Extract MySQL error classification logic (Phase 3.7, #5495)

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

Functions:
- classify_mysql_error(): classifies error codes as retryable (1047
  WSREP, 1053 shutdown) or fatal, checking retry conditions
- can_retry_on_new_connection(): checks if offline server retry is
  possible given connection state (reusable, no txn, no transfer)

include/MySQLErrorClassifier.h

@@ -0,0 +1,76 @@
+/**
+ * @file MySQLErrorClassifier.h
+ * @brief Pure MySQL error classification for retry decisions.
+ *
+ * Extracted from MySQL_Session handler_ProcessingQueryError_CheckBackendConnectionStatus()
+ * and handler_minus1_HandleErrorCodes().
+ *
+ * @see Phase 3.7 (GitHub issue #5495)
+ */
+
+#ifndef MYSQL_ERROR_CLASSIFIER_H
+#define MYSQL_ERROR_CLASSIFIER_H
+
+/**
+ * @brief Action to take after a MySQL backend query error.
+ */
+enum MySQLErrorAction {
+	MYSQL_ERROR_CONTINUE,            ///< Error handled, continue processing.
+	MYSQL_ERROR_RETRY_ON_NEW_CONN,   ///< Reconnect and retry on a new server.
+	MYSQL_ERROR_REPORT_TO_CLIENT     ///< Send error to client, no retry.
+};
+
+/**
+ * @brief Classify a MySQL error code to determine retry eligibility.
+ *
+ * Mirrors the logic in handler_minus1_HandleErrorCodes():
+ * - Error 1047 (WSREP not ready): retryable if conditions permit
+ * - Error 1053 (server shutdown): retryable if conditions permit
+ * - Other errors: report to client
+ *
+ * Retry is only possible when:
+ *   - query_retries_on_failure > 0
+ *   - connection is reusable
+ *   - no active transaction
+ *   - multiplex not disabled
+ *
+ * @param error_code              MySQL error number.
+ * @param retries_remaining       Number of retries left.
+ * @param connection_reusable     Whether the connection can be reused.
+ * @param in_active_transaction   Whether a transaction is in progress.
+ * @param multiplex_disabled      Whether multiplexing is disabled.
+ * @return MySQLErrorAction indicating what to do.
+ */
+MySQLErrorAction classify_mysql_error(
+	unsigned int error_code,
+	int retries_remaining,
+	bool connection_reusable,
+	bool in_active_transaction,
+	bool multiplex_disabled
+);
+
+/**
+ * @brief Check if a backend query can be retried on a new connection.
+ *
+ * Mirrors handler_ProcessingQueryError_CheckBackendConnectionStatus().
+ * A retry is possible when the server is offline AND all retry
+ * conditions are met.
+ *
+ * @param server_offline          Whether the backend server is offline.
+ * @param retries_remaining       Number of retries left.
+ * @param connection_reusable     Whether the connection can be reused.
+ * @param in_active_transaction   Whether a transaction is in progress.
+ * @param multiplex_disabled      Whether multiplexing is disabled.
+ * @param transfer_started        Whether result transfer has already begun.
+ * @return true if the query should be retried on a new connection.
+ */
+bool can_retry_on_new_connection(
+	bool server_offline,
+	int retries_remaining,
+	bool connection_reusable,
+	bool in_active_transaction,
+	bool multiplex_disabled,
+	bool transfer_started
+);
+
+#endif // MYSQL_ERROR_CLASSIFIER_H

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 \
+	MySQLErrorClassifier.oo \
 	proxy_sqlite3_symbols.oo
 
 # TSDB object files

lib/MySQLErrorClassifier.cpp

@@ -0,0 +1,66 @@
+/**
+ * @file MySQLErrorClassifier.cpp
+ * @brief Implementation of MySQL error classification.
+ *
+ * @see MySQLErrorClassifier.h
+ * @see Phase 3.7 (GitHub issue #5495)
+ */
+
+#include "MySQLErrorClassifier.h"
+
+MySQLErrorAction classify_mysql_error(
+	unsigned int error_code,
+	int retries_remaining,
+	bool connection_reusable,
+	bool in_active_transaction,
+	bool multiplex_disabled)
+{
+	// Check if this error code is retryable
+	bool retryable_error = false;
+	switch (error_code) {
+		case 1047:  // ER_UNKNOWN_COM_ERROR (WSREP not ready)
+		case 1053:  // ER_SERVER_SHUTDOWN
+			retryable_error = true;
+			break;
+		default:
+			break;
+	}
+
+	if (!retryable_error) {
+		return MYSQL_ERROR_REPORT_TO_CLIENT;
+	}
+
+	// Check retry conditions (mirrors handler_minus1_HandleErrorCodes)
+	if (retries_remaining > 0
+		&& connection_reusable
+		&& !in_active_transaction
+		&& !multiplex_disabled) {
+		return MYSQL_ERROR_RETRY_ON_NEW_CONN;
+	}
+
+	return MYSQL_ERROR_REPORT_TO_CLIENT;
+}
+
+bool can_retry_on_new_connection(
+	bool server_offline,
+	int retries_remaining,
+	bool connection_reusable,
+	bool in_active_transaction,
+	bool multiplex_disabled,
+	bool transfer_started)
+{
+	if (!server_offline) {
+		return false;  // server is fine, no retry needed
+	}
+
+	// Mirror handler_ProcessingQueryError_CheckBackendConnectionStatus
+	if (retries_remaining > 0
+		&& connection_reusable
+		&& !in_active_transaction
+		&& !multiplex_disabled
+		&& !transfer_started) {
+		return true;
+	}
+
+	return false;
+}