MySQL半同步复制源码解析

时间:2022-09-26 14:49:57

今天 DBA 同事问了一个问题,MySQL在半同步复制的场景下,当关闭从节点时使得从节点的数量 < rpl_semi_sync_master_wait_for_slave_count时,show full processlist 的结果不同,具体表现如下:

AFTER_SYNC表现如下:

MySQL半同步复制源码解析

 

 可以发现,只有一个查询线程处于 Waiting for semi-sync ACK from slave 状态,其他查询线程处于 query end 状态。

 

AFTER_COMMIT 表现如下:

MySQL半同步复制源码解析

 

 和 AFTER_SYNC 不同, 所有的查询线程处于 Waiting for semi-sync ACK from slave 状态;

 

之前已经了解过 MySQL半同步复制,这次从源码的角度来解析MySQL半同步复制到底是如何进行的,同时分析原因。

首先看事务的提交过程,整体的提交流程过长,切之前已经研究过源码,这里仅对关于半同步复制相关的部分做深入分析:

int MYSQL_BIN_LOG::ordered_commit(THD *thd, bool all, bool skip_commit)
{  ....
  // 执行 flush 阶段操作。
  /*
  * 1. 对 flush 队列进行 fetch, 本次处理的flush队列就固定了
    2. 在 innodb 存储引擎中 flush redo log, 做 innodb 层 redo 持久化。
    3. 为 flush 队列中每个事务生成 gtid。
    4. 将 flush队列中每个线程的 binlog cache flush 到 binlog 日志文件中。这里包含两步:
            1. 将事务的 GTID event直接写入 binlog 磁盘文件中
            2. 将事务生成的别的 event 写入 binlog file cache 中
  */
  flush_error = process_flush_stage_queue(&total_bytes, &do_rotate,
                                          &wait_queue);
  // 将 binary log cache(IO cache) flush到文件中
  if (flush_error == 0 && total_bytes > 0)
// 这里获取到 flush 队列中最后一个事务在 binlog 中的 end_pos flush_error
= flush_cache_to_file(&flush_end_pos); DBUG_EXECUTE_IF("crash_after_flush_binlog", DBUG_SUICIDE();); // sync_binlog 是否等于 1 update_binlog_end_pos_after_sync = (get_sync_period() == 1); /* If the flush finished successfully, we can call the after_flush hook. Being invoked here, we have the guarantee that the hook is executed before the before/after_send_hooks on the dump thread preventing race conditions among these plug-ins. 如果 flush 操作成功, 则调用 after_flush hook。 */ if (flush_error == 0) { const char *file_name_ptr = log_file_name + dirname_length(log_file_name); assert(flush_end_pos != 0); // 观察者模式,调用 Binlog_storage_observer 里面的repl_semi_report_binlog_update函数,将当前的 binlog 文件和最新的 pos 点记录到 active_tranxs_ 列表中 // file_name_ptr 当前写入的binlog文件 // flush_end_pos 组提交flush链表里面所有binlog最后的pos点 if (RUN_HOOK(binlog_storage, after_flush, (thd, file_name_ptr, flush_end_pos))) { sql_print_error("Failed to run 'after_flush' hooks"); flush_error = ER_ERROR_ON_WRITE; }    // 不等于 1, 通知 dump 线程 if (!update_binlog_end_pos_after_sync)
    // 更新 binlog end pos, 通知 binlog sender 线程向从库发送 event update_binlog_end_pos(); DBUG_EXECUTE_IF(
"crash_commit_after_log", DBUG_SUICIDE();); } ...... DEBUG_SYNC(thd, "bgc_after_flush_stage_before_sync_stage"); /* Stage #2: Syncing binary log file to disk */
  /** 释放 Lock_log mutex, 获取 Lock_sync mutex
   *  第一个进入的 flush 队列的 leader 为本阶段的 leader, 其他 flush 队列加入 sync 队列, 其他 flush 队列的
   * leader会被阻塞, 直到 commit 阶段被 leader 线程唤醒。
   * */
  if (change_stage(thd, Stage_manager::SYNC_STAGE, wait_queue, &LOCK_log, &LOCK_sync))
  {
    DBUG_RETURN(finish_commit(thd));
  }

