事务执行开始，先获取commit的mdl锁，然后进行2pc判断，开启binlog的就使用binlog作为协调器，binlog继承了tc_log
进入prepare，commit
xa 
if (!trn_ctx->no_2pc(trx_scope) && (trn_ctx->rw_ha_count(trx_scope) > 1))
      error= tc_log->prepare(thd, all);
      
mysqld!trx_undo_set_state_at_prepare(trx_t*, trx_undo_t*, bool, mtr_t*) (/Users/xiaoyu.bai/Downloads/mysql-5.7.29/storage/innobase/trx/trx0undo.cc:1910)
mysqld!trx_prepare_low(trx_t*, trx_undo_ptr_t*, bool) (/Users/xiaoyu.bai/Downloads/mysql-5.7.29/storage/innobase/trx/trx0trx.cc:2783)
mysqld!trx_prepare(trx_t*) (/Users/xiaoyu.bai/Downloads/mysql-5.7.29/storage/innobase/trx/trx0trx.cc:2829)
mysqld!trx_prepare_for_mysql(trx_t*) (/Users/xiaoyu.bai/Downloads/mysql-5.7.29/storage/innobase/trx/trx0trx.cc:2914)
mysqld!innobase_xa_prepare(handlerton*, THD*, bool) (/Users/xiaoyu.bai/Downloads/mysql-5.7.29/storage/innobase/handler/ha_innodb.cc:17143)
mysqld!ha_prepare_low(THD*, bool) (/Users/xiaoyu.bai/Downloads/mysql-5.7.29/sql/handler.cc:2359)
mysqld!MYSQL_BIN_LOG::prepare(THD*, bool) (/Users/xiaoyu.bai/Downloads/mysql-5.7.29/sql/binlog.cc:8607)
mysqld!ha_commit_trans(THD*, bool, bool) (/Users/xiaoyu.bai/Downloads/mysql-5.7.29/sql/handler.cc:1791)
mysqld!trans_commit_stmt(THD*) (/Users/xiaoyu.bai/Downloads/mysql-5.7.29/sql/transaction.cc:465)
mysqld!mysql_execute_command(THD*, bool) (/Users/xiaoyu.bai/Downloads/mysql-5.7.29/sql/sql_parse.cc:4995)
mysqld!mysql_parse(THD*, Parser_state*) (/Users/xiaoyu.bai/Downloads/mysql-5.7.29/sql/sql_parse.cc:5584)
mysqld!dispatch_command(THD*, COM_DATA const*, enum_server_command) (/Users/xiaoyu.bai/Downloads/mysql-5.7.29/sql/sql_parse.cc:1491)
mysqld!do_command(THD*) (/Users/xiaoyu.bai/Downloads/mysql-5.7.29/sql/sql_parse.cc:1032)
mysqld!::handle_connection(void *) (/Users/xiaoyu.bai/Downloads/mysql-5.7.29/sql/conn_handler/connection_handler_per_thread.cc:313)
mysqld!::pfs_spawn_thread(void *) (/Users/xiaoyu.bai/Downloads/mysql-5.7.29/storage/perfschema/pfs.cc:2197)
libsystem_pthread.dylib!_pthread_start (Unknown Source:0)
libsystem_pthread.dylib!thread_start (Unknown Source:0)
这部分是将undo标记prepare，将flush redo推迟到
group commit的flush阶段。
commit就是刷binlog cache,写入文件，执行ordered_commit，组提交

xa协调器部分故障处理怎么做？超时重试？

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