MySQL半同步复制--RUN_HOOK

半同步复制只是一个插件，一个动态加载的插件怎么实现事务处理线程等待、停下来呢？

这个插件只是个实现体，在MySQL源码中还是需要支撑这个实现体的框架。即在代码的某些位置，加入一些桩observer，在执行到这些桩的时候，如果有插件在这些observer中注册了实现体，就这执行这些实现体，否则什么也不执行。

RUN_HOOK就是这些桩的调用接口：

#define RUN_HOOK(group, hook, args)  (group ##_delegate->is_empty() ? 0 : group ##_delegate->hook args)

以RUN_HOOK(binlog_relay_io, thread_start, (thd, mi))为例：

binlog_relay_io_delegate->thread_start(thd,mi)是RUN_HOOK转换后的调用。

int Binlog_relay_IO_delegate::thread_start(THD *thd, Master_info *mi){  Binlog_relay_IO_param param;  init_param(&param, mi);  int ret= 0;  FOREACH_OBSERVER(ret, thread_start, thd, (¶m));  return ret;}

关键在FOREACH_OBSERVER这个宏定义：

#define FOREACH_OBSERVER(r, f, thd, args) \...for (; info; info= iter++){     if (((Observer *)info->observer)->f && ((Observer *)info->observer)->f args){        ...    }}...

到这里就很清楚了，此处用了观察者模式，最终回调typedef Binlog_relay_IO_observer Observer的thread_start.初始化的时候Binlog_relay_IO_observer是以插件的形式通过register_binlog_relay_io_observer函数注册到观察者list里面，所以最终实现io的功能是在plugin/semisync实现.

replication.h/**   Observes and extends the service of slave IO thread.*/typedef struct Binlog_relay_IO_observer {  uint32 len;  /**     This callback is called when slave IO thread starts     @param param Observer common parameter     @retval 0 Sucess     @retval 1 Failure  */  int (*thread_start)(Binlog_relay_IO_param *param);  /**     This callback is called when slave IO thread stops     @param param Observer common parameter     @retval 0 Sucess     @retval 1 Failure  */  int (*thread_stop)(Binlog_relay_IO_param *param);  /**     This callback is called before slave requesting binlog transmission from master     This is called before slave issuing BINLOG_DUMP command to master     to request binlog.     @param param Observer common parameter     @param flags binlog dump flags     @retval 0 Sucess     @retval 1 Failure  */  int (*before_request_transmit)(Binlog_relay_IO_param *param, uint32 flags);  /**     This callback is called after read an event packet from master     @param param Observer common parameter     @param packet The event packet read from master     @param len Length of the event packet read from master     @param event_buf The event packet return after process     @param event_len The length of event packet return after process     @retval 0 Sucess     @retval 1 Failure  */  int (*after_read_event)(Binlog_relay_IO_param *param,                          const char *packet, unsigned long len,                          const char **event_buf, unsigned long *event_len);  /**     This callback is called after written an event packet to relay log     @param param Observer common parameter     @param event_buf Event packet written to relay log     @param event_len Length of the event packet written to relay log     @param flags flags for relay log     @retval 0 Sucess     @retval 1 Failure  */  int (*after_queue_event)(Binlog_relay_IO_param *param,                           const char *event_buf, unsigned long event_len,                           uint32 flags);  /**     This callback is called after reset slave relay log IO status          @param param Observer common parameter     @retval 0 Sucess     @retval 1 Failure  */  int (*after_reset_slave)(Binlog_relay_IO_param *param);} Binlog_relay_IO_observer;

声明：

semisync_slave_plugin.cc：：Binlog_relay_IO_observer relay_io_observer = {  sizeof(Binlog_relay_IO_observer), // len  repl_semi_slave_io_start,// start  repl_semi_slave_io_end,// stop  repl_semi_slave_request_dump,// request_transmit  repl_semi_slave_read_event,// after_read_event  repl_semi_slave_queue_event,// after_queue_event  repl_semi_reset_slave,// reset};

即thread_start指向的是函数repl_semi_slave_io_start函数。至此run_hook调用结束。