  /*
    根据 delay 的设置来决定是否延迟一段时间, 如果 delay 的时间越久, 那么加入 sync 队列的
    事务就越多【last commit 是在 binlog prepare 时生成的, 尚未更改, 因此加入 sync 队列的
    事务是同一组事务】, 提高了从库 mts 的效率。
*/
  if (!flush_error && (sync_counter + 1 >= get_sync_period()))
    stage_manager.wait_count_or_timeout(opt_binlog_group_commit_sync_no_delay_count,
                                        opt_binlog_group_commit_sync_delay,
                                        Stage_manager::SYNC_STAGE);
    // fetch sync 队列, 对 sync 队列进行固化.
  final_queue = stage_manager.fetch_queue_for(Stage_manager::SYNC_STAGE);
    // 这里 sync_binlog file到磁盘中
if (flush_error == 0 && total_bytes > 0)
  {
      // 根据 sync_binlog 的设置决定是否刷盘
    std::pair<bool, bool> result = sync_binlog_file(false);
  }
    // 在这里 sync_binlog = 1, 更新 binlog end_pos, 通知 dump 线程发送 event
if (update_binlog_end_pos_after_sync)
  {
    THD *tmp_thd = final_queue;
    const char *binlog_file = NULL;
    my_off_t pos = 0;
    while (tmp_thd->next_to_commit != NULL)
      tmp_thd = tmp_thd->next_to_commit;
    if (flush_error == 0 && sync_error == 0)
    {
      tmp_thd->get_trans_fixed_pos(&binlog_file, &pos);
        // 更新 binlog end pos, 通知 dump 线程
      update_binlog_end_pos(binlog_file, pos);
    }
  }
  DEBUG_SYNC(thd, "bgc_after_sync_stage_before_commit_stage");
  leave_mutex_before_commit_stage = &LOCK_sync;
  /*
    Stage #3: Commit all transactions in order.
    按顺序在 Innodb 层提交所有事务。
    如果我们不需要对提交顺序进行排序, 并且每个线程必须执行 handlerton 提交, 那么这个阶段可以跳过。
    然而, 由于我们保留了前一阶段的锁, 如果我们跳过这个阶段, 则必须进行解锁。
*/
commit_stage:
    // 如果需要顺序提交
if (opt_binlog_order_commits &&
      (sync_error == 0 || binlog_error_action != ABORT_SERVER))
  {
     // SYNC队列加入 COMMIT 队列, 第一个进入的 SYNC 队列的 leader 为本阶段的 leader。其他 sync 队列
     // 加入 commit 队列的 leade 会被阻塞, 直到 COMMIT 阶段后被 leader 线程唤醒。
     // 释放 lock_sync mutex, 持有 lock_commit mutex.
if (change_stage(thd, Stage_manager::COMMIT_STAGE,
                     final_queue, leave_mutex_before_commit_stage,
                     &LOCK_commit))
    {
      DBUG_PRINT("return", ("Thread ID: %u, commit_error: %d",
                            thd->thread_id(), thd->commit_error));
      DBUG_RETURN(finish_commit(thd));
    }
    THD *commit_queue = stage_manager.fetch_queue_for(Stage_manager::COMMIT_STAGE);
    DBUG_EXECUTE_IF("semi_sync_3-way_deadlock",
                    DEBUG_SYNC(thd, "before_process_commit_stage_queue"););

