[Postfix] Abnormal Exit of Xtrabackup caused replication stop

Summary
-------
To obtain a consistent index before locking, XPAXOS replication
will be stopped. After the backup ends, XPAXOS replication will be
restarted. If Xtrabackup exits abnormally, it may cause the xpaxos
replication status to remain in a stopped state.

Fix
---
Force use FTWRL when bakcup XCluster instance to get a valid index.

storage/innobase/xtrabackup/src/backup_mysql.cc

@@ -1139,6 +1139,12 @@ bool lock_tables_maybe(MYSQL *connection, int timeout, int retry_count) {
   bool force_ftwrl = opt_slave_info && !slave_auto_position &&
                      !(server_flavor == FLAVOR_PERCONA_SERVER);
 
+  /** If it's XCluster, always use FTWRL, because commit index (for leader),
+  apply index (for follower) can not get from p_s.log_status. */
+  if (server_flavor == FLAVOR_X_CLUSTER) {
+    force_ftwrl = true;
+  }
+
   if (tables_locked || (opt_lock_ddl_per_table && !force_ftwrl)) {
     return (true);
   }
@@ -1147,8 +1153,6 @@ bool lock_tables_maybe(MYSQL *connection, int timeout, int retry_count) {
     return (true);
   }
 
-  disable_replication_for_polarx(connection);
-
   if (have_backup_locks && !force_ftwrl) {
     return lock_tables_for_backup(connection, timeout, retry_count);
   }
@@ -1172,8 +1176,6 @@ void unlock_all(MYSQL *connection) {
   }
 
   xb::info() << "All tables unlocked";
-
-  enable_replication_for_polarx(connection);
 }
 
 static int get_open_temp_tables(MYSQL *connection) {
@@ -2628,32 +2630,3 @@ bool print_instant_versioned_tables(MYSQL *connection) {
   return ret;
 }
 
-void disable_replication_for_polarx(MYSQL *connection) {
-  if (xcluster_sql_thread_started) return;
-
-  if (server_flavor == FLAVOR_X_CLUSTER) {
-    // if slave_sql_running, stop mts
-    char *slave_sql_running = NULL;
-    mysql_variable status[] = {{"Slave_SQL_Running", &slave_sql_running},
-                               {NULL, NULL}};
-    read_mysql_variables(connection, "SHOW SLAVE STATUS", status, false);
-    if (strcmp(slave_sql_running, "Yes") == 0) {
-      xb::info() << "Disable replication for polarx...";
-      xcluster_sql_thread_started = true;
-      xb_mysql_query(connection, "STOP SLAVE SQL_THREAD", false, false);
-      xb_mysql_query(connection, "STOP XPAXOS_REPLICATION", false, false);
-    }
-    free_mysql_variables(status);
-  }
-}
-
-void enable_replication_for_polarx(MYSQL *connection) {
-  if (!xcluster_sql_thread_started) return;
-
-  if (server_flavor == FLAVOR_X_CLUSTER) {
-    xb::info() << "Enable replication for polarx...";
-    xb_mysql_query(connection, "START SLAVE SQL_THREAD", false, false);
-    xb_mysql_query(connection, "START XPAXOS_REPLICATION", false, false);
-    xcluster_sql_thread_started = false;
-  }
-}

storage/innobase/xtrabackup/src/backup_mysql.h

@@ -278,8 +278,4 @@ void check_dump_innodb_buffer_pool(MYSQL *connection);
 bool print_instant_versioned_tables(MYSQL *connection);
 extern log_status_t log_status;
 
-void disable_replication_for_polarx(MYSQL *connection);
-
-void enable_replication_for_polarx(MYSQL *connection);
-
 #endif

storage/innobase/xtrabackup/src/xtrabackup.cc

@@ -7660,7 +7660,6 @@ bool xb_init() {
     }
 
     if (opt_lock_ddl) {
-      disable_replication_for_polarx(mysql_connection);
       if (!lock_tables_for_backup(mysql_connection, opt_lock_ddl_timeout, 0))
         return (false);
     }