(PSQLFFTO) $(
 default: libproxysql.a
 .PHONY: default
 
-_OBJ_CXX := ProxySQL_GloVars.oo network.oo debug.oo configfile.oo Query_Cache.oo SpookyV2.oo MySQL_Authentication.oo gen_utils.oo sqlite3db.oo mysql_connection.oo MySQL_HostGroups_Manager.oo mysql_data_stream.oo MySQL_Thread.oo MySQL_Session.oo MySQL_Protocol.oo mysql_backend.oo Query_Processor.oo MySQL_Query_Processor.oo PgSQL_Query_Processor.oo  ProxySQL_Admin.oo ProxySQL_Config.oo ProxySQL_Restapi.oo MySQL_Monitor.oo MySQL_Logger.oo log_utils.oo thread.oo MySQL_PreparedStatement.oo ProxySQL_Cluster.oo ClickHouse_Authentication.oo ClickHouse_Server.oo ProxySQL_Statistics.oo Chart_bundle_js.oo ProxySQL_HTTP_Server.oo ProxySQL_RESTAPI_Server.oo font-awesome.min.css.oo main-bundle.min.css.oo MySQL_Variables.oo c_tokenizer.oo proxysql_utils.oo proxysql_coredump.oo proxysql_sslkeylog.oo \
+_OBJ_CXX := ProxySQL_GloVars.oo network.oo debug.oo configfile.oo Query_Cache.oo SpookyV2.oo MySQL_Authentication.oo gen_utils.oo sqlite3db.oo mysql_connection.oo MySQL_HostGroups_Manager.oo mysql_data_stream.oo MySQL_Thread.oo MySQL_Session.oo MySQL_HostGroup_Routing.oo PgSQL_Session.oo PgSQL_HostGroup_Routing.oo MySQL_Protocol.oo mysql_backend.oo Query_Processor.oo MySQL_Query_Processor.oo PgSQL_Query_Processor.oo  ProxySQL_Admin.oo ProxySQL_Config.oo ProxySQL_Restapi.oo MySQL_Monitor.oo MySQL_Logger.oo log_utils.oo thread.oo MySQL_PreparedStatement.oo ProxySQL_Cluster.oo ClickHouse_Authentication.oo ClickHouse_Server.oo ProxySQL_Statistics.oo Chart_bundle_js.oo ProxySQL_HTTP_Server.oo ProxySQL_RESTAPI_Server.oo font-awesome.min.css.oo main-bundle.min.css.oo MySQL_Variables.oo c_tokenizer.oo proxysql_utils.oo proxysql_coredump.oo proxysql_sslkeylog.oo \
 	sha256crypt.oo \
 	BaseSrvList.oo BaseHGC.oo Base_HostGroups_Manager.oo \
 	QP_rule_text.oo QP_query_digest_stats.oo \

lib/MySQL_HostGroup_Routing.cpp

@@ -0,0 +1,76 @@
+#include "MySQL_HostGroup_Routing.h"
+
+MySQL_Routing_Result resolve_hostgroup_routing(
+    const MySQL_Routing_Session_State& sess_state,
+    const MySQL_Routing_QPO_State& qpo_state,
+    int set_query_lock_on_hostgroup
+) {
+    MySQL_Routing_Result result;
+    result.new_current_hostgroup = sess_state.current_hostgroup;
+    result.new_locked_on_hostgroup = sess_state.locked_on_hostgroup;
+    result.lock_hostgroup = false;
+    result.error = false;
+    result.error_msg = "";
+
+    // 1. Mirroring
+    if (sess_state.mirror) {
+        result.new_current_hostgroup = qpo_state.destination_hostgroup;
+        return result;
+    }
+
+    // 2. SHOW WARNINGS / SHOW COUNT(*) WARNINGS
+    if (qpo_state.is_show_warnings) {
+        if (sess_state.warning_in_hg > -1) {
+            result.new_current_hostgroup = sess_state.warning_in_hg;
+        }
+        return result;
+    }
+
+    // 3. LAST_INSERT_ID / @@IDENTITY
+    if (qpo_state.is_last_insert_id) {
+        if (sess_state.last_HG_affected_rows >= 0) {
+            result.new_current_hostgroup = sess_state.last_HG_affected_rows;
+            return result;
+        }
+    }
+
+    // 4. Default routing from QPO
+    if (qpo_state.destination_hostgroup >= 0) {
+        if (sess_state.transaction_persistent_hostgroup == -1) {
+            result.new_current_hostgroup = qpo_state.destination_hostgroup;
+        }
+    }
+
+    // 5. Hostgroup Locking Decisions (mysql-set_query_lock_on_hostgroup)
+    if (set_query_lock_on_hostgroup == 1) {
+        // Algorithm introduced in ProxySQL 2.0.6
+        if (result.new_locked_on_hostgroup < 0) {
+            if (qpo_state.is_set_statement) {
+                // If it's a SET statement that caused locking (determined by parser)
+                // In a pure function, we assume qpo_state.is_set_statement implies it should lock
+                result.lock_hostgroup = true;
+                result.new_locked_on_hostgroup = result.new_current_hostgroup;
+            }
+        }
+
+        if (result.new_locked_on_hostgroup >= 0) {
+            if (result.new_current_hostgroup != result.new_locked_on_hostgroup) {
+                result.error = true;
+                result.error_msg = "ProxySQL Error: connection is locked to hostgroup " + 
+                                   std::to_string(result.new_locked_on_hostgroup) + 
+                                   " but trying to reach hostgroup " + 
+                                   std::to_string(result.new_current_hostgroup);
+                return result;
+            }
+        }
+    } else {
+        // Legacy behavior before 2.0.6
+        if (sess_state.transaction_persistent_hostgroup == -1) {
+            if (qpo_state.destination_hostgroup < 0) {
+                result.new_current_hostgroup = sess_state.default_hostgroup;
+            }
+        }
+    }
+
+    return result;
+}