    if (flush_error == 0 && sync_error == 0)
      // 调用 after_sync hook.注意:对于after_sync, 这里将等待binlog dump 线程收到slave节点关于队列中事务最新的 binlog_file和 binlog_pos的ACK。
      sync_error = call_after_sync_hook(commit_queue);
     /* process_commit_stage_queue 将为队列中每个 thd 持有的 GTID
      调用 update_on_commit 或 update_on_rollback。
      这样做的目的是确保 gtid 按照顺序添加到 GTIDs中, 避免出现不必要的间隙
      如果我们只允许每个线程在完成提交时调用 update_on_commit, 则无法保证 GTID
      顺序, 并且 gtid_executed 之间可能出现空隙。发生这种情况, server必须从
      Gtid_set 中添加和删除间隔, 添加或删除间隔需要一个互斥锁, 这会降低性能。
    */
    // 在这里, 进入存储引擎中提交
    process_commit_stage_queue(thd, commit_queue);
    // 退出 Lock_commit 锁
    mysql_mutex_unlock(&LOCK_commit);
    /* 在 LOCK_commit 释放之后处理 after_commit 来避免 user thread, rotate thread 和 dump thread的
       3路死锁。
    */
    // 处理 after_commit HOOK
    process_after_commit_stage_queue(thd, commit_queue);
  }
  else
  {
      // 释放锁, 调用 after_sync hook.
if (leave_mutex_before_commit_stage)
      mysql_mutex_unlock(leave_mutex_before_commit_stage);
    if (flush_error == 0 && sync_error == 0)
      sync_error = call_after_sync_hook(final_queue);
  }
......
/* Finish the commit before executing a rotate, or run the risk of a deadlock. We don't need the return value here since it is in thd->commit_error, which is returned below. */ (void)finish_commit(thd); ...... }

在以上过程中,可以看到,在 flush 节点之后会执行 AFTER_FLUSH hook, 这个 hook 会将当前的 binlog 文件和最新的 pos 点位记录到 active_tranxs_ 链表中,这个链表在半同步复制等待 slave 节点 apply 中使用:

AFTER_FLUSH:
-----------------------------------------------------------
int Binlog_storage_delegate::after_flush(THD *thd,
                                         const char *log_file,
                                         my_off_t log_pos)
{
  DBUG_ENTER("Binlog_storage_delegate::after_flush");
  DBUG_PRINT("enter", ("log_file: %s, log_pos: %llu",
                       log_file, (ulonglong) log_pos));
  Binlog_storage_param param;
  param.server_id= thd->server_id;

  int ret= 0;
// 这里观察者模式 FOREACH_OBSERVER(ret, after_flush, thd, (
&param, log_file, log_pos)); DBUG_RETURN(ret); }
int repl_semi_report_binlog_update(Binlog_storage_param *param,
                   const char *log_file,
                   my_off_t log_pos)
{
  int  error= 0;

  if (repl_semisync.getMasterEnabled())
  {
    /*
      Let us store the binlog file name and the position, so that
      we know how long to wait for the binlog to the replicated to
      the slave in synchronous replication.
// 这里将 binlog filename & pos 写入 active_tranxs_ 链表
*/ error= repl_semisync.writeTranxInBinlog(log_file, log_pos); } return error; }

半同步复制的关键是对 after_sync 和 after_commit 的不同选择,因此这里我们主要分析 call_after_sync_hook(commit_queue) 和 process_after_commit_stage_queue(thd, commit_queue) 函数,这两个函数中分别调用了  RUN_HOOK(binlog_storage, after_sync, (queue_head, log_file, pos)) 和 RUN_HOOK(transaction, after_commit, (head, all)) 函数,其分别对应 Binlog_storage_delegate::after_sync(THD *thd, const char *log_file,my_off_t log_pos) 和 Trans_delegate::after_commit(THD *thd, bool all) 函数, 这里采用观察者模式,我们直接找到其对应的实现:

AFTER_SYNC:
-----------------------------------------------------------------
static inline int call_after_sync_hook(THD *queue_head)
{
  const char *log_file = NULL;
  my_off_t pos = 0;

  if (NO_HOOK(binlog_storage))
    return 0;

