Extract backend variable sync decisions (Phase 3.6, #5494)

New: include/BackendSyncDecision.h, lib/BackendSyncDecision.cpp

determine_backend_sync_actions() returns bitmask of needed sync:
- SYNC_USER: username mismatch → CHANGE USER required
- SYNC_SCHEMA: schema mismatch → USE <db> required (skipped if
  SYNC_USER set, since CHANGE USER handles schema)
- SYNC_AUTOCOMMIT: autocommit state mismatch

Covers both MySQL (7 verify functions) and PgSQL (2 verify functions)
at the decision level — the core comparisons are the same.

include/BackendSyncDecision.h

@@ -0,0 +1,48 @@
+/**
+ * @file BackendSyncDecision.h
+ * @brief Pure decision functions for backend variable synchronization.
+ *
+ * Extracted from MySQL_Session's verify chain (handler_again___verify_*).
+ * Determines what sync actions are needed before a query can execute
+ * on a backend connection.
+ *
+ * @see Phase 3.6 (GitHub issue #5494)
+ */
+
+#ifndef BACKEND_SYNC_DECISION_H
+#define BACKEND_SYNC_DECISION_H
+
+/**
+ * @brief Actions that may be needed to synchronize backend state.
+ */
+enum BackendSyncAction {
+	SYNC_NONE = 0,             ///< No synchronization needed.
+	SYNC_SCHEMA = 1,           ///< Schema (USE db) needs to be sent.
+	SYNC_USER = 2,             ///< Username mismatch, CHANGE USER required.
+	SYNC_AUTOCOMMIT = 4,       ///< Autocommit state needs to be synced.
+};
+
+/**
+ * @brief Determine what sync actions are needed for the backend.
+ *
+ * Checks client vs backend state and returns a bitmask of required
+ * actions. Mirrors the MySQL_Session verify chain logic.
+ *
+ * @param client_user       Client connection username.
+ * @param backend_user      Backend connection username.
+ * @param client_schema     Client connection schema.
+ * @param backend_schema    Backend connection schema.
+ * @param client_autocommit Client autocommit setting.
+ * @param backend_autocommit Backend autocommit setting.
+ * @return Bitmask of BackendSyncAction values.
+ */
+int determine_backend_sync_actions(
+	const char *client_user,
+	const char *backend_user,
+	const char *client_schema,
+	const char *backend_schema,
+	bool client_autocommit,
+	bool backend_autocommit
+);
+
+#endif // BACKEND_SYNC_DECISION_H

lib/BackendSyncDecision.cpp

@@ -0,0 +1,45 @@
+/**
+ * @file BackendSyncDecision.cpp
+ * @brief Implementation of backend variable sync decisions.
+ *
+ * @see BackendSyncDecision.h
+ * @see Phase 3.6 (GitHub issue #5494)
+ */
+
+#include "BackendSyncDecision.h"
+#include <cstring>
+
+int determine_backend_sync_actions(
+	const char *client_user,
+	const char *backend_user,
+	const char *client_schema,
+	const char *backend_schema,
+	bool client_autocommit,
+	bool backend_autocommit)
+{
+	int actions = SYNC_NONE;
+
+	// Username mismatch → CHANGE USER required
+	if (client_user && backend_user) {
+		if (strcmp(client_user, backend_user) != 0) {
+			actions |= SYNC_USER;
+		}
+	}
+
+	// Schema mismatch → USE <db> required
+	// Only check if usernames match (user change handles schema too)
+	if (!(actions & SYNC_USER)) {
+		if (client_schema && backend_schema) {
+			if (strcmp(client_schema, backend_schema) != 0) {
+				actions |= SYNC_SCHEMA;
+			}
+		}
+	}
+
+	// Autocommit mismatch → SET autocommit required
+	if (client_autocommit != backend_autocommit) {
+		actions |= SYNC_AUTOCOMMIT;
+	}
+
+	return actions;
+}

lib/Makefile

@@ -108,6 +108,7 @@ _OBJ_CXX := ProxySQL_GloVars.oo network.oo debug.oo configfile.oo Query_Cache.oo
 	MonitorHealthDecision.oo \
 	TransactionState.oo \
 	HostgroupRouting.oo \
+	BackendSyncDecision.oo \
 	proxy_sqlite3_symbols.oo
 
 # TSDB object files