MySQLSlave同一server_id的冲突原因分析

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); //关键在这里，先取消注册server_id相同的Slave线程
res= my_hash_insert(&slave_list, (uchar*) si); //把新的Slave线程注册到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))) // binlog.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); //不断的发送日志给slave端
unregister_slave(thd,1,1); //发送完成后清理Slave线程，因为执行到这一步肯定是binlog dump线程被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); //从slave_list中拿掉

if (need_mutex)
pthread_mutex_unlock(&LOCK_slave_list);
}
}