  assert(queue_head != NULL);
  for (THD *thd = queue_head; thd != NULL; thd = thd->next_to_commit)
    if (likely(thd->commit_error == THD::CE_NONE))
// 可以看到,这里获取了固化后的 commit 队列中的最新的事务的 binlog filename & pos thd
->get_trans_fixed_pos(&log_file, &pos); // 使用最新的 binlog filename & pos 调用 after_sync hook if (DBUG_EVALUATE_IF("simulate_after_sync_hook_error", 1, 0) || RUN_HOOK(binlog_storage, after_sync, (queue_head, log_file, pos))) { sql_print_error("Failed to run 'after_sync' hooks"); return ER_ERROR_ON_WRITE; } return 0; } // after_sync 函数定义 int Binlog_storage_delegate::after_sync(THD *thd, const char *log_file, my_off_t log_pos) { DBUG_ENTER("Binlog_storage_delegate::after_sync"); DBUG_PRINT("enter", ("log_file: %s, log_pos: %llu", log_file, (ulonglong) log_pos)); Binlog_storage_param param; param.server_id= thd->server_id; assert(log_pos != 0); int ret= 0; FOREACH_OBSERVER(ret, after_sync, thd, (&param, log_file, log_pos)); // 找到观察器调用, 这是是观察者模式 DEBUG_SYNC(thd, "after_call_after_sync_observer"); DBUG_RETURN(ret); }
AFTER_SYNC:
----------------------------------------------------------------------------------------------------------
// after_sync() 接口的具体实现 int repl_semi_report_binlog_sync(Binlog_storage_param *param, const char *log_file, my_off_t log_pos) { // 是否是 after_sync 模式 if (rpl_semi_sync_master_wait_point == WAIT_AFTER_SYNC) // 执行事务的线程等待从库的回复, 即等待 ACK 的实现函数 return repl_semisync.commitTrx(log_file, log_pos); return 0; }
AFTER_COMMIT:
-----------------------------------------------------------------------
void MYSQL_BIN_LOG::process_after_commit_stage_queue(THD *thd, THD *first)
{
  for (THD *head = first; head; head = head->next_to_commit)
  {
    if (head->get_transaction()->m_flags.run_hooks &&
        head->commit_error != THD::CE_COMMIT_ERROR)
    {

      /*
        TODO: This hook here should probably move outside/below this
              if and be the only after_commit invocation left in the
              code.
      */
#ifndef EMBEDDED_LIBRARY
      Thd_backup_and_restore switch_thd(thd, head);
#endif /* !EMBEDDED_LIBRARY */
      bool all = head->get_transaction()->m_flags.real_commit;
// 可以看到,这里针对固化的 commit 队列中的每一个事务都进行了 after_commit HOOK. (
void)RUN_HOOK(transaction, after_commit, (head, all)); /* When after_commit finished for the transaction, clear the run_hooks flag. This allow other parts of the system to check if after_commit was called. */ head->get_transaction()->m_flags.run_hooks = false; } } } int Trans_delegate::after_commit(THD *thd, bool all) { DBUG_ENTER("Trans_delegate::after_commit"); Trans_param param; TRANS_PARAM_ZERO(param); param.server_uuid= server_uuid; param.thread_id= thd->thread_id(); param.rpl_channel_type = thd->rpl_thd_ctx.get_rpl_channel_type(); bool is_real_trans= (all || !thd->get_transaction()->is_active(Transaction_ctx::SESSION)); if (is_real_trans) param.flags|= TRANS_IS_REAL_TRANS; thd->get_trans_fixed_pos(&param.log_file, &param.log_pos); param.server_id= thd->server_id; DBUG_PRINT("enter", ("log_file: %s, log_pos: %llu", param.log_file, param.log_pos)); DEBUG_SYNC(thd, "before_call_after_commit_observer"); int ret= 0;
// 这里观察者模式 FOREACH_OBSERVER(ret, after_commit, thd, (
&param)); DBUG_RETURN(ret); }
AFTER_COMMIT:
----------------------------------------------------------------------
// after_commit 实际调用函数
int repl_semi_report_commit(Trans_param *param)
{

  bool is_real_trans= param->flags & TRANS_IS_REAL_TRANS;
  // semi_sync 是 AFTER_COMMIT && 是真正的事务 
  if (rpl_semi_sync_master_wait_point == WAIT_AFTER_COMMIT &&
      is_real_trans && param->log_pos)
  {
    const char *binlog_name= param->log_file;
    // 执行事务的线程等待从库的回复, 即等待 ACK 的实现函数
    return repl_semisync.commitTrx(binlog_name, param->log_pos);
  }
  return 0;
}
// 执行事务的线程等待从库的回复, 即等待 ACK 的实现函数
int ReplSemiSyncMaster::commitTrx(const char* trx_wait_binlog_name,
                  my_off_t trx_wait_binlog_pos)
{
  const char *kWho = "ReplSemiSyncMaster::commitTrx";

  function_enter(kWho);
  PSI_stage_info old_stage;

