MySQL多個Slave同一server_id的沖突原因分析

 case COM_REGISTER_SLAVE:
 {
 if (!register_slave(thd, (uchar*)packet, packet_length))
 my_ok(thd);
 break;
 }

int register_slave(THD* thd, uchar* packet, uint packet_length)
{
 int res;
 SLAVE_INFO *si;
 uchar *p= packet, *p_end= packet + packet_length;
.... //省略
 if (!(si->master_id= uint4korr(p)))
 si->master_id= server_id;
 si->thd= thd;
 pthread_mutex_lock(&LOCK_slave_list);
 unregister_slave(thd,0,0); //關(guān)鍵在這里，先取消注冊server_id相同的Slave線程
 res= my_hash_insert(&slave_list, (uchar*) si); //把新的Slave線程注冊到slave_list
 pthread_mutex_unlock(&LOCK_slave_list);
 return res;
.....
}

 case COM_BINLOG_DUMP:
 {
 ulong pos;
 ushort flags;
 uint32 slave_server_id;
 
 status_var_increment(thd->status_var.com_other);
 thd->enable_slow_log= opt_log_slow_admin_statements;
 if (check_global_access(thd, REPL_SLAVE_ACL))
 break;
 
 /* TODO: The following has to be changed to an 8 byte integer */
 pos = uint4korr(packet);
 flags = uint2korr(packet + 4);
 thd->server_id=0; /* avoid suicide */
 if ((slave_server_id= uint4korr(packet+6))) // mysqlbinlog.server_id==0
 kill_zombie_dump_threads(slave_server_id);
 thd->server_id = slave_server_id;
 
 general_log_print(thd, command, "Log: '%s' Pos: %ld", packet+10,
 (long) pos);
 mysql_binlog_send(thd, thd->strdup(packet + 10), (my_off_t) pos, flags); //不斷的發(fā)送日志給slave端
 unregister_slave(thd,1,1); //發(fā)送完成后清理Slave線程，因為執(zhí)行到這一步肯定是binlog dump線程被kill了
 /* fake COM_QUIT -- if we get here, the thread needs to terminate */
 error = TRUE;
 break;
 }

void unregister_slave(THD* thd, bool only_mine, bool need_mutex)
{
 if (thd->server_id)
 {
 if (need_mutex)
 pthread_mutex_lock(&LOCK_slave_list);
 
 SLAVE_INFO* old_si;
 if ((old_si = (SLAVE_INFO*)hash_search(&slave_list,
 (uchar*)&thd->server_id, 4)) &&
 (!only_mine || old_si->thd == thd)) //拿到slave值
 hash_delete(&slave_list, (uchar*)old_si); //從slave_list中拿掉
 
 if (need_mutex)
 pthread_mutex_unlock(&LOCK_slave_list);
 }
}