Extract hostgroup routing decision logic (Phase 3.5, #5493)

New files:
- include/HostgroupRouting.h: HostgroupRoutingDecision struct +
  resolve_hostgroup_routing() declaration
- lib/HostgroupRouting.cpp: implementation mirroring the routing
  block in get_pkts_from_client() (~lines 5340-5377)

Logic covers: QP destination override, transaction affinity,
hostgroup lock acquisition/enforcement, error on HG mismatch.
Identical for both MySQL and PgSQL protocols.

include/HostgroupRouting.h

@@ -0,0 +1,52 @@
+/**
+ * @file HostgroupRouting.h
+ * @brief Pure hostgroup routing decision logic for unit testability.
+ *
+ * Extracted from MySQL_Session::get_pkts_from_client() and
+ * PgSQL_Session::get_pkts_from_client(). The logic is identical
+ * for both protocols.
+ *
+ * @see Phase 3.5 (GitHub issue #5493)
+ */
+
+#ifndef HOSTGROUP_ROUTING_H
+#define HOSTGROUP_ROUTING_H
+
+/**
+ * @brief Result of a hostgroup routing decision.
+ */
+struct HostgroupRoutingDecision {
+	int target_hostgroup;         ///< Resolved hostgroup to route to.
+	int new_locked_on_hostgroup;  ///< Updated lock state (-1 = unlocked).
+	bool error;                   ///< True if an illegal HG switch was attempted.
+};
+
+/**
+ * @brief Resolve the target hostgroup given session state and QP output.
+ *
+ * Decision logic (mirrors get_pkts_from_client()):
+ * 1. Start with default_hostgroup as the target
+ * 2. If QP provides a destination (>= 0) and no transaction lock,
+ *    use the QP destination
+ * 3. If transaction_persistent_hostgroup >= 0, override with transaction HG
+ * 4. If locking is enabled and lock_hostgroup flag is set, acquire lock
+ * 5. If already locked, verify target matches lock (error if mismatch)
+ *
+ * @param default_hostgroup           Session's default hostgroup.
+ * @param qpo_destination_hostgroup   Query Processor output destination (-1 = no override).
+ * @param transaction_persistent_hostgroup Current transaction HG (-1 = none).
+ * @param locked_on_hostgroup         Current lock state (-1 = unlocked).
+ * @param lock_hostgroup_flag         Whether the QP wants to acquire a lock.
+ * @param lock_enabled                Whether set_query_lock_on_hostgroup is enabled.
+ * @return HostgroupRoutingDecision with resolved target and updated lock.
+ */
+HostgroupRoutingDecision resolve_hostgroup_routing(
+	int default_hostgroup,
+	int qpo_destination_hostgroup,
+	int transaction_persistent_hostgroup,
+	int locked_on_hostgroup,
+	bool lock_hostgroup_flag,
+	bool lock_enabled
+);
+
+#endif // HOSTGROUP_ROUTING_H

lib/HostgroupRouting.cpp

@@ -0,0 +1,60 @@
+/**
+ * @file HostgroupRouting.cpp
+ * @brief Implementation of pure hostgroup routing decision logic.
+ *
+ * Mirrors the routing block in get_pkts_from_client() (MySQL_Session.cpp
+ * ~lines 5340-5377 and PgSQL_Session.cpp ~lines 2154-2189).
+ *
+ * @see HostgroupRouting.h
+ * @see Phase 3.5 (GitHub issue #5493)
+ */
+
+#include "HostgroupRouting.h"
+
+HostgroupRoutingDecision resolve_hostgroup_routing(
+	int default_hostgroup,
+	int qpo_destination_hostgroup,
+	int transaction_persistent_hostgroup,
+	int locked_on_hostgroup,
+	bool lock_hostgroup_flag,
+	bool lock_enabled)
+{
+	HostgroupRoutingDecision d;
+	d.error = false;
+	d.new_locked_on_hostgroup = locked_on_hostgroup;
+
+	// Start with default hostgroup
+	int current_hostgroup = default_hostgroup;
+
+	// If QP provides a valid destination and no transaction lock, use it
+	if (qpo_destination_hostgroup >= 0
+		&& transaction_persistent_hostgroup == -1) {
+		current_hostgroup = qpo_destination_hostgroup;
+	}
+
+	// Transaction affinity overrides everything
+	if (transaction_persistent_hostgroup >= 0) {
+		current_hostgroup = transaction_persistent_hostgroup;
+	}
+
+	// Hostgroup locking logic (algorithm introduced in 2.0.6)
+	if (lock_enabled) {
+		if (locked_on_hostgroup < 0) {
+			// Not yet locked
+			if (lock_hostgroup_flag) {
+				// Acquire lock on the current (already resolved) hostgroup
+				d.new_locked_on_hostgroup = current_hostgroup;
+			}
+		}
+		if (d.new_locked_on_hostgroup >= 0) {
+			// Already locked (or just acquired) — enforce
+			if (current_hostgroup != d.new_locked_on_hostgroup) {
+				d.error = true;
+			}
+			current_hostgroup = d.new_locked_on_hostgroup;
+		}
+	}
+
+	d.target_hostgroup = current_hostgroup;
+	return d;
+}

lib/Makefile

@@ -106,6 +106,7 @@ _OBJ_CXX := ProxySQL_GloVars.oo network.oo debug.oo configfile.oo Query_Cache.oo
 	PgSQL_PreparedStatement.oo PgSQL_Extended_Query_Message.oo \
 	pgsql_tokenizer.oo \
 	MonitorHealthDecision.oo \
+	HostgroupRouting.oo \
 	proxy_sqlite3_symbols.oo
 
 # TSDB object files