#if defined(ENABLED_DEBUG_SYNC)
  /* debug sync may not be initialized for a master */
  if (current_thd->debug_sync_control)
    DEBUG_SYNC(current_thd, "rpl_semisync_master_commit_trx_before_lock");
#endif
  /* Acquire the mutex. 
  获取 LOCK_binlog_ 互斥锁
  */
  lock();
  
  TranxNode* entry= NULL;
  mysql_cond_t* thd_cond= NULL;
  bool is_semi_sync_trans= true;
  // active_transx_ 为当前活跃的事务链表,在 after_flush HOOK 中会将 flush 队列中最新的事务的 binlog filename & pos 添加到该链表中
  // trx_wait_binlog_name 为固化的 commit 队列中最新的事务的 binlog filename 
  if (active_tranxs_ != NULL && trx_wait_binlog_name)
  {
    // 遍历 active_tranxs_ 活跃的事务链表, 找到大于等于 trx_wait_binlog_name 和 trx_wait_binlog_pos 
    // 的第一个事务
    entry=
      active_tranxs_->find_active_tranx_node(trx_wait_binlog_name,
                                             trx_wait_binlog_pos);
    // 如果找到了第一个事务                                         
    if (entry)
      thd_cond= &entry->cond;
  }
  /* This must be called after acquired the lock */
  // 当前线程进入 thd_cond 
  THD_ENTER_COND(NULL, thd_cond, &LOCK_binlog_,
                 & stage_waiting_for_semi_sync_ack_from_slave,
                 & old_stage);
  // 如果主库启用了半同步 
  if (getMasterEnabled() && trx_wait_binlog_name)
  {
    struct timespec start_ts;
    struct timespec abstime;
    int wait_result;
    // 设置当前时间 start_ts
    set_timespec(&start_ts, 0);
    /* This is the real check inside the mutex. */
    // 主库没有启动半同步 || 没有启动半同步复制, l_end
    if (!getMasterEnabled() || !is_on())
      goto l_end;

    if (trace_level_ & kTraceDetail)
    {
      sql_print_information("%s: wait pos (%s, %lu), repl(%d)\n", kWho,
                            trx_wait_binlog_name, (unsigned long)trx_wait_binlog_pos,
                            (int)is_on());
    }

    /* Calcuate the waiting period. */
#ifndef HAVE_STRUCT_TIMESPEC
      abstime.tv.i64 = start_ts.tv.i64 + (__int64)wait_timeout_ * TIME_THOUSAND * 10;
      abstime.max_timeout_msec= (long)wait_timeout_;
#else
      // wait_timeout 时间
      abstime.tv_sec = start_ts.tv_sec + wait_timeout_ / TIME_THOUSAND;
      abstime.tv_nsec = start_ts.tv_nsec +
        (wait_timeout_ % TIME_THOUSAND) * TIME_MILLION;
      if (abstime.tv_nsec >= TIME_BILLION)
      {
        abstime.tv_sec++;
        abstime.tv_nsec -= TIME_BILLION;
      }
#endif /* _WIN32 */
    // 打开了半同步
    while (is_on())
    {
      // 如果有从库回复
      if (reply_file_name_inited_)
      {
        // 比较从库回复的日志坐标(filename & fileops)和固化的 commit 队列中最新的事务的 binlog filename & pos
        int cmp = ActiveTranx::compare(reply_file_name_, reply_file_pos_,
                                       trx_wait_binlog_name, trx_wait_binlog_pos);
        // 如果回复的日志坐标大于当前的日志坐标                               
        if (cmp >= 0)
        {
          /* We have already sent the relevant binlog to the slave: no need to
           * wait here.
             我们已经确认将相应的 binlog 发送给了从库: 无需在此等待。
           */
          if (trace_level_ & kTraceDetail)
            sql_print_information("%s: Binlog reply is ahead (%s, %lu),",
                                  kWho, reply_file_name_, (unsigned long)reply_file_pos_);
          // 退出循环                        
          break;
        }
      }
      /*
        When code reaches here an Entry object may not be present in the
        following scenario.
        当代码到了这里, 在一下场景中可能不存在 entry。
        Semi sync was not enabled when transaction entered into ordered_commit
        process. During flush stage, semi sync was not enabled and there was no
        'Entry' object created for the transaction being committed and at a
        later stage it was enabled. In this case trx_wait_binlog_name and
        trx_wait_binlog_pos are set but the 'Entry' object is not present. Hence
        dump thread will not wait for reply from slave and it will not update
        reply_file_name. In such case the committing transaction should not wait
        for an ack from slave and it should be considered as an async
        transaction.
        事务进入 ordered_commit 时未启用半同步。
        在 flush 阶段, 没有启用半同步, 没有为提交的事务创建 entry 对象, 但是在之后的节点启用了半同步。
        在这种情况下, 设置了 trx_wait_binlog_name 和 trx_wait_binlog_pos, 但是 entry 对象并不存在。
        此时, dump 线程将不会等待 slave 节点的 reply, 并且不会更新 reply_file_name。
        在这种情况下, 提交的事务不应等待来自 slave 节点的 ack, 而应被视为异步事务。
      */
      if (!entry)
      {
        is_semi_sync_trans= false;
        goto l_end;
      }

      /* Let us update the info about the minimum binlog position of waiting
       * threads.
       * 这里更新等待线程等待的 minimum binlog pos 。
       */
      if (wait_file_name_inited_)
      {
        // 对比当前 commit 队列最后的binlog点位 和 wait_file_name_ & wait_file_pos_ 大小
        int cmp = ActiveTranx::compare(trx_wait_binlog_name, trx_wait_binlog_pos,
                                       wait_file_name_, wait_file_pos_);
        if (cmp <= 0)
          {
          /* This thd has a lower position, let's update the minimum info. 
          这里更新 wait_file_name_ & wait_file_pos_。
          */
          strncpy(wait_file_name_, trx_wait_binlog_name, sizeof(wait_file_name_) - 1);
          wait_file_name_[sizeof(wait_file_name_) - 1]= '\0';
          wait_file_pos_ = trx_wait_binlog_pos;

          rpl_semi_sync_master_wait_pos_backtraverse++;
          if (trace_level_ & kTraceDetail)
            sql_print_information("%s: move back wait position (%s, %lu),",
                                  kWho, wait_file_name_, (unsigned long)wait_file_pos_);
        }
      }
      else
      {
        strncpy(wait_file_name_, trx_wait_binlog_name, sizeof(wait_file_name_) - 1);
        wait_file_name_[sizeof(wait_file_name_) - 1]= '\0';
        wait_file_pos_ = trx_wait_binlog_pos;
        wait_file_name_inited_ = true;

        if (trace_level_ & kTraceDetail)
          sql_print_information("%s: init wait position (%s, %lu),",
                                kWho, wait_file_name_, (unsigned long)wait_file_pos_);
      }

      /* In semi-synchronous replication, we wait until the binlog-dump
       * thread has received the reply on the relevant binlog segment from the
       * replication slave.
       * 在半同步复制中, 我们等待直到 binlog dump 线程收到相关 binlog 的 reply 信息。
       * 
       * Let us suspend this thread to wait on the condition;
       * when replication has progressed far enough, we will release
       * these waiting threads.
       * 让我们暂停这个线程以等待这个条件; 
       * 当复制进展足够时, 我们将释放等待的线程。
       */
      // 判断 slave 个数和半同步是否正常
      // 当前 slave 节点的数量 == rpl_semi_sync_master_wait_for_slave_count -1 && 半同步复制正开启
      if (abort_loop && (rpl_semi_sync_master_clients ==
                         rpl_semi_sync_master_wait_for_slave_count - 1) && is_on())
      {
        sql_print_warning("SEMISYNC: Forced shutdown. Some updates might "
                          "not be replicated.");
        // 关闭半同步, 中断循环                
        switch_off();
        break;
      }
      //正式进入等待binlog同步的步骤,将rpl_semi_sync_master_wait_sessions+1
            //然后发起等待信号,进入信号等待后,只有2种情况可以退出等待。1是被其他线程唤醒(binlog dump)
            //2是等待超时时间。如果是被唤醒则返回值是0,否则是其他值
      rpl_semi_sync_master_wait_sessions++;
      
      if (trace_level_ & kTraceDetail)
        sql_print_information("%s: wait %lu ms for binlog sent (%s, %lu)",
                              kWho, wait_timeout_,
                              wait_file_name_, (unsigned long)wait_file_pos_);
      
      /* wait for the position to be ACK'ed back 
      实现 ACK 等待
      */
      assert(entry);
      entry->n_waiters++;
      // 第一个参数为条件量,第二个为等待之后释放LOCK_binlog_互斥锁,第三个为未来的超时绝对时间
      wait_result= mysql_cond_timedwait(&entry->cond, &LOCK_binlog_, &abstime);
      entry->n_waiters--;
      /*
        After we release LOCK_binlog_ above while waiting for the condition,
        it can happen that some other parallel client session executed
        RESET MASTER. That can set rpl_semi_sync_master_wait_sessions to zero.
        Hence check the value before decrementing it and decrement it only if it is
        non-zero value.
        在等待之后释放 LOCK_binlog_互斥锁, 有可能其他客户端执行 RESET MASTER 命令, 这将把 rpl_semi_sync_master_wait_sessions 重置为 0。
        因此, 在递减前需要检查该值。
      */
      if (rpl_semi_sync_master_wait_sessions > 0)
        rpl_semi_sync_master_wait_sessions--;
      // wait_result != 0, 这里表示等待超时
      if (wait_result != 0)
      {
        /* This is a real wait timeout. */
        sql_print_warning("Timeout waiting for reply of binlog (file: %s, pos: %lu), "
                          "semi-sync up to file %s, position %lu.",
                          trx_wait_binlog_name, (unsigned long)trx_wait_binlog_pos,
                          reply_file_name_, (unsigned long)reply_file_pos_);
        rpl_semi_sync_master_wait_timeouts++;
        
        /* switch semi-sync off ; 关闭 semi sync  */
        switch_off();
      }
      else
      // 等待 ACK 成功
      {
        int wait_time;
        
        wait_time = getWaitTime(start_ts);
        // wait_time < 0, 时钟错误
        if (wait_time < 0)
        {
          if (trace_level_ & kTraceGeneral)
          {
            sql_print_information("Assessment of waiting time for commitTrx "
                                  "failed at wait position (%s, %lu)",
                                  trx_wait_binlog_name,
                                  (unsigned long)trx_wait_binlog_pos);
          }
          rpl_semi_sync_master_timefunc_fails++;
        }
        else
        {
          //将等待事件与该等待计入总数  
          rpl_semi_sync_master_trx_wait_num++;
          rpl_semi_sync_master_trx_wait_time += wait_time;
        }
      }
    }

l_end:
    /* Update the status counter. 
    更新状态计数
    */
    if (is_on() && is_semi_sync_trans)
      rpl_semi_sync_master_yes_transactions++;
    else
      rpl_semi_sync_master_no_transactions++;
  }

  /* Last waiter removes the TranxNode 
  移除 active_tranxs_ 链表中 trx_wait_binlog_name & trx_wait_binlog_pos 之前的所有事务。
  */
  if (trx_wait_binlog_name && active_tranxs_
      && entry && entry->n_waiters == 0)
    active_tranxs_->clear_active_tranx_nodes(trx_wait_binlog_name,
                                             trx_wait_binlog_pos);

  unlock();
  THD_EXIT_COND(NULL, & old_stage);
  return function_exit(kWho, 0);
}

 

通过以上源码分析,可以看到在 after_sync hook 之后会释放 Lock_commit 锁,而后调用 after_commit hook。

因此当 AFTER_SYNC 时,会发现只有一个查询线程处于 Waiting for semi-sync ACK from slave 状态,其他查询线程处于 query end 状态。

而 AFTER_COMMIT 时,所有的查询线程都处于 Waiting for semi-sync ACK from slave 状态。